Convert Celsius to Kelvin
Convert Celsius (°C) to Kelvin (K) instantly by adding 273.15 — the absolute scale that chemistry and physics run on, with worked examples and the pitfalls to avoid.
- Celsius (°C)1
- Fahrenheit (°F)33.8
- Kelvin (K)274.15
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The formula: just add 273.15
Converting Celsius to Kelvin is the simplest temperature conversion there is: K = °C + 273.15. You only add — there's no multiplying or scaling, because one kelvin is exactly the same size as one degree Celsius. So water's freezing point, 0°C, is 273.15 K; a 25°C lab bench is 298.15 K; and boiling water at 100°C is 373.15 K. Body temperature, 37°C, comes to 310.15 K. The two scales run in lockstep, just shifted by 273.15 — Celsius sets its zero at water's freezing point, while Kelvin puts its zero at absolute zero.
Why Kelvin exists, and where you need it
Kelvin is the SI base unit of temperature, and its zero, 0 K, is absolute zero — the coldest anything can get, where molecular motion stops, at −273.15°C. That absolute origin is why science runs on it. The gas laws (PV = nRT, Charles's law) and nearly every thermodynamics formula demand absolute temperature: double the kelvin and you double the gas volume, but doubling Celsius is meaningless because its zero is arbitrary. Chemists quote standard conditions as 298.15 K (25°C), and cryogenics lives here too — liquid nitrogen boils at 77 K, or −196°C.
Three things people get wrong
First, Kelvin takes no degree sign and no word 'degrees': write 300 K and say 'three hundred kelvin,' never 300°K. Second, because the scales share the same step size, a temperature difference is identical in both — warming by 5°C is warming by exactly 5 K, with no 1.8 factor like Fahrenheit. Third, 273.15 is exact, so rounding to 273 introduces a 0.15 K error; keep the decimals for precise work. Finally, the US engineering counterpart is Rankine (absolute Fahrenheit), not Kelvin — don't mix the two absolute scales.
Frequently asked questions
What is 25°C in Kelvin?
25 + 273.15 = 298.15 K. That figure matters because 298.15 K (25°C) is the standard 'room temperature' chemists use when reporting reaction conditions and thermodynamic data, and it's often rounded to 298 K.
Why do I only add 273.15 instead of multiplying like Fahrenheit?
Because one kelvin and one degree Celsius are exactly the same size, the conversion is a pure shift with no scaling. The 9/5 factor in the Fahrenheit formula stretches the scale, but Celsius and Kelvin already share identical steps — you just slide the zero point from water's freezing point down to absolute zero, 273.15 degrees lower.
Is it correct to write °K or 'degrees Kelvin'?
No. Since 1967 the unit is simply the kelvin, written K with no degree symbol — 273.15 K, not 273.15°K — and you say 'kelvin,' not 'degrees Kelvin.' It's abbreviated with a capital K to honor Lord Kelvin, but spelled with a lowercase 'kelvin' when written out in full.