What is Kelvin ↔ Celsius Absolute Temperature Converter?
kelvin celsius absolute zero, SI temperature This tool runs entirely in your browser. No data is sent to any server, and no signup is needed.
Kelvin to Celsius Converter
Kelvin to Celsius: K = C + 273.15. Used in physics, chemistry, thermodynamics. 0 K = -273.15°C (absolute zero).
About the Kelvin scale
The Kelvin scale is the SI absolute thermodynamic temperature scale, with zero set at absolute zero (the lowest possible temperature). It was proposed in 1848 by William Thomson, later Lord Kelvin, in his paper "On an Absolute Thermometric Scale". The scale's defining property is that 0 K marks the theoretical point at which all classical molecular motion ceases. Real systems retain a tiny zero-point quantum motion even at 0 K, but translational thermal motion stops.
One kelvin equals one degree Celsius in size, so a 1 K change is the same magnitude as a 1 C change. Only the zero point differs: Celsius zero sits at the freezing point of water at one standard atmosphere, while Kelvin zero sits 273.15 degrees colder, at absolute zero. Converting between the two is therefore a simple additive shift with no multiplication.
How the conversion works
The Celsius scale was originally fixed by water's freezing point (0 C) and boiling point (100 C) at 1 atmosphere. Since the 2019 SI redefinition, both scales are anchored to the Boltzmann constant (k = 1.380649 x 10^-23 J/K), giving 1 K a definition independent of water at all. The practical 273.15 offset to Celsius remains exact by international agreement (BIPM, CGPM 2018 Resolution 1).
C = K - 273.15 K = C + 273.15 Kelvin has no degree symbol; the unit is just K, not deg K (SI 1968 Resolution 3).
Because the size of a kelvin equals the size of a Celsius degree, temperature differences and rates of change carry over without conversion. A heat flux of 5 W per K per square metre is identical to 5 W per C per square metre. Only absolute values require the 273.15 shift. This is why the gas constant R in PV = nRT uses K in the SI form (8.314 J per mol K) but the same R would have a very different number if expressed per degree Celsius for a non-zero baseline temperature.
Worked example
A chemistry student measures the boiling point of liquid nitrogen at 77.36 K and needs the Celsius equivalent for a lab report.
- Input: 77.36 K.
- Apply formula: C = K - 273.15.
- Subtract: 77.36 - 273.15 = -195.79.
- Reverse check: -195.79 + 273.15 = 77.36 K. Confirmed.
- Round for the report: -195.79 C (NIST tabulated value at 1 atm).
Reference table
| Phenomenon | Kelvin | Celsius |
|---|---|---|
| Absolute zero | 0 K | -273.15 C |
| Liquid helium boils (1 atm) | 4.22 K | -268.93 C |
| Liquid nitrogen boils (1 atm) | 77.36 K | -195.79 C |
| Dry ice sublimates (1 atm) | 194.65 K | -78.50 C |
| Water freezes (1 atm) | 273.15 K | 0 C |
| Room temperature | 293-296 K | 20-23 C |
| Human body core | 310.15 K | 37 C |
| Water boils (1 atm) | 373.15 K | 100 C |
| Sun surface (effective) | 5,778 K | 5,505 C |
Common pitfalls
- Writing deg K. The kelvin has no degree symbol since the 1968 SI revision. It is 273.15 K, not 273.15 deg K.
- Negative kelvins in homework. Standard thermodynamic temperature cannot be negative. If your gas-law answer returns a negative K value, you have swapped Celsius and Kelvin in the equation.
- Plugging Celsius into PV = nRT. Gas-law and Stefan-Boltzmann equations require absolute temperature. A 20 C room is 293.15 K, not 20 K.
- Confusing delta with absolute. A 10 K temperature rise equals a 10 C rise, but the absolute values differ by 273.15. Use the right one for the context.
- Rounding 273.15 to 273. Fine for back-of-envelope estimates; introduces a 0.15 K error in precise calorimetry or cryogenics.
Related tools and glossary
Frequently asked questions
What is absolute zero?
The theoretical lowest possible temperature: 0 K = -273.15 C = -459.67 F. Translational molecular motion ceases (though zero-point quantum motion persists). Approached but never reached experimentally; the coldest lab system held atoms at about 38 picokelvin (Bremen, 2021).
Why use Kelvin instead of Celsius?
Kelvin is the SI base unit for temperature. All thermodynamic equations require absolute temperature with no negative values. The ideal gas law PV = nRT, Stefan-Boltzmann radiation, and blackbody spectra all break or invert sign when fed Celsius numbers.
What is body temperature in Kelvin?
Normal core temperature 310.15 K (37 C). Room temperature is about 293 to 296 K (20 to 23 C). Comfortable refrigerator 277 K (4 C), freezer 255 K (-18 C). Water freezes at 273.15 K, boils at 373.15 K at 1 atm.
Why is there no degree symbol with Kelvin?
By 1968 SI resolution the unit is just K, not deg K. Celsius keeps the degree symbol (C) because it remains a derived unit on a relative scale. So the freezing point is written as 273.15 K and 0 C.
How do I convert Kelvin to Fahrenheit in one step?
Use F = (K - 273.15) times 9/5 + 32. The Kelvin-to-Celsius step gives the absolute drop, then the Celsius-to-Fahrenheit step rescales and shifts. Example: 300 K equals 26.85 C, which equals 80.33 F. The reverse is K = (F - 32) times 5/9 + 273.15.
Sources
- NIST, Guide for the Use of the International System of Units (SI), NIST SP 811 (2008 edition).
- BIPM, The International System of Units (SI Brochure), 9th edition (2019 redefinition).
- CODATA 2018 recommended values: Boltzmann constant k = 1.380649 x 10^-23 J/K (exact).
- 13th CGPM (1968) Resolution 3, naming the unit kelvin (symbol K).