Temperature converter for Celsius, Fahrenheit, Kelvin, Rankine and Réaumur
This temperature converter translates a reading between five scales: Celsius, Fahrenheit, Kelvin, Rankine and Réaumur. Unlike length or mass, temperature cannot be converted by multiplying by a single factor, so the tool applies the complete formula for each scale, including its offset.
Why temperature is not a simple ratio
Most units measure from a true zero: 0 m means no length and 0 kg means no mass. Celsius and Fahrenheit place zero at a convenient point instead. Celsius puts it at the freezing point of water, while Fahrenheit puts it 32 degrees below that. Converting therefore needs two things: a scale factor for the size of a degree, and an offset for where zero sits.
This is also why 20 °C is not "twice as warm" as 10 °C. In Fahrenheit the same readings are 68 °F and 50 °F, and in kelvin they are 293.15 K and 283.15 K, a rise of about 3.5%. Ratios only make sense on the absolute scales, kelvin and Rankine.
Conversion formulas
Internally the tool converts your value to Celsius, then from Celsius to the target scale.
| Scale | From Celsius | To Celsius |
|---|---|---|
| Fahrenheit | °F = °C × 9/5 + 32 | °C = (°F − 32) × 5/9 |
| Kelvin | K = °C + 273.15 | °C = K − 273.15 |
| Rankine | °R = (°C + 273.15) × 9/5 | °C = (°R − 491.67) × 5/9 |
| Réaumur | °Ré = °C × 0.8 | °C = °Ré × 1.25 |
A Celsius degree and a kelvin have the same size, and so do a Fahrenheit degree and a Rankine degree. Kelvin and Rankine both start at absolute zero, which is −273.15 °C or −459.67 °F.
Worked examples
0 C = 32 F water freezes
37 C = 98.6 F typical body temperature
100 C = 212 F water boils at sea level
-40 C = -40 F the crossover point
350 F = 176.6666667 C a common oven setting
0 K = -459.67 F absolute zeroTake 37 °C: 37 × 9/5 = 66.6, and adding 32 gives 98.6 °F. Going back, 98.6 − 32 = 66.6, and 66.6 × 5/9 = 37. The oven example shows why recipes round: 176.67 °C usually becomes 180 °C, which is 356 °F.
The −40 crossover
Celsius and Fahrenheit agree at exactly one temperature. Set C = C × 9/5 + 32 and solve: C × (−4/5) = 32, so C = −40. It makes a quick sanity check, because any conversion near −40 should give nearly the same number on both scales.
Rankine and Réaumur
Rankine is the absolute scale with Fahrenheit-sized degrees, so 0 °R is absolute zero and 32 °F is 491.67 °R. It still shows up in some US engineering thermodynamics, alongside BTUs and psi; the energy converter and pressure converter handle those units.
Réaumur splits the interval between freezing and boiling water into 80 degrees instead of 100, so 100 °C equals 80 °Ré and 20 °C equals 16 °Ré. You are most likely to meet it in historical texts and old European recipes.
Converting a temperature difference
A frequent error is running a temperature change through the full formula. If a room warms by 10 °C, it warmed by 18 °F, not 50 °F. The offset belongs to readings only; for differences, multiply by 9/5 to go from Celsius to Fahrenheit or by 5/9 to go back. A difference in kelvin equals the same difference in Celsius.
The converter treats every number as a reading, so do difference arithmetic by hand or convert both end points and subtract.
Things to watch
- Nothing can be colder than absolute zero, but the tool does not reject such input: −300 °C returns −26.85 K. Treat a negative kelvin or Rankine result as a typo in the source.
- Kelvin is written without a degree sign: 300 K, not 300 °K.
- Results like 176.6666667 are repeating decimals. Lower the significant digits for a tidier figure.
If you need other everyday conversions, the length converter uses the same approach of exact definitions.