Stone to Atomic Mass Unit Converter
Convert stones to atomic mass units with our free online weight converter.
Quick Answer
1 Stone = 3.824236e+27 atomic mass units
Formula: Stone × conversion factor = Atomic Mass Unit
Use the calculator below for instant, accurate conversions.
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All conversion formulas on UnitsConverter.io have been verified against NIST (National Institute of Standards and Technology) guidelines and international SI standards. Our calculations are accurate to 10 decimal places for standard conversions and use arbitrary precision arithmetic for astronomical units.
Stone to Atomic Mass Unit Calculator
How to Use the Stone to Atomic Mass Unit Calculator:
- Enter the value you want to convert in the 'From' field (Stone).
- The converted value in Atomic Mass Unit will appear automatically in the 'To' field.
- Use the dropdown menus to select different units within the Weight category.
- Click the swap button (⇌) to reverse the conversion direction.
How to Convert Stone to Atomic Mass Unit: Step-by-Step Guide
Converting Stone to Atomic Mass Unit involves multiplying the value by a specific conversion factor, as shown in the formula below.
Formula:
1 Stone = 3.82424e+27 atomic mass unitsExample Calculation:
Convert 5 stones: 5 × 3.82424e+27 = 1.91212e+28 atomic mass units
Disclaimer: For Reference Only
These conversion results are provided for informational purposes only. While we strive for accuracy, we make no guarantees regarding the precision of these results, especially for conversions involving extremely large or small numbers which may be subject to the inherent limitations of standard computer floating-point arithmetic.
Not for professional use. Results should be verified before use in any critical application. View our Terms of Service for more information.
Need to convert to other weight units?
View all Weight conversions →What is a Stone and a Atomic Mass Unit?
1 stone = 14 avoirdupois pounds (lb) = 6.35029318 kilograms (kg) EXACT
The stone (symbol: st) is a unit of mass in the Imperial system, legally defined in terms of pounds, which are themselves defined in terms of kilograms. The exact conversion is:
1 pound = 0.45359237 kilograms (international definition, 1959)
1 stone = 14 × 0.45359237 kg = 6.35029318 kg
Stone and Pounds Notation
The stone is almost never used alone for body weight. Instead, it's combined with additional pounds:
Format: "X stone Y pounds" or "X st Y lb"
Examples:
- 10 st 0 lb = 10 stone exactly = 140 lb = 63.5 kg
- 10 st 7 lb = 10 stone + 7 pounds = 147 lb = 66.7 kg
- 12 st 3 lb = 12 stone + 3 pounds = 171 lb = 77.6 kg
Why this format? It provides precision without unwieldy decimal places. Saying "10.5 stone" is rare—people say "10 stone 7" instead (10 stone + 7 pounds = 10.5 stone).
Stone vs. Kilogram vs. Pound
Three systems for measuring body weight:
| System | Unit | Used In | Precision | |-----------|----------|-------------|---------------| | Imperial (UK) | Stone + Pounds | UK, Ireland | "11 st 7 lb" (161 lb) | | Imperial (US) | Pounds only | United States, Canada | "161 lb" | | Metric | Kilograms | Most of the world | "73 kg" |
Cultural difference:
- Americans say "I weigh 161 pounds"
- British say "I weigh 11 stone 7" (rarely "161 pounds")
- Europeans say "I weigh 73 kilograms"
Why 14 Pounds?
The number 14 has no scientific basis—it's purely historical. Medieval England used base-12 counting (duodecimal) for some systems:
- 12 inches = 1 foot
- 12 pence = 1 shilling (pre-1971)
- But 14 pounds = 1 stone (not 12!)
Theory: The 14-pound wool stone emerged from trade practices. A "sack of wool" weighed 364 pounds = 26 stones (26 × 14 = 364), a convenient round number for taxation and commerce.
What Is an Atomic Mass Unit?
The atomic mass unit (symbol: u), also called the unified atomic mass unit or Dalton (symbol: Da), is a unit of mass used for expressing atomic and molecular masses.
Official definition: 1 u = exactly 1/12 of the mass of one unbound carbon-12 atom at rest in its ground state
Value in SI units: 1 u = 1.660 539 066 60 × 10⁻²⁷ kg (with uncertainty ±0.000 000 000 50 × 10⁻²⁷ kg)
Why Use Atomic Mass Units Instead of Kilograms?
Atomic and molecular masses in kilograms are extraordinarily small and unwieldy:
In kilograms (impractical):
- Hydrogen atom: 1.674 × 10⁻²⁷ kg
- Water molecule: 2.992 × 10⁻²⁶ kg
- Glucose molecule: 2.990 × 10⁻²⁵ kg
In atomic mass units (convenient):
- Hydrogen atom: 1.008 u
- Water molecule: 18.015 u
- Glucose molecule: 180.16 u
The atomic mass unit scales numbers to manageable sizes while maintaining precision for chemical calculations.
Carbon-12: The Reference Standard
Why carbon-12?
- Exact definition: ¹²C is defined as exactly 12 u (no uncertainty)
- Abundant: Carbon-12 comprises 98.89% of natural carbon
- Stable: Not radioactive, doesn't decay
- Central element: Carbon forms countless compounds, making it ideal for chemistry
- Integer mass: Convenient reference point (mass = 12 exactly)
Historical context: Before 1961, physicists and chemists used different oxygen-based standards, creating two incompatible atomic mass scales. Carbon-12 unified them.
