Celsius to Kelvin conversion

Celsius and Kelvin: the same size of degree, a different zero

Kelvin is the SI unit of temperature and the scale used in physics, chemistry and astronomy. Its zero point is absolute zero, the lowest temperature possible, where thermal motion is at its minimum. A kelvin is exactly the same size as a degree Celsius, so the two scales are simply offset from each other: K = °C + 273.15. Water freezes at 0 °C = 273.15 K and boils at 100 °C = 373.15 K.

Why there is no "degree" in Kelvin

Since 1967 the unit is written "kelvin" (symbol K), not "degree Kelvin", because it is an absolute unit rather than a position on an arbitrary scale. That is also why ratios make sense in kelvin but not in Celsius: 300 K really is twice as hot as 150 K in terms of thermal energy, whereas 30 °C is not twice as hot as 15 °C.

Converting

Add 273.15 to the Celsius value. Negative Celsius temperatures give kelvin values below 273.15 but never below zero — −273.15 °C is absolute zero, 0 K. For quick estimates, adding 273 is close enough.

• −273.15 °C = 0 K (absolute zero)
• −40 °C = 233.15 K
• 0 °C = 273.15 K
• 20 °C = 293.15 K (room temperature)
• 25 °C = 298.15 K (standard laboratory temperature)
• 37 °C = 310.15 K (human body)
• 100 °C = 373.15 K
• 1000 °C = 1273.15 K

Where kelvin is used

Gas laws (PV = nRT) require absolute temperature, so every chemistry student converts to kelvin before using them. Colour temperature of light bulbs and camera white balance is quoted in kelvin: 2700 K is warm white, 5600 K is daylight. Astronomers describe the surface of the Sun as about 5800 K and the cosmic microwave background as 2.7 K.

A note on 273.15

The offset is exactly 273.15 by definition. Some textbooks use 273 for simplicity; the 0.15 difference only matters when you need three-decimal accuracy.

Worked examples

Room temperature, 22 °C. 22 + 273.15 = 295.15 K. Physics problems often round this to 295 K or "about 300 K".

A freezer at −18 °C. −18 + 273.15 = 255.15 K.

An oven at 180 °C. 180 + 273.15 = 453.15 K. Kelvin makes it obvious that the oven is only about 1.5 times hotter than the room in absolute terms.

Frequently asked questions

Can kelvin be negative? No. 0 K is absolute zero; the lowest temperatures achieved in laboratories are billionths of a kelvin above it.

Is it "degrees Kelvin"? Not since 1967 — just "kelvin", symbol K, no degree sign.

Do I add 273 or 273.15? 273.15 is exact. Adding 273 introduces an error of 0.15 K, acceptable in a rough estimate, not in a lab report.

Related conversions

For Fahrenheit, convert to Celsius first (see Fahrenheit to Celsius), then add 273.15. Kelvin to Celsius is the reverse of this page.

Good to know

Colour temperature is the everyday place kelvin turns up. Candlelight is about 1 900 K, an incandescent bulb 2 700 K, "warm white" LED 3 000 K, midday sun 5 500–6 000 K, an overcast sky 7 000 K. These numbers describe the colour of light from an ideal glowing object at that temperature — the bulb's filament really is near 2 700 K (2 427 °C), but an LED labelled 2 700 K is not hot at all; it just matches the colour. The scale runs backwards from intuition: higher kelvin is bluer and looks "cooler", lower kelvin is redder and looks "warmer".