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What is Kelvin? Absolute Temperature Scale Explained

June 22, 2025 โ€ข 4 min read

You have probably heard of Celsius and Fahrenheit, but there is a third temperature scale used throughout science and engineering: Kelvin. Unlike the others, it starts at absolute zero โ€” the coldest anything can theoretically get.

The Basics

0 K = -273.15ยฐC = -459.67ยฐF

K = ยฐC + 273.15 (same degree size as Celsius, just shifted)

Kelvin uses the same increment size as Celsius โ€” a 1-degree change in Celsius equals a 1-degree change in Kelvin. The only difference is where zero sits. Note: we write "300 K" not "300ยฐK" โ€” there is no degree symbol with Kelvin.

What is Absolute Zero?

Absolute zero (0 K) is the theoretical lowest temperature possible. At this point, atoms have minimal possible motion and cannot lose any more thermal energy. In practice, absolute zero can never be perfectly reached, but scientists have cooled materials to within billionths of a degree above it.

  • โ–ถ Absolute zero: 0 K = -273.15ยฐC = -459.67ยฐF
  • โ–ถ Coldest lab temperature achieved: approximately 0.000000001 K
  • โ–ถ Outer space (cosmic background): about 2.7 K (-270.45ยฐC)

Why Scientists Use Kelvin

  • โ–ถ No negative numbers: Temperature in Kelvin is always positive (or zero), which simplifies calculations
  • โ–ถ Direct proportionality: Gas laws (like PV = nRT) only work correctly with absolute temperature
  • โ–ถ Energy relationships: Doubling the Kelvin temperature doubles the thermal energy โ€” this is not true with Celsius or Fahrenheit
  • โ–ถ SI standard: Kelvin is the official SI unit of thermodynamic temperature

Everyday Temperatures in Kelvin

  • โ–ถ Water freezes: 273.15 K (0ยฐC / 32ยฐF)
  • โ–ถ Room temperature: 293-296 K (20-23ยฐC / 68-73ยฐF)
  • โ–ถ Human body: 310 K (37ยฐC / 98.6ยฐF)
  • โ–ถ Water boils: 373.15 K (100ยฐC / 212ยฐF)
  • โ–ถ Light bulb filament: about 2,500 K
  • โ–ถ Sun surface: about 5,778 K
  • โ–ถ Sun core: about 15,000,000 K

Kelvin in Everyday Life

You might encounter Kelvin more than you think:

  • โ–ถ Light bulb colour temperature: "2700K warm white" or "5000K daylight" on LED packaging
  • โ–ถ Photography white balance: cameras measure colour temperature in Kelvin
  • โ–ถ Astrophysics news: star temperatures are always reported in Kelvin

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