Grain (gr) - Unit Information & Conversion

Quick Answer

1 grain = 64.79891 milligrams (EXACT) ≈ 0.0648 grams

The grain is the universal unit for measuring bullets and gunpowder. When you see "124 grain 9mm ammunition," it means each bullet weighs 124 grains (8.035 grams). The grain is also the basis for all traditional English weight systems—avoirdupois, troy, and apothecaries'.

Why it matters: If you reload ammunition, shoot competitively, or practice archery, you work in grains daily. A 5-grain difference in bullet weight significantly affects accuracy and recoil. In pharmaceuticals, older medications like aspirin retain grain dosages (5 grains = 325 mg).

Historical note: The grain is literally based on a grain of barley—medieval merchants used barleycorns as counterweights because they have remarkably uniform mass.

Quick Comparison Table

Item	Weight (Grains)	Weight (Grams)	Context
Barleycorn (original standard)	~1 gr	~0.065 g	Medieval weight reference
Aspirin tablet (regular strength)	5 gr	0.325 g (325 mg)	Pharmaceutical dosage
.22 LR bullet	40 gr	2.6 g	Small caliber ammunition
9mm bullet (typical)	115-124 gr	7.5-8.0 g	Handgun ammunition
.45 ACP bullet	230 gr	14.9 g	Large pistol caliber
5.56mm NATO bullet	55-77 gr	3.6-5.0 g	Rifle ammunition
Arrow (compound bow)	350-450 gr	22.7-29.2 g	Archery
Gunpowder charge (.38 Special)	3-5 gr	0.19-0.32 g	Propellant weight
Gold pennyweight	24 gr	1.555 g	Jewelry weight (troy)
Carat (gemstone)	3.086 gr	0.2 g (200 mg)	Diamond weight

Definition

1 grain = 64.79891 milligrams (mg) = 0.06479891 grams (g) EXACT

The grain (symbol: gr) is a unit of mass legally defined since 1959 as exactly 64.79891 milligrams. It is the smallest and oldest unit in the traditional English measurement systems.

Grain in Three Weight Systems

The grain is unique—it's the only unit shared identically across three different weight systems:

System	Use	Grain Relationships
Avoirdupois	General commerce, bullets	437.5 gr = 1 oz; 7,000 gr = 1 lb
Troy	Precious metals, gemstones	480 gr = 1 oz troy; 5,760 gr = 1 lb troy
Apothecaries'	Pharmacy (historical)	480 gr = 1 oz apoth; 5,760 gr = 1 lb apoth

Why this matters: The grain serves as the common denominator linking these systems. It's the conversion bridge between everyday weights and specialized applications.

The Barleycorn Origin

Historical basis: The grain was originally defined as the weight of a single grain of barley taken from the middle of the ear (not from the ends, which are lighter).

Remarkably consistent: Medieval experiments showed barleycorns have remarkably uniform mass:

• Average: 64-66 milligrams
• Modern definition: 64.79891 mg
• Variance: Only ~2-3% across different barley varieties

Length connection: King Edward I's statute (13th century) also defined 1 inch = 3 barleycorns laid end-to-end. Thus, the barleycorn defined both length and weight!

Metric Equivalents

Precise conversion:

1 grain = 64.79891 milligrams (EXACT, since 1959)

Common approximations:

1 grain ≈ 0.0648 grams (rounded)
1 grain ≈ 65 milligrams (rough)
15.43 grains ≈ 1 gram (useful for quick conversions)

Why 64.79891 mg? This exact value comes from the 1959 international yard and pound agreement:

• 1 pound = 0.45359237 kg (defined)
• 1 grain = 1/7000 pound
• 1 grain = 0.45359237 ÷ 7000 = 0.00006479891 kg = 64.79891 mg

History

Ancient Origins (3000 BCE - 500 CE)

Mesopotamian seeds: The earliest weight systems in Sumer and Babylon (circa 3000-2000 BCE) used seeds as counterweights:

• Barleycorns: Small weights
• Wheat grains: Alternative standard
• Carob seeds: Larger weights (origin of "carat" for gemstones)

Why seeds? Seeds have several advantages as weights:

1. Availability: Every agricultural community had grain
2. Uniformity: Grains from the same species have consistent mass
3. Portability: Easy to carry, store, and count
4. Natural standard: Self-evident, no authority needed to verify

Babylonian system:

• 180 barleycorns = 1 shekel (~8.4 grams)
• Shekels formed the basis for Mesopotamian commerce

Egyptian weights: Ancient Egypt used wheat grains similarly, though their system developed independently.