Dalton vs. Unified Atomic Mass Unit
Two names, same unit:
Unified atomic mass unit (u):
- Official SI-accepted name
- Used primarily in chemistry and physics
- Symbol: u
Dalton (Da):
- Alternative name honoring John Dalton
- Used primarily in biochemistry and molecular biology
- Symbol: Da
- Convenient for large molecules (kilodaltons, kDa)
Relationship: 1 u = 1 Da (exactly equivalent)
Usage patterns:
- "The oxygen atom has a mass of 16.0 u" (chemistry)
- "The antibody protein has a mass of 150 kDa" (biochemistry)
Both refer to the same fundamental unit.
Note: The Stone is part of the imperial/US customary system, primarily used in the US, UK, and Canada for everyday measurements. The Atomic Mass Unit belongs to the imperial/US customary system.
History of the Stone and Atomic Mass Unit
Ancient and Medieval Origins (Pre-1300)
The concept of standardized stones: Before precise metallic weights, communities used stones as trade counterweights. A merchant would keep a reference stone in the marketplace, verified by local authorities, against which goods were weighed.
Advantages:
- Durability: Stones don't corrode or wear like metal
- Availability: Every village had stones
- Tamper-resistance: Hard to secretly shave weight off a stone
Problem: Every region had different stones! The "stone of wool" in Yorkshire differed from the "stone of wool" in Kent.
Medieval Standardization Attempts (1300-1824)
Edward III's wool stone (1340): King Edward III standardized the wool stone at 14 pounds as part of regulating the lucrative wool trade (England's economic backbone in the Middle Ages). The "sack of wool" was defined as 364 pounds = 26 stones.
Commodity-specific stones: Different goods had different stone weights:
| Commodity | Stone Weight | Reasoning | |--------------|-----------------|---------------| | Wool | 14 lb (6.35 kg) | Trade standard | | Meat | 8 lb (3.63 kg) | Butcher's stone | | Glass | 5 lb (2.27 kg) | Fragile goods | | Cheese | 16 lb (7.26 kg) | Agricultural products | | Iron | Variable (8-15 lb) | Regional differences |
Why different weights? Practical reasons:
- Heavy commodities (iron, lead): Smaller stone weight made counting easier
- Light, valuable goods (wool, spices): Larger stone weight reduced fractions
- Tradition: Each guild jealously guarded its customary weights
The Weights and Measures Act 1824
The problem: By 1800, Britain had dozens of incompatible stone definitions, creating chaos in trade and taxation.
The solution: The 1824 Act standardized British weights and measures:
- 14 pounds = 1 stone (for general use, not tied to specific commodities)
- Stone officially defined in relation to the pound
- Commodity-specific stones discouraged (but not banned)
Imperial standardization: The Act also defined:
- 1 pound = 7,000 grains
- 16 ounces = 1 pound
- 14 pounds = 1 stone
- 8 stone = 1 hundredweight (112 pounds)
- 20 hundredweight = 1 ton (2,240 pounds)
Body weight adoption: The Victorian era (1837-1901) saw the stone become the standard for human weighing. Bathroom scales, medical records, and public health data used stones and pounds.
Metrication and Persistence (1965-Present)
The Weights and Measures Act 1965: The UK officially adopted the metric system, making kilograms the legal unit for trade. However, the Act exempted personal weighing—bathroom scales could continue showing stones.
Why the exemption?
- Cultural resistance: Brits refused to abandon stones for body weight
- Economic lobbying: Scale manufacturers didn't want to retool
- Medical inertia: NHS records already used stones; conversion would be costly
The result: 60+ years later, the stone persists:
- Bathroom scales: Default to stones in the UK (even modern digital ones)
- NHS medical records: Still record patient weight in stones/pounds
- Weight loss programs: Slimming World, Weight Watchers UK use stones
- Media: British newspapers report celebrity weight in stones
- Sports: Boxing, horse racing, rowing use stones for weight classes
Ireland's experience: Ireland officially adopted metric units in 2005, but the stone remains common for body weight, especially among older generations.
Generational divide:
- Older Brits (60+): Think exclusively in stones
- Middle-aged (30-60): Bilingual (stones and kilograms)
- Younger (<30): Increasingly use kilograms, but still understand stones
Cultural Tenacity
The stone is the most persistent Imperial unit in British daily life, outlasting:
- Fahrenheit: Replaced by Celsius (weather, ovens)
- Inches/feet for height: Partially replaced by metres (though feet still common)
- Pints: Milk sold in litres (though beer still sold in pints!)
- Miles: Road signs still use miles (the UK never fully switched)
Why the stone survives:
- Emotional connection: Body weight is personal; changing units feels invasive
- Convenient range: For adults, weight is 8-20 stones (easy to remember vs. 50-127 kg)
- Medical exemption: Doctors use stones, so patients use stones
- Social reinforcement: Everyone around you uses stones, so you do too
John Dalton and Atomic Theory (1803-1808)
John Dalton (1766-1844), an English chemist and physicist, revolutionized chemistry with his atomic theory (1803):
Dalton's key postulates:
- All matter consists of indivisible atoms
- Atoms of the same element are identical in mass and properties
- Atoms of different elements have different masses
- Chemical compounds form when atoms combine in simple whole-number ratios
Relative atomic masses: Dalton created the first table of atomic weights (1805-1808), assigning hydrogen a mass of 1 and expressing other elements relative to it:
- Hydrogen: 1
- Oxygen: 7 (incorrect; should be ~16, but Dalton thought water was HO, not H₂O)
- Carbon: 5 (incorrect)
Though Dalton's numerical values were often wrong (he didn't yet know correct chemical formulas), his conceptual framework established that elements have characteristic atomic masses.
