Celsius to Kelvin Converter
Convert °C to K for physics, chemistry and engineering work. The converter rejects temperatures below absolute zero.
K = °C + 273.15
Why the Conversion Is Just + 273.15
The kelvin is the SI unit of thermodynamic temperature, and the Celsius scale is defined from it. By definition, a Celsius temperature is the kelvin temperature minus exactly 273.15. So converting means adding or subtracting that offset. Nothing is rescaled, because a Celsius degree and a kelvin are the same size.
Since the 2019 redefinition of the SI, the kelvin itself is fixed by the Boltzmann constant, k = 1.380649 × 10⁻²³ J/K. Before that, it was tied to the triple point of water. Water's freezing and boiling points (0°C and 100°C at standard pressure) are now measured values, not definitions, although they still land within a few thousandths of a degree of those round numbers.
Temperature Differences: 1°C = 1 K
Only add 273.15 to a temperature reading, never to a temperature change. If a sample warms from 20°C to 35°C, it has warmed by 15°C, which is also 15 K. Adding 273.15 to the difference is a common mistake in heat-transfer problems. Specific heat capacities quoted in J/(kg·K) can be used directly with a temperature change measured in °C.
Celsius to Kelvin Table
From absolute zero to furnace temperatures, including common lab conditions (4°C for maximum water density, 20°C and 25°C for reference temperatures).
| °C | K |
|---|---|
| -273.15°C | 0 K |
| -200°C | 73.15 K |
| -100°C | 173.15 K |
| -40°C | 233.15 K |
| 0°C | 273.15 K |
| 4°C | 277.15 K |
| 20°C | 293.15 K |
| 25°C | 298.15 K |
| 37°C | 310.15 K |
| 100°C | 373.15 K |
| 200°C | 473.15 K |
| 500°C | 773.15 K |
| 1,000°C | 1,273.15 K |
Worked Example: Why Gas Laws Need Kelvin
A sealed, rigid container of gas is heated from 20°C to 40°C. By how much does the pressure rise? For a fixed volume, pressure is proportional to absolute temperature (Gay-Lussac's law), so the temperatures must be in kelvin.
Step 1: Convert both temperatures
Step 2: Take the ratio
The pressure rises by 6.8%. Using Celsius directly (40 ÷ 20 = 2) would wrongly predict that the pressure doubles.
Reference Temperatures in Kelvin
Fixed points from cryogenics to astrophysics.
| Reference point | K | °C |
|---|---|---|
| Absolute zero | 0 | -273.15 |
| Cosmic microwave background | 2.725 | -270.425 |
| Liquid nitrogen boils (1 atm) | 77.36 | -195.79 |
| Dry ice sublimates (1 atm) | 194.65 | -78.5 |
| Water freezes (1 atm) | 273.15 | 0 |
| Average human body temperature | 310.15 | 37 |
| Water boils (1 atm) | 373.15 | 100 |
| Sun's effective surface temperature | 5,772 | 5,498.85 |
The cosmic microwave background and solar figures are measured astrophysical values (the solar value is the IAU nominal effective temperature), not exact constants.
How to Write Kelvin Correctly
- No degree sign. Write 298.15 K, not 298.15 °K. The "degree Kelvin" was renamed the kelvin in 1967.
- Space before the symbol. Write 300 K, not 300K, the same as with other SI units.
- Lowercase unit name. The unit is the "kelvin" (plural "kelvins"). Only the symbol K and the scientist's name, Lord Kelvin, are capitalized.
Frequently asked questions
Is it 273 or 273.15?
273.15 exactly. Many textbook problems round to 273 for convenience, which introduces a 0.15 K error. That matters little for room-temperature estimates but adds up in precise work.
Can a kelvin temperature be negative?
No. 0 K is absolute zero, the lowest possible thermodynamic temperature, so this converter rejects negative kelvin values and Celsius values below −273.15°C.
What is room temperature in kelvin?
About 293–298 K (20–25°C). Chemistry data tables usually use 298.15 K (25°C) as their standard reference temperature.
Has anything reached absolute zero?
No. The third law of thermodynamics means absolute zero can be approached but not reached. Laboratory experiments have cooled atoms to within a billionth of a kelvin of it.
Sources
- BIPM, The International System of Units (SI Brochure): definition of the kelvin and the Celsius temperature.
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI): unit naming and notation rules.
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