Kelvin (K) - Unit Information & Conversion

Quick Answer

What is Kelvin? Kelvin (K) is the SI base unit of temperature and an absolute scale starting at absolute zero (0 K = -273.15°C = -459.67°F). It's used in scientific research, physics, chemistry, and astronomy. One kelvin equals one degree Celsius in magnitude (temperature difference). Room temperature is about 293 K (20°C). Use our temperature converter to convert Kelvin to Celsius, Fahrenheit, and more instantly.

Quick Comparison Table

Kelvin	Celsius	Fahrenheit	Description	Convert Now
0 K	-273.15°C	-459.67°F	Absolute zero	Convert →
77 K	-196°C	-321°F	Liquid nitrogen	Convert →
273.15 K	0°C	32°F	Water freezes	Convert →
293 K	20°C	68°F	Room temperature	Convert →
310 K	37°C	98.6°F	Human body temperature	Convert →
373.15 K	100°C	212°F	Water boils	Convert →
5,778 K	5,505°C	9,941°F	Sun's surface	Convert →

Need a different conversion? Try our temperature converter for all temperature units.

Definition

Kelvin (symbol: K, not °K) is the base unit of thermodynamic temperature in the International System of Units (SI). It is an absolute temperature scale, meaning its zero point (0 K) represents the lowest theoretically possible temperature.

Key characteristics:

• Absolute zero: 0 K = -273.15°C = -459.67°F
• No negative temperatures: (in ordinary matter)
• No degree symbol: Write "273 K" not "273°K"
• Same magnitude as Celsius: 1 K change = 1°C change

Modern Definition (2019): The kelvin is defined by fixing the numerical value of the Boltzmann constant (k) to exactly 1.380649×10⁻²³ joules per kelvin (J/K). This definition links temperature to energy at the atomic level.

Conversion formulas:

• From Celsius: K = °C + 273.15 - Convert C to K
• From Fahrenheit: K = (°F - 32) × 5/9 + 273.15 - Convert F to K
• To Celsius: °C = K - 273.15 - Convert K to C
• To Fahrenheit: °F = (K - 273.15) × 9/5 + 32 - Convert K to F

Important fixed points:

• Absolute zero: 0 K (exactly)
• Water triple point: 273.16 K (0.01°C) - where ice, water, and vapor coexist
• Water freezing: 273.15 K (0°C)
• Water boiling: 373.15 K (100°C)
• Room temperature: ~293 K (20°C)
• Human body: ~310 K (37°C)

Why no degree symbol? Kelvin is an absolute scale starting from a fundamental physical limit (absolute zero), not an arbitrary reference point like Celsius or Fahrenheit. The unit is "kelvin" (lowercase when spelled out), not "degrees Kelvin."

Convert between temperature units: Kelvin converter

History

• Early Thermodynamics (1840s): Scientists studying heat engines and thermodynamics realized that there must be a lowest possible temperature, where thermal energy reaches its minimum.

• William Thomson (Lord Kelvin) (1824-1907): British physicist and engineer who proposed the absolute temperature scale in 1848. He later became Baron Kelvin, and the unit was named in his honor.

• Original Proposal (1848): Thomson proposed an absolute thermodynamic temperature scale based on:

  • Carnot's theorem on heat engines
  • The idea that there exists a temperature at which thermal motion ceases
  • Independence from the properties of any particular substance

• Determination of Absolute Zero: By studying the thermal expansion of gases, scientists extrapolated that gases would theoretically have zero volume at approximately -273°C. This temperature was identified as absolute zero.

• Original Scale (1848-1954): Thomson's scale was initially called the "absolute scale" or "thermodynamic temperature scale." It used the same degree size as Celsius but started at absolute zero.

• Triple Point Definition (1954): The 10th CGPM (General Conference on Weights and Measures) officially named the unit "kelvin" (symbol K) and defined it based on the triple point of water:

  • Triple point of water = exactly 273.16 K
  • This made the kelvin equal in magnitude to the Celsius degree
  • Eliminated need for a physical artifact

• Why 273.15?: This value was chosen to maintain compatibility with the Celsius scale, ensuring that the freezing point of water remained at 0°C (273.15 K) and boiling point at 100°C (373.15 K).