Berzelius and Improved Atomic Weights (1810s-1820s)
Jöns Jacob Berzelius (Swedish chemist, 1779-1848) refined Dalton's work with meticulous experiments:
Achievements:
- Determined accurate atomic weights for over 40 elements by 1818
- Established oxygen = 100 as the standard (for convenience in calculation)
- Introduced modern chemical notation (H, O, C, etc.)
Berzelius' atomic weights were remarkably accurate, many within 1% of modern values.
Cannizzaro and Avogadro's Number (1860)
Stanislao Cannizzaro (Italian chemist, 1826-1910) resolved confusion about atomic vs. molecular weights at the Karlsruhe Congress (1860):
Key insight: Avogadro's hypothesis (1811)—equal volumes of gases contain equal numbers of molecules—allows distinguishing atomic from molecular masses
Result: By 1860s, chemists adopted consistent atomic weights based on oxygen = 16
The Oxygen Standard Era (1890s-1960)
Chemist's standard (1890s onward):
- Natural oxygen (mixture of ¹⁶O, ¹⁷O, ¹⁸O) = 16.0000 exactly
- Practical for analytical chemistry
- Used in atomic weight tables
Physicist's standard (1900s onward):
- Oxygen-16 isotope (¹⁶O) = 16.0000 exactly
- Used in mass spectrometry and nuclear physics
- More precise for isotope work
The problem: Natural oxygen is 99.757% ¹⁶O, 0.038% ¹⁷O, and 0.205% ¹⁸O
- Chemist's scale and physicist's scale differed by ~0.0003 (0.03%)
- Small but significant for precision work
Unification: Carbon-12 Standard (1961)
1960 IUPAP resolution (International Union of Pure and Applied Physics):
- Proposed carbon-12 as the new standard
- Physicist Alfred Nier championed the change
1961 IUPAC resolution (International Union of Pure and Applied Chemistry):
- Adopted carbon-12 standard
- Defined: 1 atomic mass unit = 1/12 the mass of ¹²C atom
Advantages of carbon-12:
- Unified physics and chemistry scales
- Carbon is central to organic chemistry
- Mass spectrometry reference (carbon calibration)
- Abundant, stable, non-radioactive
Notation evolution:
- Old: amu (atomic mass unit, ambiguous—which standard?)
- New: u (unified atomic mass unit, unambiguous—carbon-12 standard)
The Dalton Name (1960s-1980s)
1960s proposal: Several scientists suggested naming the unit after John Dalton
1980s acceptance: The name "Dalton" (Da) gained widespread use in biochemistry
1993 IUPAC endorsement: Officially recognized "Dalton" as an alternative name for the unified atomic mass unit
Modern usage:
- Chemistry/physics: Prefer "u" (atomic mass unit)
- Biochemistry: Prefer "Da" (Dalton), especially with kDa (kilodaltons) for proteins
Mass Spectrometry and Precision (1900s-Present)
Mass spectrometry (developed 1910s-1920s, refined continuously):
Thomson and Aston (1910s-1920s):
- J.J. Thomson and Francis Aston developed early mass spectrographs
- Discovered isotopes by precise mass measurement
- Aston won 1922 Nobel Prize in Chemistry
Modern precision:
- Mass spectrometry now measures atomic masses to 8-10 decimal places
- Essential for determining isotopic compositions
- Used to measure the carbon-12 standard with extraordinary accuracy
CODATA values: The Committee on Data for Science and Technology (CODATA) publishes official atomic mass unit values every few years, incorporating latest measurements:
- 2018 value: 1 u = 1.660 539 066 60(50) × 10⁻²⁷ kg
2019 SI Redefinition
Historic change: On May 20, 2019, the International System of Units (SI) was redefined based on fundamental physical constants rather than physical artifacts (like the kilogram prototype)
New kilogram definition: Based on the Planck constant (h = 6.626 070 15 × 10⁻³⁴ J·s, exact)
Impact on atomic mass unit: The atomic mass unit is now indirectly tied to fundamental constants through the kilogram's new definition, though it remains defined as 1/12 the mass of carbon-12
Practical effect: Minimal—atomic masses remain effectively unchanged, but now rooted in unchanging physical constants
Common Uses and Applications: stones vs atomic mass units
Explore the typical applications for both Stone (imperial/US) and Atomic Mass Unit (imperial/US) to understand their common contexts.
Common Uses for stones
1. Body Weight Measurement
The stone is the unit for body weight in the UK and Ireland.
Bathroom scales:
- Display: "11 st 7 lb" (digital) or analog dial with stone markings
- Dual units: Many scales toggle between st/lb and kg
- Default: Stones for UK-sold scales, even from international brands
Weighing yourself:
- British: "I'm 12 stone 3"
- American: "I'm 171 pounds"
- European: "I'm 78 kilograms"
Weight goals:
- "I want to lose a stone" = 14-pound goal
- "I'm aiming for 10 stone" = target weight
- "I've gained half a stone" = 7-pound increase
2. Medical and Healthcare
NHS patient records: British hospitals and GPs record weight in stones/pounds (with kg conversion).