Greek and Roman Adoption (500 BCE - 500 CE)

Roman grain (granum): Romans used grains of wheat as small weight standards:

• 1 siliqua (carob seed) = 3 grains of wheat
• 24 siliquae = 1 solidus (Roman gold coin, ~4.5 grams)

Classical pharmacy: Greek and Roman physicians (Hippocrates, Galen) prescribed medicines in grain weights, establishing the apothecaries' tradition.

Medieval England (1000-1500 CE)

Barleycorn statutes: English law formalized the barleycorn as both length and weight standard.

King Edward I (1272-1307): His statute defined:

• 1 inch = 3 barleycorns laid end-to-end
• 1 grain = weight of 1 barleycorn from the middle of the ear

Establishing the pound: The avoirdupois pound was defined as 7,000 grains, making the grain the fundamental unit.

Why 7,000? Likely evolved from trade practices. 7,000 is divisible by many numbers (1, 2, 4, 5, 7, 8, 10, 14, 20, etc.), making fractional calculations easier.

Troy vs. Avoirdupois:

• Troy pound: 5,760 grains (12 troy ounces × 480 grains)
• Avoirdupois pound: 7,000 grains (16 avoirdupois ounces × 437.5 grains)
• Grain: Identical in both systems (the common unit)

Gunpowder and Firearms (1300-1800)

Black powder measurement: The invention of gunpowder (China, 9th century; Europe, 13th century) required precise measurement. Early gunners measured powder charges in grains for consistency.

Why grains for gunpowder?

1. Precision: Small unit allows fine-tuning of powder charges
2. Safety: Overcharging a cannon or musket could cause explosion
3. Consistency: Uniform charges improve accuracy

Development of firearms: As firearms evolved from cannons to muskets to rifles (1400s-1800s), grain measurement became standard:

• Musket ball: 400-500 grains (26-32 grams)
• Powder charge: 70-100 grains (4.5-6.5 grams)

Ballistics science: By the 18th century, ballistics became a science, with detailed tables relating bullet weight (grains), powder charge (grains), and muzzle velocity.

Apothecaries' and Pharmacy (1500-1900)

Apothecaries' system: Pharmacists adopted the grain from medieval medicine, using it alongside drams, scruples, and ounces.

System structure:

• 20 grains = 1 scruple
• 3 scruples = 1 dram
• 8 drams = 1 ounce (apothecaries')
• 12 ounces = 1 pound (apothecaries')

Why grains for medicine?

• Precision: Many drugs are potent at milligram doses (grain scale)
• Safety: Overdosing could be fatal; grains allowed careful measurement
• Tradition: Galen, Hippocrates used grains; continuity mattered

Common medications:

• Aspirin: 5 grains (325 mg) — "standard dose"
• Morphine: 1/4 to 1 grain (16-65 mg) — pain relief
• Digitalis: 1/60 to 1/30 grain (1-2 mg) — heart medication

Modern Standardization (1900-Present)

The 1959 Agreement: The international yard and pound agreement fixed the grain:

• 1 pound = 0.45359237 kilograms (EXACT)
• 1 grain = 1/7000 pound = 64.79891 mg (EXACT)

This ended slight variations between British and US grains.

Metrication in pharmacy: Most countries switched to milligrams for drug dosing (1950s-1980s). However:

• United States: Some medications retain grain labels (aspirin, thyroid hormone)
• UK: Fully metric in pharmacy by 1970s

Persistence in ammunition: Unlike pharmacy, the ammunition industry never metricated:

• US ammunition: Grains (dominant globally)
• European ammunition: Some metric (grams), but grains still common for export

Modern shooting sports: Competitive shooting, reloading, and ballistics all use grains:

• Bullet weight: Grains
• Powder charge: Grains
• Arrow weight: Grains (archery)

Real-World Examples

1. Ammunition and Ballistics

Handgun ammunition:

9mm Luger (9×19mm):

• Light: 115 grain (7.45 g) — higher velocity, less recoil
• Standard: 124 grain (8.04 g) — NATO standard
• Heavy: 147 grain (9.52 g) — subsonic, suppressor-friendly

. 45 ACP:

• Standard: 230 grain (14.9 g) — classic "hardball"
• Light: 185 grain (12.0 g) — faster, flatter trajectory

.38 Special:

• Light: 110 grain (7.1 g) — self-defense, +P loads
• Standard: 158 grain (10.2 g) — traditional police round

Rifle ammunition:

5.56×45mm NATO (.223 Remington):

