Convert between Celsius (°C), Fahrenheit (°F), Kelvin (K) and more
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Solution Steps
Pro feature°F = °C × 9/5 + 32Temperature conversions use offset/scale formulas rather than a single multiplier.
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How it works
Two of these three scales put their zero in a different place, so converting a temperature is a scaling and a shift rather than the single multiplication every other unit conversion reduces to. That offset is also the source of the one mistake people reliably make with it.
°F = °C × 9/5 + 32 °C = (°F − 32) × 5/9 K = °C + 273.15 °R = °F + 459.67
9/5ratio of degree sizes: one Celsius degree spans 1.8 Fahrenheit degrees32the Fahrenheit reading at the Celsius zero point273.15the offset to absolute zero, which sits at minus 273.15 °CKkelvin, written without a degree sign and without the word degrees
- 37 × 9/5 = 66.6
- 66.6 + 32 = 98.6
- Kelvin: 37 + 273.15 = 310.15
- A temperature and a temperature difference convert differently. A room at 20 °C is 68 °F, but a rise of 20 °C is a rise of 36 °F, because the 32 offset belongs to the reading and not to the interval. Every HVAC delta-T, thermal expansion coefficient and specific-heat value is an interval, so use only the 9/5 factor there and drop the offset.
- The two scales cross at exactly -40, the single reading that is the same number in both. Setting C = C × 9/5 + 32 and collecting terms gives -0.8 × C = 32, so C = -40. It is the standard sanity check for a conversion you have just written.
- The famous 98.6 °F is a conversion artefact. Wunderlich published 37 °C as a round figure in 1868, and converting it manufactured a spurious extra digit of precision that has been quoted as clinical fact ever since. Modern samples put the mean closer to 36.6 °C, and a healthy individual swings roughly a full degree Celsius across the day.
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Frequently Asked Questions
Multiply the Celsius temperature by 9/5 (or 1.8) and add 32. For example, 25°C × 1.8 + 32 = 77°F.
Absolute zero is 0 Kelvin (-273.15°C or -459.67°F). It is the lowest possible temperature where all molecular motion ceases.
Kelvin is used in scientific contexts, especially in physics and chemistry, where calculations require an absolute temperature scale with no negative values.