How to convert Celsius, Fahrenheit and Kelvin
To convert Celsius to Fahrenheit, multiply by 1.8 and add 32. To go from Fahrenheit to Celsius, subtract 32 and divide by 1.8. Kelvin is Celsius plus 273.15. This converter applies those exact formulas: type a temperature, pick the From and To scales, and press Convert. Negative values are fine.
| From | To | Formula |
|---|---|---|
| °C | °F | °F = °C × 9/5 + 32 |
| °F | °C | °C = (°F − 32) × 5/9 |
| °C | K | K = °C + 273.15 |
| K | °C | °C = K − 273.15 |
| °F | K | K = (°F − 32) × 5/9 + 273.15 |
| K | °F | °F = (K − 273.15) × 9/5 + 32 |
Why temperature is different from other units
Most converters on this site multiply by a single factor, because zero meters is zero feet and zero kilograms is zero pounds. Temperature scales don't share a zero. Celsius puts zero at the freezing point of water, Fahrenheit puts it 32 degrees below that, and Kelvin starts at absolute zero, the coldest anything can be.
So each unit here carries a factor and an offset, and the converter goes through kelvin as its base:
| Unit | Factor to kelvin | Offset (K) |
|---|---|---|
| Kelvin | 1 | 0 |
| Celsius | 1 | 273.15 |
| Fahrenheit | 5/9 | 273.15 − 32 × 5/9 ≈ 255.372 |
A value becomes kelvin as value × factor + offset, then the reverse step turns it into the target scale. The Fahrenheit offset is derived from the same 32 and 5/9 as the textbook formula, so 32 °F lands on exactly 273.15 K. Results are shown to at most 8 decimal places, so a value like 0 °F reads as −17.77777778 °C.
The converter doesn't stop you from entering a temperature below absolute zero (−273.15 °C, −459.67 °F, 0 K). It will return a number, but that temperature can't physically exist.
Where the definitions come from
Since the 2019 revision of the SI, the kelvin is defined by fixing the Boltzmann constant at exactly 1.380649 × 10⁻²³ J/K, and the degree Celsius is defined as the kelvin shifted by exactly 273.15. Both are in the BIPM SI Brochure. The Fahrenheit relationship, a degree of exactly 5/9 kelvin with 32 °F at 0 °C, is given in NIST Special Publication 811.
Common temperatures
| Description | °C | °F | K |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Celsius equals Fahrenheit | −40 | −40 | 233.15 |
| Home freezer | −18 | −0.4 | 255.15 |
| Water freezes | 0 | 32 | 273.15 |
| Mild room temperature | 20 | 68 | 293.15 |
| Normal body temperature | 37 | 98.6 | 310.15 |
| Water boils (sea level) | 100 | 212 | 373.15 |
| Moderate oven | 180 | 356 | 453.15 |
Worked examples
- An American recipe says 350 °F: (350 − 32) × 5/9 = 176.67 °C. European recipes usually round this to 180 °C.
- A lab sample at 25 °C in kelvin: 25 + 273.15 = 298.15 K.
- A fever reading of 38.5 °C: 38.5 × 1.8 + 32 = 101.3 °F.
Temperature readings vs temperature differences
The formulas above convert a reading on a thermometer. A difference between two readings converts without the offset. If the weather forecast says it will be 10 °C warmer tomorrow, that's 18 °F warmer, not 50 °F. One kelvin and one degree Celsius are the same size, so a difference of 5 °C is also a difference of 5 K. For differences, multiply or divide by 1.8 yourself rather than using the converter.
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