• Adoption as SI Base Unit (1960): When the International System of Units (SI) was established, the kelvin was designated as one of the seven SI base units for thermodynamic temperature.

• Symbol Change (1967): The symbol was changed from "°K" (degree Kelvin) to just "K" (kelvin) to emphasize its absolute nature and distinguish it from relative scales.

• 2019 Redefinition: On May 20, 2019, the kelvin was redefined based on the Boltzmann constant:

  • Old definition: Based on triple point of water (273.16 K)
  • New definition: Boltzmann constant fixed at exactly 1.380649×10⁻²³ J/K
  • Why: Links temperature to fundamental physics (energy per particle)
  • Advantage: Can be reproduced in any properly equipped laboratory
  • Impact: No change to the scale's size or zero point, only how it's realized

• Boltzmann Connection: The Boltzmann constant (k) relates the average kinetic energy of particles to temperature: E = (3/2)kT. By fixing k, temperature is now defined through energy.

• Global Scientific Standard: The kelvin is the only SI base unit for temperature. It's used universally in:

  • Physics research
  • Chemistry
  • Astronomy and astrophysics
  • Engineering
  • Materials science
  • Climate science

• Practical Usage: While Celsius dominates everyday life in most countries and Fahrenheit in the US, scientists worldwide use kelvin for research, ensuring universal compatibility and precision.

Real-World Examples

Cryogenic Temperatures (Very Cold)

Near Absolute Zero:

• Absolute zero: 0 K (-273.15°C) - theoretically unattainable
• Coldest achieved in lab: ~100 picokelvin (0.0000000001 K) - ultra-cold atoms
• Bose-Einstein condensate: <1 microkelvin (0.000001 K)
• Dilution refrigerator: ~0.002 K

Liquefied Gases:

• Liquid helium: 4.2 K (-269°C) at atmospheric pressure
• Liquid hydrogen: 20 K (-253°C)
• Liquid nitrogen: 77 K (-196°C) - common cryogenic coolant
• Liquid oxygen: 90 K (-183°C)
• Liquid air: ~79 K (-194°C)

Space and Cryogenics:

• Cosmic microwave background: 2.7 K (-270.45°C) - temperature of space
• Pluto surface: ~40 K (-233°C)
• Superconducting magnets: 4-10 K (MRI, particle accelerators)
• Infrared telescopes: ~6 K (to reduce thermal noise)

---

Everyday Temperatures

Human Comfort Range:

• Room temperature: 293-295 K (20-22°C, 68-72°F)
• Human body: 310 K (37°C, 98.6°F)
• Comfortable sleep: 288-291 K (15-18°C, 59-64°F)
• Hot summer day: 303-313 K (30-40°C, 86-104°F)
• Cold winter day: 253-273 K (-20 to 0°C, -4 to 32°F)

Water Temperatures:

• Ice melting/water freezing: 273.15 K (0°C, 32°F)
• Cool tap water: 283-288 K (10-15°C, 50-59°F)
• Warm shower: 310-315 K (37-42°C, 98-108°F)
• Hot coffee: 333-353 K (60-80°C, 140-176°F)
• Water boiling: 373.15 K (100°C, 212°F) at sea level

Home and Climate:

• Refrigerator: 275-278 K (2-5°C, 36-41°F)
• Freezer: 253 K (-20°C, -4°F) or below
• Oven baking: 450-500 K (177-227°C, 350-440°F)

---

Industrial and High Temperatures

Manufacturing:

• Lead melting: 600 K (327°C, 621°F)
• Aluminum melting: 933 K (660°C, 1,221°F)
• Glass working: 1,300-1,800 K (1,027-1,527°C)
• Steel melting: 1,800-1,900 K (1,527-1,627°C)
• Iron melting: 1,811 K (1,538°C, 2,800°F)
• Tungsten melting: 3,695 K (3,422°C, 6,192°F) - highest melting point metal