Medical forms:
- Pre-op questionnaires: "Weight: __ st __ lb"
- Prescription dosing: Sometimes based on weight (converted to kg for calculations)
- Anesthesia planning: Weight in stones converted to kg for drug dosages
Maternity care:
- Booking appointment: "What was your pre-pregnancy weight?" (stones)
- Pregnancy weight tracking: "You've gained 2 stone, which is healthy"
- Post-natal: "Most women lose 1-2 stone in the first weeks"
Mental health context: Eating disorder treatment tracks weight changes in stones (e.g., anorexia recovery: "gained 1 stone to 7 stone 10").
3. Weight Loss and Fitness
Slimming clubs:
- Slimming World, Weight Watchers UK: Weigh-ins in stones
- Awards: "Half-stone hero," "Stone club," "3-stone milestone"
- Targets: "Lose 10% of body weight" (e.g., 1.5 stone from 15 stone start)
Fitness apps (UK versions):
- MyFitnessPal UK: Input weight in stones
- Fitbit/Garmin: UK users set goals in stones
- Weight tracking graphs: Y-axis shows stones, not kg
Personal trainers: British trainers discuss client progress in stones: "You've dropped from 14 stone to 12 stone 8—fantastic!"
4. Sports Weight Classes
Boxing: British boxing traditionally used stones for weight classes (now officially kilograms, but stones still common in commentary).
Horse racing:
- Jockey weights: Includes jockey + saddle + lead weights to meet required "riding weight"
- Handicapping: Horses carry different weights (in stones) to equalize competition
- Penalties: "Carrying 9 stone 7" vs. "Carrying 10 stone" affects race outcomes
Rowing: Lightweight rowers must weigh under certain stone limits (now metric, but historically stones).
5. Everyday Conversation
The stone pervades British informal speech:
Common phrases:
- "I'm 11 stone, give or take" = approximate weight
- "She must be 10 stone soaking wet" = very light
- "He's put on a stone since Christmas" = seasonal weight gain
- "I haven't been 9 stone since I was 15!" = nostalgic reference
Social etiquette:
- It's impolite to ask someone's weight directly, but acceptable to discuss your own
- Women might say "I'm trying to get back to 9 stone" (goal weight)
6. Media and Entertainment
British TV shows:
- Reality TV: "Love Island" contestants' weights discussed in tabloids (stones)
- Medical shows: "Embarrassing Bodies" references patient weight in stones
- Game shows: "The Biggest Loser UK" tracked loss in stones
Newspapers and magazines:
- Celebrity weight speculation: "Has she lost 2 stone?"
- Health articles: "How to lose half a stone by summer"
- Success stories: "I lost 8 stone and transformed my life!"
7. Historical and Cultural References
Literature: Victorian novels reference weight in stones:
- Dickens, Austen rarely mention specific weights (impolite)
- 20th-century literature: "She was a strapping girl of 12 stone"
British humor: Comedians joke about weight in stones:
- "I'm not overweight, I'm just undertall for my 14 stone!"
Generational markers:
- Older Brits: "When I got married, I was 8 stone"
- Modern comparison: "That's only 112 pounds—too thin by today's standards!"
When to Use atomic mass units
1. Atomic Weights and Periodic Table
The periodic table lists atomic weights (average masses) of elements in atomic mass units:
Example: Carbon:
- Natural carbon contains 98.89% ¹²C (12.0000 u) and 1.11% ¹³C (13.0034 u)
- Weighted average: 0.9889 × 12.0000 + 0.0111 × 13.0034 = 12.0107 u
- Periodic table lists carbon's atomic weight as 12.011 u
Why atomic weights aren't integers: Most elements are mixtures of isotopes with different masses, so the average is non-integer
Usage: Every stoichiometry calculation in chemistry depends on atomic weights expressed in u or g/mol (numerically equal)
2. Molecular Mass Calculations
Molecular mass = sum of atomic masses of all atoms in the molecule
Example: Glucose (C₆H₁₂O₆):
- 6 carbon atoms: 6 × 12.011 = 72.066 u
- 12 hydrogen atoms: 12 × 1.008 = 12.096 u
- 6 oxygen atoms: 6 × 15.999 = 95.994 u
- Total: 72.066 + 12.096 + 95.994 = 180.156 u
Molar mass connection: 180.156 u per molecule = 180.156 g/mol (numerically identical!)
