Celsius to Kelvin Converter
Type in either box. K = °C + 273.15 — no multiplication, because the degrees are the same size.
—
The one temperature conversion with no multiplication in it
Celsius and kelvin were deliberately built to rise at the same rate. A kelvin and a degree Celsius are the same size, so converting between them never involves stretching anything — you only move the starting point.
K = °C + 273.15
That single number, 273.15, is the distance from absolute zero to water's freezing point. It is why the conversion is pure addition and why a temperature difference is identical on both scales: a rise of 5 degrees Celsius is a rise of exactly 5 kelvin, with no conversion at all.
The kelvin is the SI base unit and the only scale here with a real zero. 0 K is absolute zero, the point at which a system has no thermal energy left to give up. Nothing is colder, which is why a negative kelvin reading is not a small number but an impossible one.
Two conventions catch people out. Kelvin takes no degree sign — write 300 K, never 300 °K — and the unit is lower-case (“three hundred kelvin”) even though the symbol is a capital K, because it is named after a person.
The working, step by step
Every line updates with whatever you type above. Temperature is the one conversion where the method genuinely matters, because two things change at once: the size of a degree and the position of zero.
Why it is a line that misses the origin
Plotting one scale against the other shows the whole relationship at a glance. The slope is the size of a degree. The height at which the line crosses the vertical axis is the offset — the reason multiplying alone never works. Converting kilograms to pounds gives a line through (0, 0); this one does not.
The same temperature, two different numbers
One physical temperature, read off two scales. The tick marks are spaced differently and shifted along, which is exactly what the two numbers in the formula are doing.
Where does that sit?
A reading is easier to judge against temperatures you already know.
Your turn
Reading a method and being able to use it are different things. Answer within half a per cent and it counts as correct.
Score: 0 of 0
How to use this converter
- Type into either box. The other side updates as you go. There is no button to press.
- Negative readings are fine. Type a minus sign and the graph and scale below follow the value below zero.
- Swap the direction with the button between the boxes if you want to work the other way round.
- Set the decimals if the default is more or less precision than you need.
- Copy the answer with its unit, ready to paste.
- Share the result. The address bar tracks what you type, so the link you copy opens on the same reading.
If you enter something colder than absolute zero, the tool will say so rather than quietly converting a temperature that cannot exist.
Common uses and tips
Where this comes up
- Gas laws and thermodynamics. Any formula with a temperature ratio in it — the ideal gas law, Carnot efficiency, Stefan–Boltzmann — needs absolute temperature. Feeding Celsius into them gives nonsense, including negative pressures.
- Colour temperature. Light-bulb boxes quote 2700 K for warm white and 6500 K for daylight. These are genuine temperatures, describing the colour a perfect radiator would glow at, not a marketing scale.
- Astronomy and cryogenics. The cosmic microwave background sits at 2.7 K; liquid nitrogen boils at 77 K; liquid helium at 4.2 K. At these temperatures Celsius figures are all cumbersome negatives, which is precisely why kelvin is used.
- Laboratory reporting. Scientific papers use kelvin by convention, so converting a room-temperature measurement of 25 °C to 298.15 K is routine.
Tips worth knowing
- Differences need no conversion. Δ5 °C is Δ5 K. This is the practical payoff of the two scales sharing a degree size.
- 273 or 273.15? Use 273.15 when the figure is going into a calculation, and 273 only for rough mental work. The 0.15 matters at low temperatures, where it can be a large fraction of the reading.
- Ratios only mean something in kelvin. 600 K really is twice the absolute temperature of 300 K. 60 °C is not twice 30 °C in any physical sense.
Readings people look up most
Each one opens the converter with the number already in it.
- -273.15 °C to K
- -196 °C to K
- -78.5 °C to K
- -18 °C to K
- 0 °C to K
- 20 °C to K
- 25 °C to K
- 37 °C to K
- 100 °C to K
- 200 °C to K
Quick reference
| Celsius | Kelvin | Usually this is… |
|---|---|---|
| −273.15 °C | 0 K | absolute zero |
| −269 °C | 4.2 K | liquid helium boils |
| −196 °C | 77 K | liquid nitrogen boils |
| −78.5 °C | 194.65 K | dry ice |
| −18 °C | 255.15 K | home freezer |
| 0 °C | 273.15 K | water freezes |
| 20 °C | 293.15 K | room temperature |
| 25 °C | 298.15 K | standard lab conditions |
| 37 °C | 310.15 K | body temperature |
| 100 °C | 373.15 K | water boils |
| 2427 °C | 2700 K | warm white bulb |
| 6227 °C | 6500 K | daylight bulb |
Questions people ask
What is the formula for Celsius to kelvin?
Add 273.15. There is no multiplication, because a kelvin and a degree Celsius are the same size.
Why is there no degree sign on kelvin?
Because it is an absolute unit rather than a scale of degrees. Write 300 K, not 300 °K. The unit name is also lower-case: three hundred kelvin.
Can kelvin be negative?
No. 0 K is absolute zero, the coldest anything can be. A negative kelvin figure means an arithmetic mistake.
Is a change of 1 °C the same as a change of 1 K?
Yes, exactly. The two scales differ only in where they start, so any temperature difference is identical in both.
Should I use 273 or 273.15?
273.15 for anything that goes into a calculation. 273 is fine for mental arithmetic near room temperature but is a meaningful error at cryogenic temperatures.