How to use
- Type the temperature you have.
- Choose the scale it is on: Celsius, Fahrenheit or kelvin.
- The other two appear underneath, each with a copy button. Nothing is shown until the box has a number in it.
💡 Exponents can be entered with e — for example, 1.5×10⁻⁵ is entered as 1.5e-5.
Formula & notes
Three scales, and the whole difficulty is that they disagree about where zero is and how big a degree is. The kelvin starts at absolute zero; Celsius starts 273.15 K above it and counts in the same size of step; Fahrenheit starts lower still and counts in steps five-ninths as large.
Everything the tool does is these three relations, applied in whichever direction is needed.
- Celsius and kelvin
- T/K = t/°C + 273.15
- Fahrenheit and Celsius
- t/°C = (t/°F − 32) × 5/9
- Fahrenheit and kelvin
- T/K = (t/°F + 459.67) × 5/9
Points worth knowing by heart
| °C | °F | K | |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| The scales cross | −40 | −40 | 233.15 |
| Water freezes, 1 atm | 0 | 32 | 273.15 |
| Body temperature | 37 | 98.6 | 310.15 |
| Water boils, 1 atm | 100 | 212 | 373.15 |
−40 is the one place the two degree scales agree, and it is a useful check: if a conversion you have done by hand does not reproduce it, the arithmetic is wrong somewhere.
Practical notes
- 273.15 is a definition, not a measurement. JIS Z 8000-5:2014 (ISO 80000-5) 5-2 fixes the Celsius scale as t = T − T₀ with T₀ = 273.15 K exactly, and notes that T₀ sits exactly 0.01 K below the triple point of water. Since the 2019 revision of the SI the kelvin itself is fixed by the Boltzmann constant, and 273.15 stays as the definition of Celsius.
- A temperature and a temperature difference do not convert the same way. This page converts temperatures, which is why it applies the offsets. For a difference, drop them: 1 °C of change is 1 K and 1.8 °F.
- The unit is the kelvin, written K. There is no degree sign on it and it is not capitalised when spelled out.
- Below absolute zero the tool refuses instead of answering. −300 °C is not a very cold temperature; it is not a temperature.
- Water freezing at 0 °C and boiling at 100 °C are where the scale came from historically. Neither defines it now — both are measured values that come out very close to the round numbers, and the boiling point depends on the pressure anyway.
FAQs
Because that is 26.85 K below absolute zero, and there is nothing there. Absolute zero is 0 K, −273.15 °C, −459.67 °F. A converter that quietly returns −26.85 K for it would be doing arithmetic on something that is not a temperature.
Yes — the two scales count in the same size of step, so a difference is the same number in both. It is not 50 °F, though. A difference of 10 °C is a difference of 18 °F, because the Fahrenheit degree is five-ninths the size. This page converts temperatures rather than differences, so it applies the +32 that a difference must not have.
300 K. The degree sign was dropped in 1967 when the unit was renamed from the degree Kelvin to the kelvin. Written out it is lower-case — three hundred kelvin — while the symbol stays capital, after Lord Kelvin.
98.6 is the exact conversion of 37, and that is the problem with it: 37 is a round figure good to about a degree, and converting it produces a number that looks good to a tenth of a degree. The precision is an artefact of the arithmetic, not of anyone's thermometer.
Because 5/9 is what the standard writes, and it is exact. 0.5556 is short by about one part in 20 000, which is invisible at room temperature and reaches a tenth of a degree by the time you are converting a furnace. The tool carries the fraction.
Not here. A rate is built from differences, so it follows the difference rule: multiply by 9/5 for °F per minute, and leave it alone for K per minute. Putting it through this page would add 32 to it.