About the Temperature Converter
This temperature converter switches between Celsius, Fahrenheit, Kelvin and Rankine and shows you the exact formula being applied for the pair you picked, rather than just an answer. Temperature is the odd one out among unit conversions, because three of these four scales have their zero point in a different place. That means you cannot simply multiply by a factor as you would for length or mass — an offset has to be applied as well, which is precisely where mental arithmetic tends to go wrong. All four values are also listed together beneath the result.
How to use the Temperature Converter
- 1
Enter the temperature reading in the Value box, using a minus sign for sub-zero figures.
- 2
Choose the scale your reading is on in the From list.
- 3
Choose the scale you want it converted to in the To list.
- 4
Read the converted figure, note the exact formula shown in the conversion details, and check the table for all four scales.
What people use it for
Cooking from foreign recipes
An American recipe calling for 350 °F needs 176.7 °C, which most ovens round to 180. A 425 °F hot oven is 218.3 °C, usually set to 220.
Weather and travel
A forecast of 95 °F is 35 °C, and 15 °F is minus 9.4 °C. Knowing the crossover points makes packing decisions far quicker than converting every figure.
Science and engineering work
Gas law calculations must use absolute temperature. Converting 25 °C to 298.15 K before applying the ideal gas equation avoids one of the most common errors in the subject.
The formulas, and the crossover at minus 40
Fahrenheit and Celsius differ in both the size of a degree and where zero sits. A Fahrenheit degree is five ninths the size of a Celsius degree, and the scales are offset by 32 at the freezing point of water. That gives °F = °C × 9/5 + 32 and, rearranged, °C = (°F − 32) × 5/9. Kelvin uses Celsius-sized degrees but starts at absolute zero, so K = °C + 273.15. Rankine uses Fahrenheit-sized degrees from the same absolute zero, so °R = °F + 459.67, or equivalently K × 1.8. A satisfying consequence of the Celsius and Fahrenheit formulas is that the two scales cross. Set °C equal to °F and solve: x = 9x/5 + 32 gives −4x/5 = 32, so x = −40. Minus 40 °C and minus 40 °F are the same physical temperature, which makes it the fastest sanity check on any converter or piece of code. Absolute zero is 0 K, 0 °R, −273.15 °C and −459.67 °F; anything below those is a typo rather than a temperature.
Where the scales came from, and the 2019 kelvin
Daniel Fahrenheit set his 1724 scale using a brine freezing mixture at zero and human body temperature near 96, values chosen because they let him subdivide by repeated halving. Water's freezing and boiling points landed at 32 and 212, an inconvenient 180 degrees apart. Anders Celsius proposed his centigrade scale in 1742 with 0 at boiling and 100 at freezing, and it was inverted shortly after his death to the arrangement used today. Lord Kelvin's absolute scale of 1848 kept the Celsius degree but moved zero to the point where molecular motion ceases. Since May 2019 the kelvin is no longer defined by the triple point of water at all, but by fixing the Boltzmann constant at exactly 1.380649 × 10⁻²³ joules per kelvin, which ties temperature directly to energy. Rankine, proposed in 1859, is the Fahrenheit equivalent of Kelvin and survives mainly in American thermodynamics and HVAC work. For mental conversion, double the Celsius figure, subtract a tenth of the result, then add 32 — for 20 °C that is 40 minus 4 plus 32, or exactly 68 °F.
Tips
- Learn three anchor points and you can estimate the rest: 0 °C is 32 °F, 10 °C is 50 °F, and 28 °C is about 82 °F.
- Kelvin is written without a degree symbol — 300 K, never 300 °K.
- A temperature difference converts differently from a temperature: a rise of 10 °C is a rise of 18 °F, with no offset applied.