3. Mass Spectrometry
Mass spectrometry measures the mass-to-charge ratio (m/z) of ions:
Technique:
- Ionize molecules (add or remove electrons)
- Accelerate ions through electric/magnetic fields
- Separate by mass-to-charge ratio
- Detect and measure abundances
Output: Mass spectrum showing peaks at specific m/z values (in u/e or Da/e, where e = elementary charge)
Applications:
- Determining molecular formulas
- Identifying unknown compounds
- Measuring isotope ratios
- Protein identification in proteomics
- Drug testing and forensics
Example: A peak at m/z = 180 for glucose (C₆H₁₂O₆ = 180 u, charge = +1e)
4. Protein Characterization (Biochemistry)
Biochemists routinely express protein masses in kilodaltons (kDa):
SDS-PAGE (sodium dodecyl sulfate polyacrylamide gel electrophoresis):
- Separates proteins by molecular weight
- Gels calibrated with protein standards of known kDa
- "The unknown protein band migrates at ~50 kDa"
Protein databases:
- UniProt, PDB (Protein Data Bank) list protein masses in Da or kDa
- Essential for identifying proteins by mass
Clinical diagnostics:
- "Elevated levels of 150 kDa IgG antibodies detected" (immune response)
- Tumor markers identified by protein mass
5. Stoichiometry and Chemical Equations
Stoichiometry: Calculating quantities in chemical reactions
Example: Combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O
Molecular masses:
- CH₄: 16.043 u
- O₂: 31.998 u
- CO₂: 44.010 u
- H₂O: 18.015 u
Mass balance: 16.043 + 2(31.998) = 44.010 + 2(18.015) = 80.039 u (both sides equal, confirming conservation of mass)
Practical calculation: To produce 44 grams of CO₂, you need 16 grams of CH₄ and 64 grams of O₂
6. Isotope Analysis
Isotopes: Atoms of the same element with different numbers of neutrons (different masses)
Examples:
- ¹²C: 12.0000 u (6 protons, 6 neutrons) — 98.89% of natural carbon
- ¹³C: 13.0034 u (6 protons, 7 neutrons) — 1.11% of natural carbon
- ¹⁴C: 14.0032 u (6 protons, 8 neutrons) — radioactive, trace amounts
Applications:
- Radiocarbon dating: ¹⁴C decay measures age of organic materials
- Climate science: ¹³C/¹²C ratios in ice cores track ancient temperatures
- Medical tracers: ¹³C-labeled compounds track metabolic pathways
- Forensics: Isotope ratios identify geographic origins of materials
7. Nuclear Physics and Mass Defect
Mass-energy equivalence (E = mc²): Mass and energy are interconvertible
Mass defect: The mass of a nucleus is slightly less than the sum of its individual protons and neutrons
Example: Helium-4 (⁴He):
- 2 protons: 2 × 1.007276 = 2.014552 u
- 2 neutrons: 2 × 1.008665 = 2.017330 u
- Sum: 4.031882 u
- Actual ⁴He nucleus mass: 4.001506 u
- Mass defect: 4.031882 - 4.001506 = 0.030376 u
Interpretation: The "missing" 0.030376 u was converted to binding energy that holds the nucleus together
Calculation: 0.030376 u × c² = 28.3 MeV (million electron volts)
This is the energy released when helium-4 forms from protons and neutrons (nuclear fusion).
Additional Unit Information
About Stone (st)
1. How many pounds are in a stone?
Exactly 14 pounds.
This is a defined constant. There are no regional variations—1 stone always equals 14 pounds in any context.
Calculation examples:
- 5 stone = 5 × 14 = 70 pounds
- 12 stone = 12 × 14 = 168 pounds
- 0.5 stone = 0.5 × 14 = 7 pounds
2. Is the stone used outside the UK and Ireland?
Rarely. The stone is almost exclusive to the UK and Ireland.
Usage by country:
- UK: Dominant for body weight (even with official metrication)
- Ireland: Common, especially among older generations
- Canada, Australia, New Zealand: Not used (fully metric)
- United States: Not used (pounds only)
- Rest of world: Not used (metric)
Exception: British expats abroad sometimes use stones, and international weight loss forums may reference stones when discussing UK participants.
3. Why is it called a stone?
Historical practice: Actual stones were used as standardized weights in medieval markets.
How it worked:
- A community selected a reference stone of agreed weight
- The stone was kept in the marketplace (sometimes literally built into a wall)
- Merchants used the reference stone on balance scales to verify weights
- Different stones existed for different commodities (wool stone, meat stone, etc.)
Modern name: The unit name "stone" is a fossil of this practice, long after actual stones stopped being used.
4. How do you convert stone to kilograms?
Formula:
Kilograms = Stone × 6.35029318
Quick approximation:
Kilograms ≈ Stone × 6.35 (good to 3 decimal places)
Examples:
- 10 stone × 6.35 = 63.5 kg
- 12 stone × 6.35 = 76.2 kg
- 15 stone × 6.35 = 95.25 kg
Online tools: Most conversion sites and apps include stone ↔ kilogram calculators.
5. How do British people talk about their weight?
Typical format: "I'm X stone Y pounds" or "I'm X stone Y"
Examples:
- "I'm 11 stone 7" = 11 stone + 7 pounds = 161 lb = 73 kg
- "I'm just over 12 stone" = slightly more than 168 lb
- "I'm nearly 10 stone" = approaching 140 lb
Rarely said:
- "I'm 11.5 stone" (uncommon—people say "11 stone 7" instead)
- "I'm 161 pounds" (too American—Brits don't think in pounds alone)
- "I'm 73 kilograms" (used by younger generations, but less common)
Conversational weight: Discussing weight is somewhat taboo, so people often avoid specifics: "I need to lose a bit of weight" rather than "I need to drop from 13 to 11 stone."
6. Do British bathroom scales show kilograms?
Yes, most modern scales show both.
Typical features:
- Default: Stones and pounds (st/lb)
- Toggle button: Switch to kilograms
- Dual display: Some show both simultaneously
Older scales: Analog scales from before 2000 often show stones only.
Buying scales in the UK: Even international brands (Fitbit, Garmin) sell UK-specific versions that default to stones.
7. Will the UK ever stop using stones?
Unlikely in the near future.