• Light: 55 grain (3.56 g) — high velocity (3,240 fps)
• Standard: 62 grain (4.02 g) — M855 "green tip"
• Heavy: 77 grain (4.99 g) — long-range accuracy

.308 Winchester (7.62×51mm):

• Light: 147 grain (9.5 g) — M80 ball
• Standard: 168 grain (10.9 g) — match-grade accuracy
• Heavy: 175-180 grain (11.3-11.7 g) — long-range precision

.50 BMG:

• Standard: 660-750 grain (42.8-48.6 g) — armor-piercing, explosive

Why bullet weight matters:

• Velocity: Lighter bullets = higher velocity (assuming same powder)
• Energy: Heavier bullets = more momentum, better penetration
• Recoil: Heavier bullets = more recoil (Newton's third law)
• Accuracy: Bullet weight affects stability and wind drift

2. Gunpowder and Reloading

Reloaders measure powder charges in grains:

.38 Special reload:

• Case: .38 Special brass
• Bullet: 158 grain lead round-nose
• Powder: 4.0 grains Bullseye (0.259 g)
• Primer: Small pistol

Why precision matters:

• Too little powder: Bullet stuck in barrel (dangerous!)
• Too much powder: Overpressure, case rupture, injury
• Typical tolerance: ±0.1 grain (0.0065 g)

Powder types (examples):

• Bullseye: Fast-burning pistol powder (3-6 grains per charge)
• Unique: Medium-burning pistol powder (5-10 grains)
• IMR 4064: Rifle powder (40-50 grains per charge)

Reloading scales: Digital scales measure in grains (or grams). Accuracy: ±0.1 grain essential for safety.

3. Archery

Arrow weight (total mass):

Compound bow arrows:

• Light: 350 grains (22.7 g) — target shooting, flat trajectory
• Medium: 400-450 grains (25.9-29.2 g) — hunting, all-around
• Heavy: 500+ grains (32.4+ g) — large game, penetration

Components (350-grain arrow):

• Shaft: 200 grains (12.96 g)
• Point/broadhead: 100 grains (6.48 g)
• Nock: 10 grains (0.65 g)
• Fletching: 40 grains (2.59 g)

Recurve bow arrows:

• Typically 400-550 grains (25.9-35.6 g)
• Olympic recurve: ~425 grains (27.5 g)

Why arrow weight matters:

• Kinetic energy: Heavier arrows hit harder (better for hunting)
• Trajectory: Lighter arrows fly flatter (better for target shooting)
• Bow stress: Arrows too light can damage the bow (dry-fire effect)
• Penetration: Heavier arrows penetrate better (momentum)

GPP (Grains Per Pound of draw weight): Safe minimum: 5-6 GPP

• 50 lb bow × 6 GPP = 300 grain minimum arrow weight

4. Pharmaceutical Dosages

Aspirin:

• Regular strength: 5 grains (325 mg)
• Extra strength: 7.7 grains (500 mg)
• Low-dose (baby aspirin): 1.2 grains (81 mg)

Why 325 mg? 5 grains was the traditional dose; 325 mg is the metric equivalent (5 × 64.79891 ≈ 324 mg, rounded to 325).

Thyroid hormone (levothyroxine): Historically dosed in grains of desiccated thyroid:

• 1/4 grain = 15 mg
• 1/2 grain = 30 mg
• 1 grain = 60 mg
• 2 grains = 120 mg

Modern synthetic levothyroxine uses micrograms (mcg), but some patients and doctors still reference grain equivalents.

Nitroglycerin (historical):

• 1/100 grain = 0.6 mg (sublingual tablet for angina)

Morphine (historical):

• 1/8 grain = 8 mg
• 1/4 grain = 16 mg
• 1/2 grain = 32 mg

Pharmaceutical metrication: Most countries abandoned grains in favor of milligrams by the 1970s-1990s. The US still labels some medications in grains alongside metric.

5. Precious Metals (Troy System)

Grain as base unit:

Troy ounce:

• 1 troy ounce = 480 grains (31.1 g)
• Gold, silver, platinum weighed in troy ounces

Pennyweight (dwt):

• 1 pennyweight = 24 grains (1.555 g)
• Jewelers buy/sell scrap gold in pennyweights

Example: Gold ring

• Weight: 120 grains (7.78 g total weight)
• 14K gold = 58.3% pure
• Pure gold content: 120 × 0.583 = 70 grains (4.54 g)

Gemstones (carat):

• 1 carat = 200 mg = 3.086 grains
• A 5-carat diamond = 15.43 grains

6. Historical Measurements

Medieval coins:

