Kelvin Converter

The absolute scale: 0 K is absolute zero, and nothing exists below it. Convert to Celsius, Fahrenheit and Rankine.

Conversion table

Temperature	In Celsius
10	-263.15 °C
25	-248.15 °C
50	-223.15 °C
75	-198.15 °C
100	-173.15 °C
150	-123.15 °C
200	-73.15 °C
250	-23.15 °C
300	26.85 °C
400	126.85 °C
500	226.85 °C
600	326.85 °C
750	476.85 °C
1,000	726.85 °C
1,500	1,226.85 °C

🔬 Fixed points

0 K−273.15 °C = −459.67 °F
273.15 K0 °C, water freezes
293.15 K20 °C, room temperature
373.15 K100 °C, water boils

Writing and using kelvin correctly

The kelvin takes no degree sign and no word "degrees": write 300 K and say "three hundred kelvin". It is the only major temperature scale that works this way.
A step of one kelvin is exactly the same size as a step of one degree Celsius, so a temperature difference of 5 K and 5 °C are the same difference.
To convert, add or subtract 273.15 — never multiply. K = °C + 273.15 and °C = K − 273.15.
Anything below 0 K is not a cold reading, it is an impossible one; this converter refuses it rather than returning a number.

Why an absolute scale exists at all

Celsius and Fahrenheit both start from something chosen: the freezing point of water, a brine mixture, a body temperature. That is fine for weather but useless for physics, because ratios stop making sense — 20 °C is not twice as hot as 10 °C in any meaningful way. The kelvin fixes its zero at the point where thermal motion is at its minimum, so temperatures become genuine quantities you can multiply and divide. Gas laws, blackbody radiation and thermodynamic efficiency are all written in kelvin for that reason, and the size of the degree was kept identical to the Celsius degree so the two scales differ only by an offset of 273.15.

What changed in 2019

Until 2019 the kelvin was defined by the triple point of water: that specific temperature was declared to be exactly 273.16 K. The trouble is that water is a substance, with isotopic composition and impurities that vary, so the definition depended on a sample. The 2019 revision of the SI cut the tie by fixing the Boltzmann constant at exactly 1.380649 × 10⁻²³ joules per kelvin, which turns the kelvin into a statement about energy per particle instead. Everyday numbers did not move at all — water still freezes at 273.15 K — but the scale is now reproducible in any laboratory without a bottle of water.

The four scales side by side

Kelvin: absolute, degree the size of a Celsius degree. Zero at absolute zero. The SI base unit.
Celsius: same degree size, zero at the freezing point of water. K = °C + 273.15.
Fahrenheit: smaller degrees, zero at a brine freezing mixture. °F = °C × 9/5 + 32.
Rankine: absolute like kelvin but with Fahrenheit-sized degrees. 0 °R is absolute zero and 491.67 °R is 0 °F.
A difference of 1 K equals a difference of 1 °C, and a difference of 1 °R equals 1 °F.
−40 is the one place Celsius and Fahrenheit agree; in kelvin that is 233.15 K.

Where you meet kelvin outside physics class

Light bulbs. A "warm white" lamp is 2,700 K and a "daylight" one 6,500 K — counterintuitively, warmer-looking light has the lower number.
Photography. White balance is set in kelvin: candlelight around 1,900 K, overcast sky around 7,000 K.
Astronomy. The Sun's surface is about 5,772 K, and the cosmic microwave background sits at 2.725 K.
Cryogenics. Liquid nitrogen boils at 77 K, liquid helium at 4.2 K, and superconductors are described by their critical temperature in kelvin.
Semiconductors and electronics specify noise and drift per kelvin of temperature change.
Weather and cooking never use kelvin — an oven at 200 °C would be 473 K, which nobody writes on a dial.

Worked examples

Adding and subtracting 273.15

Convert 300 K to Celsius and Fahrenheit

300 − 273.15 = 26.85 °C
26.85 × 9/5 + 32 = 80.33 °F
A warm room, not a laboratory extreme

300 K = 26.85 °C = 80.33 °F

Convert −10 °C to kelvin

−10 + 273.15 = 263.15 K
Note the kelvin figure is still positive
In Fahrenheit: 14 °F

−10 °C = 263.15 K

Frequently Asked Questions

How do I convert kelvin to Celsius?

Subtract 273.15. So 300 K is 26.85 °C and 273.15 K is exactly 0 °C. Going the other way, add 273.15 to the Celsius figure. There is no multiplication involved, because a kelvin and a Celsius degree are the same size.

What is absolute zero?

0 kelvin, which is −273.15 °C or −459.67 °F: the temperature at which thermal motion reaches its quantum minimum. It cannot be reached, only approached — laboratories have got within billionths of a kelvin — and no temperature below it exists, which is why a negative kelvin value is not cold but meaningless.

Why is there no degree symbol on kelvin?

Because the kelvin is a unit in its own right rather than a position on a graduated scale. Since 1967 the correct form has been "300 K", read as three hundred kelvin, with no degree sign and no capital on the unit name. Celsius and Fahrenheit keep the degree symbol because they mark points on a chosen scale.

Is a change of 1 K the same as a change of 1 °C?

Yes, exactly. The two scales were deliberately built with the same interval size and differ only by the offset of 273.15, so any temperature difference is numerically identical in both. That is why heat capacities are quoted in joules per kelvin and the same number works for Celsius.

What does 2700 K mean on a light bulb?

It is the colour temperature: the temperature a theoretical blackbody would need to glow that colour. 2,700 K is the warm yellowish light of an old incandescent bulb, 4,000 K is neutral, and 6,500 K is the bluish white of daylight. The scale runs opposite to intuition, because higher numbers look colder.

What is Rankine and who uses it?

An absolute scale with Fahrenheit-sized degrees: zero is absolute zero, and 0 °F is 459.67 °R. It survives in American engineering thermodynamics, where it lets absolute temperatures be used in equations without leaving the Fahrenheit world. To convert, °R = K × 9/5.