Reasons for persistence:
- Cultural attachment: Body weight is personal; people resist change
- Legal exemption: Personal weighing scales exempt from trade regulations
- NHS inertia: Changing medical records costly
- Generational use: Older generations use stones exclusively
- No enforcement: No push to mandate kilograms for personal use
Trend: Younger Brits (under 30) increasingly use kilograms, especially those who travel or use fitness apps with international audiences. However, the stone will likely persist for decades among older populations.
Comparison: Similar to Fahrenheit in the US—officially discouraged but culturally entrenched.
8. What is a "half stone"?
Half stone = 7 pounds = 3.175 kg
Usage:
- Weight loss: "I've lost half a stone" = 7 lb loss
- Weight gain: "I've put on half a stone over Christmas" = 7 lb gain
- Milestones: "Half-stone club" in weight loss programs
Why significant? Half a stone is a noticeable weight change—enough to affect how clothes fit and how you feel, but achievable in 3-7 weeks of dieting (at 1-2 lb/week loss).
9. How do you write stone and pounds?
Common formats:
Formal:
- "11 stone 7 pounds"
- "11 st 7 lb"
Informal:
- "11 stone 7"
- "11st 7lb" (no spaces)
- "11-7" (very casual, context-dependent)
Avoid:
- "11.7 stone" (ambiguous—could mean 11 stone 7 lb or 11 stone 9.8 lb)
- "11/7 st" (confusing notation)
Medical records: NHS typically uses "st/lb" format: "Patient weight: 12 st 3 lb"
10. Why do Americans not use stone?
The United States never adopted the stone for body weight.
Historical reasons:
- Colonial divergence: By the time the stone standardized in Britain (1824), the US had already established pounds as the body weight unit
- Decimal preference: Americans favored simpler base-10 systems where possible
- No cultural push: No equivalent to UK's Victorian-era adoption of stones for weighing people
Result: Americans think in pounds only:
- "I weigh 180 pounds" (no stones)
- Weight loss: "I lost 30 pounds" (not "2 stone 2 pounds")
Canadian note: Canada officially metricated in the 1970s and uses kilograms, not stones or pounds (though older Canadians may still think in pounds).
11. Is stone a legal unit?
Yes, in the UK and Ireland, but with restrictions.
Legal status:
- Personal use: Fully legal (bathroom scales, self-weighing)
- Trade: Must use metric (kilograms) for selling goods by weight
- Medical: Allowed in patient records (NHS uses stones)
Weights and Measures Act: Kilograms are the legal unit for commerce, but stones remain legal for "non-trade" purposes (personal weighing, medical records).
Comparison: Similar to miles on UK road signs—officially metric, but exceptions preserve traditional units in specific contexts.
12. How much is a stone in other historical weight units?
Stone in troy and apothecary systems:
Troy weight (precious metals):
- 1 stone (avoirdupois) = 14 pounds (avoirdupois)
- 1 pound (avoirdupois) = 7,000 grains
- 1 stone = 98,000 grains (troy)
- 1 troy pound = 5,760 grains
- 1 stone ≈ 17.01 troy pounds
Apothecaries' weight (pharmacy):
- Same grain as troy and avoirdupois (64.79891 mg)
- 1 stone = 98,000 grains (apothecaries')
Why this matters: Historically, pharmacists used apothecaries' weights, so understanding stone conversions was important for dosing medicines based on body weight.
About Atomic Mass Unit (u)
What is the value of 1 u (or Da) in kilograms?
Answer: 1 u = 1.660 539 066 60 × 10⁻²⁷ kg (with standard uncertainty ±0.000 000 000 50 × 10⁻²⁷ kg)
This extraordinarily precise value comes from measurements of carbon-12 atoms using mass spectrometry and relates to the newly defined kilogram (based on Planck's constant as of 2019).
Approximate value: 1 u ≈ 1.6605 × 10⁻²⁷ kg
In grams: 1 u ≈ 1.6605 × 10⁻²⁴ g
Memorization tip: "1.66 and exponent −27"
Uncertainty: The precision is about 0.3 parts per billion (extremely accurate!)
Source: CODATA 2018 recommended values (Committee on Data for Science and Technology)
Is the atomic mass unit (amu) the same as the Dalton (Da)?
Answer: Yes—in modern usage, u (unified atomic mass unit), amu, and Da (Dalton) all refer to the same unit
Historical context:
Pre-1961 (ambiguous):
- "amu" could mean the oxygen-based physics scale (¹⁶O = 16) or chemistry scale (natural O = 16)
- These differed by ~0.03%, causing confusion
1961 unification:
- IUPAC/IUPAP adopted carbon-12 standard
- "u" (unified atomic mass unit) replaced ambiguous "amu"
- 1 u = 1/12 mass of ¹²C atom
1970s-1993:
- "Dalton" (Da) proposed as an alternative name honoring John Dalton
- Gained popularity in biochemistry
Today:
- u: Official name, preferred in chemistry and physics
- Da: Alternative name, preferred in biochemistry (especially kDa for proteins)
- amu: Informal, but understood to mean "u" in modern contexts
Bottom line: 1 u = 1 Da = 1 amu (modern) — all identical
Why was Carbon-12 chosen as the standard for atomic mass?
