Celsius to Kelvin Converter

Celsius to kelvin — quick reference

The formula, worked line by line

The conversion is a single addition, and the reason it is that simple is written into the definition of the Celsius scale. The SI Brochure gives it as t/°C = T/K − 273.15, and states that the degree Celsius is equal in magnitude to the kelvin, so no stretching of the scale is needed.

Rearranged for this direction that is K = °C + 273.15. There is no order-of-operations trap and no factor to forget. The only thing to get right is whether you are converting a temperature or a temperature difference, and that distinction changes the answer completely.

K = °C + 273.15
°C = K − 273.15
25 °C → 25 + 273.15 = 298.15 K
For a temperature difference, add nothing: ΔK = Δ°C

Why 273.15 is exact, and 273.16 no longer is

The offset you use here is exact by definition and has not changed. What did change is the number one hundredth above it. Until 2019 the kelvin was defined as one 273.16th of the thermodynamic temperature of the triple point of water, which made 273.16 K 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 triple point became something to be measured rather than declared — the best estimate remains 273.16 K, now with a small uncertainty — while the 273.15 offset was deliberately preserved and stayed exact.

How to use the celsius to kelvin converter

Type a Celsius figure and the kelvin equivalent appears as you type, to two decimal places. Swap turns the page into a kelvin to Celsius converter without retyping. Nothing below −273.15 °C is treated as an ordinary temperature reading, because that is absolute zero.

Almost everybody who needs this conversion needs it because a formula has refused their number. The ideal gas law, the combined gas law, Charles’s law, the Stefan–Boltzmann radiation law and every equilibrium constant in thermodynamics all take an absolute temperature, and feeding Celsius into any of them produces an answer that is not merely imprecise but meaningless.

The arithmetic is the easy half. Add 273.15. There is no scale factor, because the SI Brochure defines the degree Celsius as equal in magnitude to the kelvin, so the two scales are the same ruler with the zero mark moved. The interesting half is why the position of that zero matters so much.

Here is the reason in one line. On a scale with an arbitrary zero, ratios are nonsense: 20 °C is not twice as hot as 10 °C in any physical sense, and 0 °C does not mean an absence of anything. On the Kelvin scale, 200 K genuinely does carry twice the thermodynamic temperature of 100 K, and zero means zero.

A gas law is a ratio, which is why it insists. Double the absolute temperature of a gas at constant pressure and its volume doubles. Try the same reasoning in Celsius — heat a gas from 10 °C to 20 °C — and you predict the volume doubling when the true ratio is 293.15 to 283.15, an increase of about 3.5 percent.

Where the conversion actually gets used

The single most common place this appears is a gas-law problem stated in Celsius and solved in kelvin. The conversion happens twice — once on each temperature — and the whole answer turns on doing both before any dividing begins, not after.

Take a fixed volume of gas at 27 °C and 1.0 atmospheres, warmed to 127 °C. Convert first: 300.15 K and 400.15 K. The pressure ratio is 400.15 divided by 300.15, giving about 1.333, so the new pressure is roughly 1.33 atmospheres.

Do it in Celsius instead and 127 divided by 27 suggests the pressure should rise by a factor of 4.7. The two answers are not close, and the gap is not a rounding error — it is what happens when a ratio is taken on a scale whose zero was placed for human convenience.

298.15 K and the other numbers worth memorising

Thermodynamic tables are quoted at a standard temperature, and for chemistry that temperature is 298.15 K — exactly 25 °C. Enthalpies of formation, entropies and Gibbs energies in a data book almost all carry that subscript, which is why the number turns up so often it is worth knowing by sight.

A different 273.15 K, or 0 °C, is the standard temperature for gas-volume work, and mixing the two conventions is a common source of a small, stubborn discrepancy. If a molar volume does not match the book, check which standard temperature the book used before checking the arithmetic.

Three more are worth carrying: 310.15 K for body temperature, 293.15 K for the room temperature many physical constants are quoted at, and 77.4 K for liquid nitrogen, which is the reference point most laboratory cold work is described against.

Every Celsius value, worked out

96 common Celsius 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