Celsius to kelvin converter, add 273.15 and write K.

To convert Celsius to kelvin, add 273.15. So 0 °C is 273.15 K, 25 °C is 298.15 K and 100 °C is 373.15 K. Write the answer with a K and no degree sign. A kelvin and a degree Celsius are the same size, so only the starting point moves.

How to use the converter

1. Type the temperature in the Degrees Celsius (°C) box. A minus sign works.
2. The big result shows Kelvins (K). A card under it shows the same temperature in Fahrenheit.
3. Type kelvin in the second box to go back, or open Kelvin to Celsius.
4. Set Decimals to 2 to keep the .15, or 0 for a rough figure. Click a row in Common values to load it.
5. Use Copy result, Copy link or Print chart.

The exact rule

NIST, SI units: temperature says one Celsius degree is an interval of 1 K and zero degrees Celsius is 273.15 K, and its conversion table gives °C + 273.15 = K. NIST Handbook 44 (2026), Appendix C lists the same equation, kelvin = degree Celsius + 273.15, under "Conversion Equations for Units of Temperature (Exact)". NIST Guide to the SI, chapter 4 states T0 = 273.15 K by definition. All pages checked 19 Oct 2026. Since 273.15 is exact, the answer here is exact up to the decimals you choose.

Why a change of 1 °C is a change of 1 K

NIST, Kelvin: Introduction says a change of one kelvin is the same amount of temperature change as one degree Celsius. That means you add 273.15 only for a temperature reading. For a difference you add nothing. If a sample warms from 20 °C to 35 °C, it has risen 15 °C, and that is a rise of 15 K. The two readings in kelvin are 293.15 K and 308.15 K, and the gap is still 15.

Worked examples

• 20 °C. 20 + 273.15 = 293.15 K.
• 37 °C. 37 + 273.15 = 310.15 K.
• -40 °C. -40 + 273.15 = 233.15 K. A negative Celsius number still gives a positive kelvin number while it stays above -273.15.
• 1000 °C. 1000 + 273.15 = 1,273.15 K.
• -273.15 °C. -273.15 + 273.15 = 0 K, which is the bottom of the kelvin scale.

The bottom of the scale

NIST, SI units: temperature says 0 K is commonly called absolute zero. In Celsius that is -273.15 °C, so -273.15 °C is as low as the scale goes. Type a smaller number and the tool shows a warning, because a kelvin value below 0 is not a temperature. The same page gives the triple point of water as 273.16 K, which is 0.01 °C when you turn it back.

273 or 273.15

Some books round the constant to 273. It saves a keystroke and it costs 0.15 K every time. The table shows what the short version does.

For a school sum the short version may be accepted, so read the question. For anything you report or compare, use 273.15.

One temperature on three scales

Fahrenheit comes from the exact rule in the same NIST Handbook table: °F = °C × 1.8 + 32. The kelvin column only adds 273.15, so a jump of 10 °C is a jump of 10 in the kelvin column too.

Doing it without the tool

Add 273, then add 0.15 more. For 52 °C: 52 + 273 = 325, and 325 + 0.15 = 325.15 K. If you only need a rough figure, stop at 325. The tool keeps the .15 so a long chain of sums does not drift.

Mistakes that are specific to Celsius to kelvin

The first is writing the result as degrees kelvin or with a degree sign. NIST says the kelvin unit is not expressed in degrees like Celsius or Fahrenheit are, so 298.15 K is correct and 298.15 °K is not. The second is adding 273.15 to a difference, which is covered above. The third is subtracting by mistake. Celsius to kelvin always gives a bigger number. The fourth is starting from Fahrenheit. Change that to Celsius first, or open Celsius to Fahrenheit and read the other way.

Who uses Celsius to kelvin

Students who meet kelvin in science classes. Anyone reading a data sheet that gives a temperature in K. To go the other way use Kelvin to Celsius. For Fahrenheit see Fahrenheit to Celsius. This page does the maths only.