Answer: Carbon-12 unified divergent physics and chemistry scales while being abundant, stable, and convenient
Historical problem (pre-1961):
- Physicists used ¹⁶O = 16.0000 exactly (pure isotope)
- Chemists used natural oxygen = 16.0000 exactly (isotope mixture)
- Natural oxygen is 99.757% ¹⁶O, 0.038% ¹⁷O, 0.205% ¹⁸O
- Result: Two incompatible atomic mass scales differing by ~0.03%
Carbon-12 advantages:
1. Unification: Resolved the physics-chemistry discrepancy with a single standard
2. Abundance: ¹²C comprises 98.89% of natural carbon (readily available)
3. Stability: Not radioactive (unlike ¹⁴C); doesn't decay
4. Integer mass: Defining ¹²C = 12 exactly gives a clean reference point
5. Chemical importance: Carbon is the basis of organic chemistry—central to life and synthetic compounds
6. Mass spectrometry: Carbon compounds are ubiquitous calibration standards
7. Convenience: Most atomic masses end up close to integers (approximately equal to mass number A)
Alternative considered: Hydrogen was Dalton's original choice, but hydrogen's mass (1.008 u) isn't exactly 1, and hydrogen forms fewer compounds than carbon or oxygen.
Result: Since 1961, all atomic weights worldwide are based on ¹²C = 12.0000 u (exact)
How does the atomic mass unit relate to Avogadro's number?
Answer: The atomic mass unit and Avogadro's number are defined such that mass in u equals molar mass in g/mol numerically
The elegant relationship:
Avogadro's constant: N_A = 6.022 140 76 × 10²³ mol⁻¹ (exact, as of 2019 SI redefinition)
Atomic mass unit: 1 u = 1/12 the mass of one ¹²C atom
Molar mass constant: M_u = 1 g/mol (by definition of the mole)
Mathematical relationship:
1 u = 1 g / N_A
Example:
- One carbon-12 atom: 12 u
- One mole of carbon-12 atoms: 12 g
- Number of atoms: 6.022 × 10²³
Practical consequence: To convert molecular mass (u) to grams, multiply by Avogadro's number:
- 1 water molecule: 18 u
- 1 mole of water: 18 g
- 18 g ÷ (6.022 × 10²³) = 2.99 × 10⁻²³ g per molecule ✓
Why this works: The definition of the mole (amount containing N_A entities) is coordinated with the definition of the atomic mass unit to make this numerical equality hold.
What is the difference between atomic mass and atomic weight?
Answer: Atomic mass refers to a specific isotope; atomic weight is the weighted average of all isotopes in natural abundance
Atomic mass (isotope-specific):
- Mass of one specific isotope
- Example: ¹²C has atomic mass = 12.0000 u (exact)
- Example: ¹³C has atomic mass = 13.0034 u
Atomic weight (element average):
- Weighted average of all naturally occurring isotopes
- Example: Natural carbon (98.89% ¹²C, 1.11% ¹³C) has atomic weight = 12.0107 u
- Listed on the periodic table
Calculation for carbon: Atomic weight = (0.9889 × 12.0000) + (0.0111 × 13.0034) = 12.0107 u
Why "weight" instead of "mass"? Historical naming; "atomic weight" actually refers to mass, not weight (force). The term persists despite being technically incorrect.
Relative atomic mass: Modern term preferred over "atomic weight" (same meaning, less confusing)
Important distinction: When doing precise isotope work (mass spectrometry, nuclear chemistry), use atomic masses of specific isotopes, not elemental atomic weights.
Can I use atomic mass units for objects larger than molecules?
Answer: Technically yes, but it's impractical—atomic mass units are too small for macroscopic objects
Practical range for atomic mass units:
- Atoms: 1-300 u (hydrogen to heaviest elements)
- Small molecules: 10-1,000 u
- Proteins: 1,000-10,000,000 u (1 kDa - 10 MDa)
- Viruses: up to ~1,000 MDa (1 gigadalton, GDa)
Beyond this: Use conventional mass units (grams, kilograms)
Example (why it's impractical):
- A grain of sand (~1 mg = 10⁻⁶ kg)
- In atomic mass units: 10⁻⁶ kg ÷ (1.66 × 10⁻²⁷ kg/u) ≈ 6 × 10²⁰ u
- This number is unwieldy!
Rule of thumb: Use atomic mass units for individual molecules or molecular complexes; switch to grams/kilograms for anything visible to the eye.
Extreme example: A 70 kg human = 4.2 × 10²⁸ u (42,000 trillion trillion u—utterly impractical!)
How accurate are modern atomic mass measurements?
Answer: Extraordinarily accurate—often 8-10 decimal places (parts per billion precision)
Modern mass spectrometry precision:
- Typical: 1 part per million (ppm) — 6 decimal places
- High-resolution: 1 part per billion (ppb) — 9 decimal places
- Ultra-high-resolution: 0.1 ppb — 10 decimal places
Example: Carbon-12:
- Defined as exactly 12.00000000000... u (infinite precision by definition)
Example: Hydrogen-1:
- Measured value: 1.00782503207 u (11 significant figures!)
- Uncertainty: ±0.00000000077 u
Why such precision matters:
1. Isotope identification: Distinguishing ¹²C¹H₄ (16.0313 u) from ¹³C¹H₃ (16.0344 u) requires high precision
2. Mass defect measurements: Nuclear binding energies calculated from tiny mass differences (0.1% of nuclear mass)
3. Molecular formula determination: Mass spectrometry can distinguish C₁₃H₁₂ from C₁₂H₁₂O from C₁₁H₁₆N (all ~168 u) with sufficient precision
4. Fundamental physics: Testing mass-energy equivalence, searching for physics beyond the Standard Model
Limitation: Even with extreme precision, natural isotopic variation (different ¹²C/¹³C ratios in different samples) limits practical accuracy to ~4-5 decimal places for most chemical applications.
