Celsius to kelvin
Convert Celsius to kelvin by adding 273.15. Why kelvin has no degree sign, where absolute zero sits, and why light bulbs are labelled in K.
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The formula
kelvin = celsius + 273.15There is no multiplication. A kelvin and a degree Celsius are the same size — a rise of one is a rise of the other — and the scales differ only in where they put zero. That makes this the simplest conversion on the site, and the only one where a temperature difference needs no conversion at all: a 5 °C rise is a 5 K rise.
Quick reference
| Celsius | Kelvin | |
|---|---|---|
| −273.15 °C | 0 K | absolute zero |
| −196 °C | 77.15 K | liquid nitrogen boils |
| −40 °C | 233.15 K | |
| 0 °C | 273.15 K | water freezes |
| 20 °C | 293.15 K | room temperature |
| 25 °C | 298.15 K | standard laboratory conditions |
| 37 °C | 310.15 K | body temperature |
| 100 °C | 373.15 K | water boils at sea level |
No degree sign
It is 300 K, not 300 °K. Kelvin is an absolute scale rather than a scale of degrees
above an arbitrary point, and the degree symbol was formally dropped in 1967. The unit is
also written in lower case when spelled out — “three hundred kelvin”, never “Kelvins” — but
capitalised as a symbol, because it is named after a person.
Absolute zero
0 K is the point at which a system holds no thermal energy that can be removed. It is not a temperature anything has ever reached: laboratories have got within billionths of a kelvin of it, and the third law of thermodynamics says the last step cannot be taken in a finite number of operations.
Everything above 0 K is a positive number, which is the point of the scale. Ratios mean something on it in a way they do not in Celsius — 600 K really is twice as hot as 300 K in terms of thermal energy, while 60 °C is not twice 30 °C in any useful sense.
Why light bulbs are labelled in kelvin
A bulb described as “2700 K” is not 2700 kelvin hot. That is its correlated colour temperature: the temperature to which you would have to heat an ideal black body for it to glow that colour. A tungsten filament genuinely does run at a few thousand kelvin, so for incandescent bulbs the number was almost literal. For an LED it is a description of appearance only.
| Rating | Appearance |
|---|---|
| 2200 K | very warm, candle-like |
| 2700 K | warm white — the classic incandescent look |
| 3000 K | soft white |
| 4000 K | neutral white |
| 5000 K | daylight |
| 6500 K | cool daylight, typical of a screen’s default white |
Counter-intuitively, higher numbers look colder. The scale describes the physics of a glowing object, not how the light feels.
Estimating in your head
Add 273 and be done. The .15 matters in a laboratory and nowhere else.