English penny (pre-1816):

• Weight: 22.5 grains (1.46 g) — silver
• 240 pennies = 1 pound (tower pound, 5,400 grains)

Gold sovereign:

• Weight: 123.27 grains (7.99 g) — 22K gold

Roman solidus:

• Weight: ~70 grains (4.5 g) — pure gold coin

Trade: Merchants used grain weights for spices, medicines, and precious materials:

• Saffron: Sold by the grain (expensive!)
• Pearls: Measured in grains (now carats)

7. Scientific and Precision Applications

Balances and scales: Analytical balances (pre-digital) often had grain markings:

• 1 grain = 0.0648 g
• Used in chemistry, pharmacy, metallurgy

Bullet casting: Home bullet casters measure lead bullets in grains to ensure consistency:

• Lead density: ~11.34 g/cm³
• A 230-grain .45 bullet = 14.9 g

Propellant chemistry: Rocket propellants and explosives measured in grains during development (now grams/kilograms).

Common Uses

1. Ammunition Manufacturing and Reloading

The grain is the universal standard for bullet and powder measurement.

Bullet weight: Every ammunition box lists bullet weight in grains:

• 9mm: "115 gr FMJ" = 115-grain full metal jacket
• .308: "168 gr HPBT" = 168-grain hollow-point boat-tail

Powder charges: Reloaders measure powder in grains using precision scales:

• Typical pistol charge: 3-10 grains
• Typical rifle charge: 20-60 grains

Why grains persist:

1. Ballistics tables: Decades of data in grains
2. Reloading manuals: All recipes in grains
3. International standard: Even metric countries use grains for export ammo
4. Precision: Grain scale appropriate for small differences that matter

2. Archery

Arrow selection: Archers match arrow weight (grains) to bow draw weight:

• Too light: Bow damage risk
• Too heavy: Poor trajectory

Broadheads: Hunting broadheads sold by weight:

• 75 grain, 100 grain, 125 grain, etc.

Tuning: Archers adjust arrow weight by changing point weight (grains) to fine-tune flight.

3. Pharmaceuticals (Historical and Residual)

United States: Some medications still list grain dosages:

• Aspirin: 5 grain (325 mg)
• Thyroid medication: Grain equivalents

Medical history: Understanding grain dosages important for:

• Historical medical research
• Old prescriptions
• Classic pharmaceutical formulations

4. Jewelry and Precious Metals

Troy system: Grains underpin the troy weight system used for gold, silver, platinum.

Jeweler's usage:

• Weighing scrap: Pennyweights (24 grains)
• Gold purity calculations: Grain-based math
• Stone setting: Small gemstones sometimes measured in grains

5. Historical and Collectors' Context

Numismatics (coin collecting): Historical coins' weights recorded in grains:

• Helps identify counterfeits (wrong weight)
• Documents wear (lost grains over time)

Antique firearms: Black powder firearms measured in grains:

• "This musket took 90 grains of powder and a 450-grain ball"

6. Scientific and Educational

Teaching weight systems: The grain demonstrates the connection between avoirdupois, troy, and apothecaries' systems.

Historical science: Understanding old experiments and recipes requires grain knowledge:

• 18th-century chemistry
• Medieval alchemy
• Renaissance medicine

7. International Trade

Ammunition export: US and European manufacturers use grains globally:

• 124 gr 9mm NATO standard (worldwide)
• Even metric-preferring countries import grain-labeled ammo

Conversion Guide

Basic Conversions

Grain to Metric:

1 grain = 64.79891 milligrams (EXACT)
1 grain = 0.06479891 grams

Grain to Avoirdupois:

437.5 grains = 1 ounce (avoirdupois)
7,000 grains = 1 pound (avoirdupois)
1 grain = 1/7000 pound

Grain to Troy:

480 grains = 1 ounce (troy)
5,760 grains = 1 pound (troy)
24 grains = 1 pennyweight

Grain to Apothecaries':

480 grains = 1 ounce (apothecaries')
20 grains = 1 scruple
60 grains = 1 dram

Grams to Grains:

1 gram = 15.43236 grains

Milligrams to Grains:

1 milligram = 0.015432 grains

Conversion Tables

Grains → Milligrams

Grains	Milligrams
1 gr	64.8 mg
5 gr (aspirin)	324 mg
10 gr	648 mg
50 gr	3,240 mg (3.24 g)
100 gr	6,480 mg (6.48 g)
115 gr (9mm bullet)	7,452 mg (7.45 g)
230 gr (.45 bullet)	14,904 mg (14.9 g)

