Chemistry and physics formulas that involve temperature — the gas laws, reaction rates, radiation, thermal noise — almost always need it in kelvins. Lab thermometers, however, read in Celsius. The conversion is the simplest of any temperature pair: add 273.15. This page also explains why skipping that step produces answers that are wrong by a wide margin.
How to use the Celsius to Kelvin converter
- Type your reading into Temperature in Celsius (°C) — anything from −273.15 °C upward.
- The result appears in kelvins, alongside Fahrenheit and Rankine values for the same temperature.
- The reference table marks useful fixed points: absolute zero, the boiling points of liquid nitrogen and oxygen, the triple point of water, standard ambient temperature and several metal melting points.
Celsius to Kelvin formula
Since 2019 the kelvin has been defined by fixing the Boltzmann constant at exactly 1.380649 × 10⁻²³ J/K, and the Celsius scale is defined from the kelvin by this same offset. The size of one kelvin and one Celsius degree is identical, so a temperature difference is the same number on both scales.
Worked example: room temperature in a gas-law problem
Step 1. A sealed flask is at 20 °C. In kelvins: 20 + 273.15 = 293.15 K.
Step 2. It is warmed to 40 °C, which is 313.15 K.
Step 3. At constant volume, pressure scales with absolute temperature: 313.15 ÷ 293.15 = 1.0682, so the pressure rises by about 6.8%.
If you had used Celsius directly, 40 ÷ 20 would suggest the pressure doubles — about fifteen times the real change. That is the whole reason absolute scales exist: ratios only make sense when zero means “no thermal energy”. The ideal gas law calculator handles these conversions for you.
Standard conditions, defined
Textbooks and data tables quote “standard” temperatures that are easy to mix up:
| Name | Temperature | Pressure | Used by |
|---|---|---|---|
| STP (IUPAC) | 0 °C = 273.15 K | 100 kPa | Gas volumes in chemistry |
| SATP | 25 °C = 298.15 K | 100 kPa | Thermodynamic data tables |
| NTP | 20 °C = 293.15 K | 101.325 kPa | Some engineering and lab references |
Older textbooks define STP at 1 atm (101.325 kPa) instead of 100 kPa, which changes the molar volume of an ideal gas from 22.711 L to 22.414 L. Always check which definition a problem uses.
Notable fixed points
| Event | °C | K |
|---|---|---|
| Absolute zero | −273.15 | 0 |
| Liquid nitrogen boils (1 atm) | −195.79 | 77.36 |
| Dry ice sublimes (1 atm) | −78.46 | 194.69 |
| Water freezes | 0 | 273.15 |
| Triple point of water | 0.01 | 273.16 |
| Human body | 37 | 310.15 |
| Gold melts | 1,064.18 | 1,337.33 |
The triple point of water — where ice, liquid water and vapor coexist — used to define the kelvin before 2019. It is still 273.16 K to within measurement precision.
Writing kelvin correctly
The unit is “kelvin”, lowercase, with the symbol K and no degree sign. Write 300 K and read it as “three hundred kelvins”. The Celsius scale keeps its degree sign: °C. To convert back, use Kelvin to Celsius; to see all four scales at once, use the temperature converter.
Frequently asked questions
How do you convert Celsius to Kelvin?
Add 273.15. For example, 25 °C + 273.15 = 298.15 K. No multiplication is needed, because a kelvin and a degree Celsius are the same size.
Why 273.15 and not 273?
Absolute zero lies 273.15 degrees below 0 °C. Using 273 introduces a 0.15 K error, which is harmless for rough work but matters in precise thermodynamics and in published data.
Do you write a degree sign with kelvin?
No. Since 1967–68 the unit has been simply 'kelvin' (symbol K), so you write 298.15 K, not 298.15 °K. The plural is kelvins.
Can a kelvin temperature be negative?
Not for ordinary thermodynamic temperature. 0 K is absolute zero, the lowest possible temperature, so −273.15 °C is the lowest value this converter accepts.