Celsius to Kelvin is just one addition
Of all the temperature conversions, this is the friendliest. There’s no scaling factor and no fractions to wrangle. K = C + 273.15. Add 273.15 to your Celsius value and you have Kelvin. Done.
Type a Celsius number and the Kelvin reading appears at once. The dropdown reverses it (Kelvin back to Celsius) or pulls in Fahrenheit if you need all three.
Why 273.15, and why no minus signs
Kelvin is the science world’s temperature scale, and it’s built so that zero means the coldest anything can possibly get: absolute zero, the point where molecular motion stops. That sits at -273.15C. So Kelvin is really just Celsius shifted up by 273.15 degrees, which moves that absolute floor to a clean 0 K.
The degrees themselves are the same size. A change of one Kelvin equals a change of one Celsius degree, so the scales rise in lockstep. The only difference is where each one calls zero. One nice consequence: Kelvin values are never negative for real temperatures, which is exactly what physicists want when they plug temperature into equations.
A small style note: Kelvin uses no degree symbol. You write 300 K, not 300 degrees K.
Key anchor points
| Celsius | Kelvin | What it is |
|---|---|---|
| -273.15 C | 0 K | absolute zero, the coldest possible |
| -40 C | 233.15 K | a brutal winter |
| 0 C | 273.15 K | water freezes |
| 20 C | 293.15 K | room temperature |
| 37 C | 310.15 K | human body temperature |
| 100 C | 373.15 K | water boils at sea level |
The two endpoints to remember are the easy ones: water freezes at 273.15 K and boils at 373.15 K, exactly 100 K apart, same as the 100-degree gap in Celsius. And 0 K is the hard wall, the temperature nothing can drop below.
Where Kelvin actually shows up
Physics and chemistry homework is the big one. The gas laws, thermodynamics, and the Stefan-Boltzmann equation all demand temperature in Kelvin, because plugging in a negative Celsius value would break the math. A reaction “run at 25C” goes into the equation as 298.15 K.
Astronomy and engineering use it constantly too. Star surface temperatures, blackbody radiation, cryogenics, and superconductor research are all quoted in Kelvin. Liquid nitrogen boils at about 77 K, which is -196C.
Lighting is the sneaky everyday one. The “color temperature” of a bulb is given in Kelvin: a warm white bulb is around 2700 K, daylight is roughly 5500 K. That’s a different concept from air temperature, but it’s the same unit, and it’s why the number on a lightbulb box looks so large.
Worth keeping straight
Don’t add a degree symbol or the word “degrees” to Kelvin. It’s just “kelvin,” like 273.15 K. This is the one temperature scale where that’s the convention.
Also remember the offset is 273.15, not 273. Rounding to 273 is fine for a rough classroom estimate, but for anything precise, the .15 matters. The converter uses the full value.
Because Kelvin and Celsius share the same degree size, a temperature difference is identical in both. A 10-degree rise in Celsius is a 10 K rise. You only add 273.15 when converting an actual temperature, not a change in temperature.
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FAQ
What’s the Celsius to Kelvin formula?
K = C + 273.15. Just add 273.15. So 25C is 298.15 K.
What is absolute zero?
The coldest temperature possible, where molecular motion stops. It’s 0 K, which equals -273.15C.
Why is there no degree symbol on Kelvin?
By convention, Kelvin is written without one. You say 300 K, not 300 degrees K.
Is the offset 273 or 273.15?
It’s 273.15. The .15 matters for precise work, though 273 is close enough for a rough estimate.
Does it upload anything?
No. The numbers crunch on your own device, never a server, so nothing leaves it.