Grains → Grams

Grains	Grams
1 gr	0.065 g
10 gr	0.648 g
100 gr	6.48 g
115 gr	7.45 g
230 gr	14.9 g
350 gr (arrow)	22.7 g
500 gr	32.4 g

Grams → Grains

Grams	Grains
0.1 g	1.54 gr
1 g	15.43 gr
5 g	77.16 gr
10 g	154.32 gr
25 g	385.81 gr

Common Ammunition Conversions

Caliber	Bullet Weight (Grains)	Grams
.22 LR	40 gr	2.6 g
9mm	115-147 gr	7.5-9.5 g
.38 Special	110-158 gr	7.1-10.2 g
.45 ACP	185-230 gr	12.0-14.9 g
5.56mm NATO	55-77 gr	3.6-5.0 g
.308 Win	147-180 gr	9.5-11.7 g

Practical Conversion Examples

Example 1: Aspirin dose A tablet is labeled "5 grains." How many milligrams?

5 grains × 64.79891 mg/grain = 323.99 mg ≈ 325 mg

Example 2: 9mm bullet weight A 115-grain bullet weighs how many grams?

115 gr × 0.06479891 g/gr = 7.45 grams

Example 3: Arrow weight An arrow weighs 28 grams. How many grains?

28 g × 15.43236 gr/g = 432.1 grains

Example 4: Powder charge A reloading recipe calls for 4.5 grains of powder. How many milligrams?

4.5 gr × 64.79891 mg/gr = 291.6 milligrams (0.292 grams)

Common Conversion Mistakes

1. Confusing Grains with Grams

The mistake: Thinking "grains" and "grams" are the same or similar.

Reality:

• 1 grain = 0.0648 grams (much smaller!)
• 1 gram = 15.43 grains

Example error: A recipe calls for "5 grains of powder." Someone uses 5 grams instead:

• 5 grains = 0.324 grams
• 5 grams = 77.16 grains
• Error: 15× too much powder! (Dangerous in reloading!)

How to avoid: Always check units carefully. Grains are abbreviated "gr" (or "grn"), grams are "g."

2. Using Wrong Ounce Conversion

The mistake: Converting grains to ounces using the wrong system (troy vs. avoirdupois).

Reality:

• 1 ounce (avoirdupois) = 437.5 grains
• 1 ounce (troy) = 480 grains
• Different values!

Example error: Converting 480 grains to avoirdupois ounces:

• Wrong: 480 ÷ 480 = 1 oz (using troy conversion)
• Correct: 480 ÷ 437.5 = 1.097 oz avoirdupois

How to avoid: Specify which ounce system you're using. Context matters:

• Bullets, general items: avoirdupois
• Gold, silver, gems: troy

3. Rounding Errors in Metric Conversion

The mistake: Using "1 grain ≈ 65 mg" (too rounded).

Reality:

• 1 grain = 64.79891 mg (EXACT)
• Rounding to 65 mg introduces 0.3% error

Compounding errors:

• 500 grains × 65 mg = 32,500 mg (wrong)
• 500 grains × 64.79891 mg = 32,399 mg (correct)
• Error: 101 mg (3.1% error after compounding)

How to avoid: Use the precise value (64.79891 mg) or at least 64.8 mg for better accuracy.

4. Assuming Grain Measurements Are Obsolete

The mistake: Thinking grains are only historical and no longer used.

Reality: Grains remain the standard in:

• Ammunition (worldwide)
• Reloading (all countries)
• Archery (arrows, broadheads)
• Some pharmaceuticals (U.S.)

Example: A shooter in Europe buys "9mm 124 grain" ammunition—grains are used globally for ammo, even in metric countries.

5. Confusing Grain (Mass) with Grain (Seed)

The mistake: Thinking a grain is a literal seed weight that varies.

Reality:

• Historically based on barleycorns, but now precisely defined: 64.79891 mg
• No variation—it's a fixed unit, not dependent on actual barley

Example: Someone thinks "grains vary by barley type." No! The grain is now a defined constant, not tied to actual seeds.

6. Forgetting Grain Precision in Reloading

The mistake: Measuring powder charges with ±1 grain tolerance.

Reality:

• Safe tolerance: ±0.1 grain (0.0065 g)
• ±1 grain can mean 20-30% variation in pistol charges (dangerous!)

Example:

• Recipe: 4.0 grains Bullseye
• Measured: 5.0 grains (1 grain over)
• Result: 25% overpressure—potential case rupture!

How to avoid: Use precision scales (digital, ±0.1 grain). Reloading requires exactness.