Combustion:

• Candle flame: 1,400 K (1,127°C) outer, 1,700 K (1,427°C) inner
• Propane flame: 2,200 K (1,927°C, 3,500°F)
• Acetylene flame: 3,600 K (3,327°C, 6,020°F) - welding torch
• Wood fire: 900-1,200 K (627-927°C)

---

Astronomical Temperatures

Planets and Moons:

• Mercury (night): 100 K (-173°C)
• Mercury (day): 700 K (427°C)
• Venus surface: 737 K (464°C) - hottest planet
• Earth average: 288 K (15°C, 59°F)
• Mars average: 210 K (-63°C, -81°F)
• Jupiter cloud tops: 165 K (-108°C)
• Saturn: 134 K (-139°C)
• Uranus: 76 K (-197°C)
• Neptune: 72 K (-201°C)

Stars:

• Sun's surface (photosphere): 5,778 K (5,505°C, 9,941°F)
• Sun's core: ~15,000,000 K (15 million K)
• Red dwarf stars: 2,500-4,000 K
• Sun-like stars: 5,000-6,000 K
• Blue giant stars: 10,000-50,000 K
• Hottest known stars: >100,000 K

Extreme Astronomical:

• Neutron star surface: 1,000,000 K (1 million K)
• Supernova core: 100,000,000,000 K (100 billion K)
• Big Bang (1 second after): 10,000,000,000 K (10 billion K)
• Hottest temperature created: 5,500,000,000,000 K (5.5 trillion K) - Large Hadron Collider

---

Color Temperature (Lighting)

Light Bulb Color Temperatures:

• Candle flame: 1,800-2,000 K - warm, orange glow
• Incandescent bulb: 2,700-3,000 K - warm white
• Sunrise/sunset: 2,000-3,000 K - golden hour
• Halogen bulb: 3,200 K - warm white
• Fluorescent: 4,000-5,000 K - cool white
• Daylight: 5,500-6,500 K - neutral white
• Overcast sky: 6,500-7,500 K - cool white
• Clear blue sky: 10,000-15,000 K - very cool blue

Photography and Video:

• Tungsten lighting: 3,200 K
• Daylight balanced: 5,600 K
• Studio flash: 5,500-6,000 K
• LED panels: 3,200-5,600 K (adjustable)

---

Materials Science

Phase Transitions:

• Helium liquefies: 4.2 K
• Nitrogen liquefies: 77 K
• CO₂ sublimes (dry ice): 194.7 K (-78.5°C)
• Mercury freezes: 234 K (-39°C)
• Water freezes: 273.15 K (0°C)
• Gallium melts: 303 K (30°C) - melts in hand

Superconductivity:

• Classic superconductors: <30 K
• High-temperature superconductors: 90-138 K
• Record (at high pressure): 250 K (-23°C)

Common Uses

The kelvin is the standard temperature unit in scientific and technical fields worldwide:

Scientific Research

The universal temperature unit in physics, chemistry, and all scientific disciplines. Essential for ensuring reproducibility and international collaboration.

Scientific applications:

• Thermodynamics and statistical mechanics
• Quantum mechanics and atomic physics
• Chemical kinetics and equilibrium
• Materials science research
• Particle physics experiments
• Cryogenics and low-temperature physics

Why kelvin in science:

• SI base unit (international standard)
• Absolute scale (no negative temperatures)
• Direct relationship to energy (via Boltzmann constant)
• Universal reproducibility
• Required for scientific publications

Convert for scientific work: kelvins to other units

---

Astronomy and Astrophysics

Standard for measuring stellar temperatures, cosmic phenomena, and space science.