Do protons and neutrons have exactly the same mass?
Answer: No—neutrons are slightly heavier than protons by about 0.14%
Precise values:
- Proton mass: 1.007276466621 u
- Neutron mass: 1.00866491595 u
- Difference: 0.00138845 u (neutron is heavier by ~1.4 MeV/c²)
Why this matters:
1. Neutron decay: Free neutrons decay into protons + electrons + antineutrinos with a half-life of ~10 minutes (neutron → proton + e⁻ + ν̄ₑ)
2. Nuclear stability: The mass difference affects which isotopes are stable vs. radioactive
3. Element synthesis: Mass differences determine which nuclear reactions can occur spontaneously in stars
Fun fact: Both are close to 1 u (within 1%), which is why atomic mass numbers (protons + neutrons) approximately equal atomic masses in u
Electron mass: Much lighter—only 0.000548580 u (~1/1836 of a proton)
Consequence: Atomic mass is almost entirely due to protons and neutrons; electrons contribute negligibly (<0.03%)
Why is the atomic mass of hydrogen 1.008 u instead of 1 u?
Answer: Because protons are slightly heavier than 1/12 of a carbon-12 atom, plus hydrogen atoms include an electron
Breakdown of hydrogen atom (¹H):
- Proton: 1.007276 u
- Electron: 0.000549 u
- Binding energy (negligible): −0.000015 u
- Total: 1.007825 u ≈ 1.008 u
Why isn't a proton exactly 1 u?
The atomic mass unit is defined as 1/12 the mass of carbon-12, which contains 6 protons + 6 neutrons + 6 electrons, minus the nuclear binding energy:
¹²C mass: 12 u (exact) = 6 protons + 6 neutrons + 6 electrons − binding energy
Solving: 1 nucleon (proton or neutron) ≈ 1.007-1.009 u (slightly more than 1 u)
Why the carbon-12 nucleus is lighter than 12 individual nucleons: Nuclear binding energy (E = mc²) converts ~0.1 u of mass into energy that holds the nucleus together
Result: Hydrogen (1 proton + 1 electron) ends up at 1.008 u, not 1.000 u
Will the definition of the atomic mass unit ever change?
Answer: Unlikely—the carbon-12 standard is stable, internationally accepted, and fundamental to chemistry
Why it's stable:
1. International agreement: IUPAC, IUPAP, and NIST all recognize ¹²C standard (since 1961)
2. Infrastructure: All atomic weight tables, databases, lab equipment calibrated to carbon-12
3. No compelling alternative: Carbon-12 works perfectly for chemistry and biochemistry
4. Historical continuity: Changing standards disrupts 60+ years of data
Recent change (2019 SI redefinition):
- The kilogram was redefined based on Planck's constant
- This indirectly affects the atomic mass unit (since 1 u is expressed in kg)
- But the change is at the 9th decimal place—completely negligible for chemistry
Future refinement: Values like 1.660539066(50) × 10⁻²⁷ kg will get more decimal places as measurements improve, but the carbon-12 definition (1 u = 1/12 m(¹²C)) won't change
Contrast with other standards:
- Meter: Redefined from physical bar to speed of light (1983)
- Kilogram: Redefined from physical cylinder to Planck constant (2019)
- Atomic mass unit: Based on fundamental particle (¹²C atom)—already a natural standard
Conclusion: The carbon-12 definition is here to stay for the foreseeable future (decades to centuries).
Conversion Table: Stone to Atomic Mass Unit
| Stone (st) | Atomic Mass Unit (u) |
|---|---|
| 0.5 | 1,912,117,970,522,187,800,000,000,000 |
| 1 | 3,824,235,941,044,375,600,000,000,000 |
| 1.5 | 5,736,353,911,566,563,000,000,000,000 |
| 2 | 7,648,471,882,088,751,000,000,000,000 |
| 5 | 19,121,179,705,221,880,000,000,000,000 |
| 10 | 38,242,359,410,443,760,000,000,000,000 |
| 25 | 95,605,898,526,109,400,000,000,000,000 |
| 50 | 191,211,797,052,218,800,000,000,000,000 |
| 100 | 382,423,594,104,437,600,000,000,000,000 |
| 250 | 956,058,985,261,094,000,000,000,000,000 |
| 500 | 1,912,117,970,522,188,000,000,000,000,000 |
| 1,000 | 3,824,235,941,044,376,400,000,000,000,000 |
People Also Ask
How do I convert Stone to Atomic Mass Unit?
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Learn more →What is the conversion factor from Stone to Atomic Mass Unit?
The conversion factor depends on the specific relationship between Stone and Atomic Mass Unit. You can find the exact conversion formula and factor on this page. Our calculator handles all calculations automatically. See the conversion table above for common values.
Can I convert Atomic Mass Unit back to Stone?
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Learn more →What are common uses for Stone and Atomic Mass Unit?
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Verified Against Authority Standards
All conversion formulas have been verified against international standards and authoritative sources to ensure maximum accuracy and reliability.
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Last verified: December 3, 2025