Kelvin to Celsius Converter

kelvin to Celsius — quick reference

The formula, worked line by line

Every other temperature conversion on this site needs two operations, because the two scales disagree about two separate things: how big one degree is, and where zero sits. Kelvin and Celsius disagree about only the second. The degree is the same size on both, so the scale factor is exactly 1.

That is not a coincidence or a convenient approximation. The Celsius scale is defined in terms of the kelvin — t/°C = T/K − 273.15 — with the degree Celsius equal in magnitude to the kelvin. The two scales are the same ruler with the zero mark in a different place.

°C = K − 273.15
K = °C + 273.15
300 K → 300 − 273.15 = 26.85 °C
For a temperature difference, subtract nothing: Δ°C = ΔK

Readings and intervals are different quantities

This is the mistake behind almost every wrong Kelvin conversion, and it is not a slip in the arithmetic. A reading of 1 K is −272.15 °C. An interval of 1 K is an interval of 1 °C. Both statements are correct, and they look like they contradict each other.

They do not, because they describe different quantities. The 273.15 shifts where the scale starts, and an interval does not care where the scale starts — it only cares how far apart the marks are, and on these two scales the marks are identically spaced. So a rise of 20 K is a rise of 20 °C, while a temperature of 20 K is −253.15 °C.

How to use the kelvin to celsius converter

Type a kelvin figure and the Celsius equivalent appears as you type, to two decimal places. Swap turns the page into a Celsius to kelvin converter without retyping, and the converter refuses to treat anything below 0 K as an ordinary temperature reading.

Kelvin reaches most people from four places: a chemistry or physics problem where a gas law demands an absolute temperature, an astronomy article giving the surface of a star, a cryogenics or superconductor specification, and the colour temperature printed on a light bulb. Only three of those four are genuinely temperatures, which is a trap worth knowing about before you convert anything.

The arithmetic itself is the simplest of any temperature pair on this site. There is no scale factor, no 9/5, and no order-of-operations trap. Subtract 273.15. That is the entire conversion, and the reason it is so simple is worth more than the formula itself.

A kelvin and a degree Celsius are the same size by definition. The SI Brochure defines the Celsius scale as t/°C = T/K − 273.15 and states that the degree Celsius is equal in magnitude to the kelvin, which is why the conversion is a pure offset with no stretching of the scale at all.

That equality of size has a consequence people miss constantly. A temperature difference of 10 K is a temperature difference of 10 °C — not 283.15 °C. The offset belongs to readings, not to changes, tolerances or rates. Warming something by 5 K and warming it by 5 °C are the same instruction.

What absolute zero actually means

Absolute zero is 0 K, which the SI definition of the Celsius scale fixes at exactly −273.15 °C. It is the bottom of the ordinary temperature scale: a system there sits in its lowest possible energy state, and there is no thermal energy left to remove.

The familiar description — that all motion stops — is a simplification, and a widespread one. Thermal motion does go to zero, but quantum zero-point motion does not disappear, which is why helium stays liquid at absolute zero under its own vapour pressure rather than freezing solid.

The cryogenic ladder below 100 K is where the scale earns its keep, because Celsius readings there are all large negative numbers that convey nothing. Nitrogen boils at about 77.4 K and oxygen at about 90.2 K, a gap of thirteen degrees that industry uses to separate the two.

273.15, 273.16, and which one changed

Two numbers sit one hundredth apart and almost everything written about them has them the wrong way round. 273.15 is the Celsius offset. 273.16 is the triple point of water, the single condition where ice, liquid water and water vapour coexist.

Until 2019 the second was the definition: the kelvin was one 273.16th of the thermodynamic temperature of that triple point, so 273.16 K was exact by construction. On 20 May 2019 the 26th General Conference on Weights and Measures redefined the kelvin by fixing the Boltzmann constant instead.

The consequence is a reversal of which figure you can trust. The triple point is now measured rather than defined — best estimate 273.16 K, with a standard uncertainty near a ten-thousandth of a kelvin — while 273.15, the offset you actually use, was deliberately left exact and is unaffected.

Every kelvin value, worked out

84 common kelvin figures each get their own page, with the answer at full precision, the arithmetic, what rounding costs, and the nearest real-world reference point on the scale.

Questions people ask