Celsius to Kelvin Converter

The formula is one addition: K = °C + 273.15

Celsius and kelvin use the same size degree. Only the zero point moves, and it moves by 273.15 degrees, so the conversion is a single addition:

K = °C + 273.15

There is no multiplication and no second offset, which makes this the easiest temperature conversion on the site. The number comes from absolute zero, the coldest temperature possible, which sits at 0 K and at -273.15 °C. The BIPM SI Brochure defines the kelvin this way and fixes the triple point of water at 273.16 K, and NIST SP 811, Appendix B.9 prints the Celsius to kelvin relation as exactly that offset.

Every row is the Celsius figure plus 273.15, and the 0.15 is not decoration. Drop it and 100 °C becomes 373 K instead of 373.15 K, an error of 0.15 K that shows up in any lab notebook.

Worked examples from the forecast, the oven and the lab

Start with a weather forecast. A 25 °C summer afternoon is 25 + 273.15 = 298.15 K. A 5 °C morning is 278.15 K, and a -10 °C cold snap is 263.15 K. The Celsius number can be negative, and the kelvin number never is, which is the whole point of the scale.

In the kitchen, an oven set to 180 °C is 180 + 273.15 = 453.15 K. A 220 °C roasting temperature is 493.15 K. Recipes never ask for kelvin, but a food science paper on browning or on sugar stages will, and the conversion is the same one line.

In the lab, a 37 °C incubator is 310.15 K, and liquid nitrogen at -196 °C is 77.15 K. A freezer at -20 °C is 253.15 K. Note how the negative sign disappears as soon as the value crosses into kelvin: -20 + 273.15 = 253.15, a positive number.

Going the other way, subtract. A 300 K reading is 300 - 273.15 = 26.85 °C, and 273.15 K is exactly 0 °C. The subtraction is the only step, so a kelvin value below 273.15 always lands on a negative Celsius value.

Celsius to kelvin chart from absolute zero to a furnace

These values come from adding 273.15 to each Celsius figure, then rounding to two decimals. The last column gives the Fahrenheit equivalent so you can cross-check against a US thermometer.

The gap between the two columns stays at 273.15 all the way up. That constant gap is the difference between a scale offset and a scale factor: Celsius and kelvin share the factor, so only the offset changes.

Absolute zero is 0 K, and nothing goes below it

Kelvin starts at the coldest temperature physics allows. At 0 K, or -273.15 °C, molecular motion reaches its minimum, and no object has been cooled to that point. Labs have reached a few billionths of a kelvin above it, but never zero.

That makes kelvin the natural scale for anything where the ratio of temperatures matters. A gas at 300 K has twice the thermal energy of the same gas at 150 K, and the volume in Charles's law scales with the kelvin value, not the Celsius one. Doubling 20 °C to 40 °C does not double anything physical, because 20 °C is 293.15 K and 40 °C is 313.15 K, a change of only 6.8 percent.

This is why chemistry problems insist on kelvin before you touch a gas law. The NIST page on SI temperature units states the same rule: the kelvin is the base unit, and Celsius is derived from it by the offset.

Kelvin takes no degree sign and a capital K

The unit is written K, not °K, and not "degrees Kelvin" in modern usage. The degree symbol belongs to Celsius and Fahrenheit, which are interval scales named after people; the kelvin is an SI base unit, and the SI style rules drop the symbol. The 9th edition of the SI Brochure, published in 2019, also redefined the kelvin through the Boltzmann constant, but the size of the unit did not change, so every conversion on this page still holds.

Write 273.15 K with a space between the number and the symbol, the same way you write 5 kg or 10 m. "273.15K" without the space is common in code and wrong in a report.

The name is lowercase kelvin and the symbol is uppercase K. A sentence that says "273.15 degrees Kelvin" reads as dated to any editor, and a sentence that says "273.15 °K" is wrong twice.

From kelvin to Fahrenheit and Rankine

Kelvin sits in the middle of the temperature family, so the other scales are one step away.

• Celsius: °C = K - 273.15
• Fahrenheit: °F = K x 9/5 - 459.67
• Rankine: °R = K x 9/5

Rankine is the Fahrenheit-sized absolute scale, so 0 K is 0 °R and the two zeros line up. That makes Rankine useful in US engineering work where the absolute scale has to match Fahrenheit degrees. Kelvin and Rankine both start at absolute zero, so neither can go negative.

A 25 °C room is 298.15 K, 77 °F and 536.67 °R. The Fahrenheit and Rankine columns differ by exactly 459.67, just as the Celsius and kelvin columns differ by 273.15.

Where each scale shows up in daily life

Weather reports, ovens and thermostats in most of the world speak Celsius. Science, engineering and anything that multiplies temperatures speak kelvin. A few everyday products have gone kelvin without announcing it.

Light bulbs are the clearest case. A warm white bulb is sold as 2700 K, a neutral one as 4000 K, and daylight as 6500 K. Those numbers are the color temperature of an ideal radiator, not the temperature of the glass, so a 6500 K bulb is not hot enough to burn you. In Celsius terms the same bulbs are 2426.85 °C, 3726.85 °C and 6226.85 °C, which is why nobody prints the Celsius figure on the box.

Astronomy and plasma physics use kelvin for the same reason: the numbers get large and the zero point matters. A star at 5,800 K is 5,526.85 °C. Climate science reports global averages in Celsius but runs its models in kelvin, because a 1.5 K rise and a 1.5 °C rise are the same physical change, and the model equations need the absolute scale.

Three errors that flip the answer

Using 273 instead of 273.15. The shortcut is fine for a weather app and wrong for a lab. At 0 °C it gives 273 K instead of 273.15 K, an error of 0.15 K. At 1,000 °C the same shortcut is still 0.15 K off, so the error does not grow with temperature, but it never disappears either.

Adding 273.15 to a temperature difference. A difference of 10 °C is a difference of 10 K, with no offset at all. The offset applies to a reading on the scale, not to a gap between two readings. If a room warms from 20 °C to 30 °C, it warmed by 10 K, not by 283.15 K.

Converting a kelvin value twice. In the gas law PV = nRT, the temperature must be in kelvin. If the problem already gives 350 K, adding 273.15 turns it into 623.15 K and doubles the answer. Check the unit before you touch the number.

One more habit saves trouble: keep the sign. A -5 °C reading becomes 268.15 K, not 278.15 K. Dropping the minus turns a winter morning into a spring afternoon.