The temperature formulas
Temperature scales do not just differ in size; they also start from different zero points. That is why converting needs an addition or subtraction as well as a multiplication, and why the answer is never a simple ratio.
°C = (°F − 32) × 5/9
K = °C + 273.15
°R = °F + 459.67 = K × 9/5
All of these constants are exact. A Celsius degree and a kelvin are the same size; a Fahrenheit degree and a Rankine degree are both 5/9 of that. The converter shows the formula for the pair you choose, filled in with your own number.
Worked examples
- Weather: 25 °C × 9/5 + 32 = 45 + 32 = 77 °F.
- Baking: a US recipe says 350 °F. (350 − 32) × 5/9 = 318 × 5/9 = 176.7 °C, usually rounded to 180 °C on European ovens.
- Science: room temperature of 20 °C is 20 + 273.15 = 293.15 K.
- Engineering: 70 °F + 459.67 = 529.67 °R.
A quick mental shortcut for Celsius to Fahrenheit is “double it and add 30”: 25 °C gives about 80 °F. It is close for everyday weather but drifts at the extremes, so use the exact formula for cooking, medicine and science.
Common temperatures
| What | °C | °F | K |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Celsius equals Fahrenheit | −40 | −40 | 233.15 |
| Home freezer | −18 | about 0 | 255.15 |
| Water freezes | 0 | 32 | 273.15 |
| Room temperature | 20 | 68 | 293.15 |
| Body temperature | 37 | 98.6 | 310.15 |
| Water boils (sea level) | 100 | 212 | 373.15 |
| Moderate oven | 180 | 356 | 453.15 |
The four scales
- Celsius (°C) is used for weather and daily life almost everywhere. Its zero is the freezing point of water.
- Fahrenheit (°F) is used for weather and cooking in the United States and a few other places. Water freezes at 32 °F and boils at 212 °F, 180 degrees apart.
- Kelvin (K) is the SI unit used in science. It starts at absolute zero, the coldest possible temperature, and is written without the word “degree” or the ° sign.
- Rankine (°R) is an absolute scale like kelvin but with Fahrenheit-sized degrees. It still appears in some US engineering work, such as thermodynamics tables.
Nothing can be colder than absolute zero, so the converter warns you if you enter a value below 0 K, −273.15 °C or −459.67 °F.
Temperature differences
Converting a temperature difference is not the same as converting a temperature. A rise of 10 °C is a rise of 18 °F (10 × 9/5), with no +32, because the zero points cancel out. Likewise a change of 10 °C is a change of 10 K. Keep this in mind for weather reports (“5 degrees warmer”) and lab work.
Tips
- Fan ovens usually need about 20 °C less than conventional ovens; check your recipe and oven manual.
- Body temperature varies from person to person and through the day, so 37 °C (98.6 °F) is a typical value rather than a strict rule.
- Converting a whole recipe? Use the volume converter for cups, tablespoons and millilitres, and the weight converter for ounces and grams. For lengths such as cake tin sizes, try the length converter.