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What is a kelvin?

The SI temperature unit — starts at absolute zero

Definition

The kelvin is the SI base unit of thermodynamic temperature. Its zero point is absolute zero — the theoretical coldest possible temperature. Water freezes at 273.15 K and boils at 373.15 K. Note: there is no "degree" symbol with kelvin, only "K".

Common uses

  • Scientific and engineering thermodynamics
  • Astronomy and astrophysics temperature reporting
  • Colour temperature of light sources (e.g., 6500 K daylight)

🌍 Real-world scale

Room temperature ≈ 293 K. Water boils at 373 K. The Sun's surface is 5778 K. Deep space is about 2.7 K.

Convert from kelvins

3 conversions
Kelvin to Celsius1 K = -272.15 °CKelvin to Fahrenheit1 K = -457.87 °FKelvin to Rankine1 K = 1.8 °R

Convert to kelvins

3 conversions
Celsius to Kelvin1 °C = 274.15 KFahrenheit to Kelvin1 °F = 255.928 KRankine to Kelvin1 °R = 0.5556 K

Tips

  • K = °C + 273.15. Identical degree size; only the zero point differs.
  • There are no "degrees Kelvin" — just "kelvins". The symbol is K, not °K.
  • Colour temperature: 3000 K warm/yellow; 5000 K daylight; 6500+ K cool/blue.

Common mistakes

  • Writing °K — incorrect. Kelvin uses no degree symbol.
  • Trying to use negative Kelvin values — impossible physically.
  • Confusing "kelvin" (unit) with "Kelvin" (the scientist William Thomson).

FAQ about the kelvin

What is a kelvin?
The kelvin is the SI base unit of thermodynamic temperature. Its zero point is absolute zero — the theoretical coldest possible temperature. Water freezes at 273.15 K and boils at 373.15 K. Note: there is no "degree" symbol with kelvin, only "K".
Where is the kelvin used?
The kelvin is commonly used for: Scientific and engineering thermodynamics; Astronomy and astrophysics temperature reporting; Colour temperature of light sources (e.g., 6500 K daylight).
What is an example of a kelvin in real life?
Room temperature ≈ 293 K. Water boils at 373 K. The Sun's surface is 5778 K. Deep space is about 2.7 K.
What category does the kelvin belong to?
The kelvin is a unit of temperature, part of the 4-unit temperature category on UnitConv.
How accurate are kelvin conversions?
All conversion factors follow NIST and BIPM international standards to 8 significant figures — the highest practical accuracy.
Explore all 4 temperature units and their conversions.🌡️ Temperature hub →