Astronomical uses:

• Star surface temperatures (spectral classification)
• Stellar core temperatures
• Planetary atmosphere temperatures
• Cosmic microwave background (2.7 K)
• Interstellar medium temperature
• Black hole thermodynamics
• Big Bang cosmology

Why kelvin in astronomy:

• Suitable for extreme temperatures (millions of kelvins)
• No confusion with negative values
• International astronomical standard
• Links to blackbody radiation physics

---

Color Temperature

Standard for describing the color of light sources in photography, cinematography, and lighting design.

Color temperature uses:

• Light bulb specifications (2,700-6,500 K)
• Camera white balance settings
• Video production lighting
• Architectural lighting design
• Display calibration
• Stage and theater lighting

Common values:

• Warm light: 2,700-3,500 K
• Neutral/daylight: 5,000-6,500 K
• Cool light: 6,500-10,000 K

---

Cryogenics

Essential for ultra-low temperature applications and liquefied gas handling.

Cryogenic applications:

• Liquid nitrogen storage (77 K)
• Liquid helium systems (4 K)
• Superconducting magnets (MRI, particle accelerators)
• Cryopreservation (biological samples)
• Rocket fuel (liquid hydrogen, liquid oxygen)
• Low-temperature physics research

Why kelvin in cryogenics:

• Natural scale for very low temperatures
• Avoids large negative numbers
• Direct relationship to thermal energy
• Industry standard

---

Materials Science

Critical for studying phase transitions, material properties, and thermal behavior.

Materials applications:

• Melting and boiling points
• Glass transition temperatures
• Superconductor critical temperatures
• Thermal expansion studies
• Heat capacity measurements
• Crystal structure studies

---

Engineering and Industry

Used in technical specifications where absolute temperature is important.

Engineering uses:

• Thermodynamic calculations (heat engines, refrigeration)
• Gas laws and ideal gas calculations
• Chemical reactor design
• Aerospace engineering (re-entry heat)
• Semiconductor manufacturing
• Industrial process control

Ideal gas law: PV = nRT (where T must be in kelvins)

---

Climate Science

Standard for scientific climate modeling and atmospheric research.

Climate uses:

• Atmospheric temperature profiles
• Ocean temperature measurements
• Climate model simulations
• Radiative transfer calculations
• Greenhouse gas physics
• Ice core data analysis

Use our kelvin converter for scientific conversions.

Common Conversion Mistakes to Avoid

❌ Mistake #1: Using degree symbol with kelvin

• Wrong: 300°K
• Correct: 300 K (no degree symbol)
• Why: Kelvin is an absolute scale, not measured in "degrees"
• Remember: Write "kelvin" (lowercase) or "K", never "°K"

❌ Mistake #2: Forgetting to add 273.15

• Wrong: 20°C = 20 K
• Correct: 20°C = 293.15 K (add 273.15)
• Formula: K = °C + 273.15
• Example: 0°C = 273.15 K, NOT 0 K

❌ Mistake #3: Confusing temperature difference vs absolute

• Wrong: "Temperature increased by 10 K" = "new temperature is 10 K"
• Correct: A 10 K increase means a change of 10 kelvins
• Example: Room temp (293 K) + 10 K increase = 303 K
• Note: 1 K difference = 1°C difference (same magnitude)

❌ Mistake #4: Using Fahrenheit formula directly

• Wrong: K = (°F + 273.15)
• Correct: K = (°F - 32) × 5/9 + 273.15
• Example: 68°F = (68-32) × 5/9 + 273.15 = 293.15 K
• Must convert F→C first, then C→K
• Tool: Fahrenheit to Kelvin converter

❌ Mistake #5: Thinking kelvin can be negative

• Wrong: -50 K is colder than 0 K
• Correct: 0 K is absolute zero, the lowest possible temperature
• No negative kelvins: (in ordinary matter)
• If you calculate negative K, you made an error

❌ Mistake #6: Rounding 273.15 to 273

• Wrong: K = °C + 273
• Correct: K = °C + 273.15
• Why it matters: Precision in scientific work
• Example: 0°C = 273.15 K (not 273 K)
• Water triple point: 273.16 K (even more precise)