The two exact formulas
The Celsius and Fahrenheit scales are related by an offset of 32 and a ratio of 9 to 5. To go from Fahrenheit to Celsius: °C = (°F − 32) × 5/9. To go from Celsius to Fahrenheit: °F = °C × 9/5 + 32. Since 9/5 is 1.8, the second one can be written °F = °C × 1.8 + 32.
The 32 is there because water freezes at 0 °C and at 32 °F. The 5/9 comes from the span between freezing and boiling water (at sea level): 100 Celsius degrees but 180 Fahrenheit degrees, and 180/100 = 9/5. One Fahrenheit degree is therefore a bit smaller: 5/9 of a Celsius degree.
Worked examples, step by step
Going from Fahrenheit to Celsius, subtract 32 first and then multiply by 5/9. Going from Celsius to Fahrenheit, multiply by 1.8 first and then add 32. The order matters.
- 77 °F: 77 − 32 = 45; 45 × 5/9 = 25 °C.
- 98.6 °F: 98.6 − 32 = 66.6; 66.6 × 5/9 = 37 °C.
- 100 °F: 100 − 32 = 68; 68 × 5/9 = 340/9 ≈ 37.8 °C.
- 0 °F: 0 − 32 = −32; −32 × 5/9 ≈ −17.8 °C.
- 18 °C: 18 × 1.8 = 32.4; 32.4 + 32 = 64.4 °F.
- −10 °C: −10 × 1.8 = −18; −18 + 32 = 14 °F.
- 37.5 °C: 37.5 × 1.8 = 67.5; 67.5 + 32 = 99.5 °F.
Chart from 0 to 40 °C
These values are exact, not rounded. Use them as a quick reference for weather, a thermostat or water temperature.
| Celsius (°C) | Fahrenheit (°F) |
|---|---|
| 0 | 32 |
| 5 | 41 |
| 10 | 50 |
| 15 | 59 |
| 20 | 68 |
| 25 | 77 |
| 30 | 86 |
| 35 | 95 |
| 40 | 104 |
A few fixed points are worth remembering: water boils at 100 °C = 212 °F at sea level (at higher altitude it boils at a lower temperature), typical body temperature is 37 °C ≈ 98.6 °F, and −40 is the one value that is the same on both scales: −40 °C = −40 °F.
A mental shortcut: subtract 30 and halve
To estimate Fahrenheit to Celsius without a calculator, subtract 30 and divide by 2. To go the other way, double and add 30. It is an approximation that works well in everyday weather temperatures and gets worse outside that range.
| °F | Exact Celsius | Shortcut (°F − 30) ÷ 2 | Difference |
|---|---|---|---|
| 41 | 5 | 5.5 | 0.5 |
| 50 | 10 | 10 | 0 |
| 59 | 15 | 14.5 | 0.5 |
| 68 | 20 | 19 | 1 |
| 86 | 30 | 28 | 2 |
| 104 | 40 | 37 | 3 |
| 14 | −10 | −8 | 2 |
| 212 | 100 | 91 | 9 |
Between 40 and 60 °F the shortcut is nearly right; above 80 °F the error passes 2 degrees, and for cooking temperatures it is useless (212 °F gives 91 °C instead of 100). A more precise version for Celsius to Fahrenheit: double the number, subtract 10 percent of that double, then add 32. With 30 °C: 60 − 6 = 54; 54 + 32 = 86 °F, exact.
Oven temperatures
Recipes from countries that use Fahrenheit usually round to the numbers printed on oven dials. These are the exact conversions.
| Celsius | Exact Fahrenheit | How recipes list it |
|---|---|---|
| 160 °C | 320 °F | 320 °F |
| 180 °C | 356 °F | 350 °F |
| 190 °C | 374 °F | 375 °F |
| 200 °C | 392 °F | 400 °F |
| 220 °C | 428 °F | 425 °F |
| 230 °C | 446 °F | 450 °F |
A 5 to 10 °F difference won't ruin a sponge cake or roast potatoes, but keep in mind that many ovens drift from what the dial says; an oven thermometer is cheap and avoids surprises.
Weather and fever readings
For weather, useful reference points are: 0 °C = 32 °F (freezing), 10 °C = 50 °F (cool), 20 °C = 68 °F (mild), 30 °C = 86 °F (hot) and 35 °C = 95 °F (very hot). For cold, −5 °C = 23 °F and −10 °C = 14 °F.
For body thermometers, 37 °C = 98.6 °F, 38 °C = 100.4 °F and 39 °C = 102.2 °F. These are only conversions; what to do about a high or low temperature is a question for a health professional, not a converter.
Common mistakes
- Forgetting the 32: applying 5/9 to 68 °F without subtracting first gives 37.8 °C, when the correct answer is 20 °C. Always subtract 32 first.
- Multiplying before subtracting: 68 × 5/9 = 37.8 and 37.8 − 32 = 5.8 °C, which is not 20 °C either. The subtraction comes first.
- Using the fraction upside down: (68 − 32) × 9/5 = 64.8 is not Celsius. From Fahrenheit to Celsius you use 5/9, which is less than 1.
- Mixing up a temperature with a difference: a rise of 10 °C is 18 °F, not 50 °F. The 32 is added or subtracted only when converting a temperature, not a change.
- Rounding too early: if you will keep calculating with the result, hold on to the decimals until the end.
Kelvin and absolute zero
Kelvin is the scientific scale. One kelvin is the same size as one Celsius degree, and K = °C + 273.15. So 0 °C = 273.15 K, 25 °C = 298.15 K and 100 °C = 373.15 K. Absolute zero is 0 K, which equals −273.15 °C or −459.67 °F; no lower temperature exists.
How to use the ABC SyG tools
To skip the math, open Fahrenheit to Celsius or Celsius to Fahrenheit. Everything runs in your browser: what you type is not uploaded or stored.
- Type the temperature in the "Value" field.
- See the result, highlighted at the top, and the equivalent in Kelvin.
- Change the number to try other temperatures.
- Tap "Copy result" if you want to take the value with you.
If you also need other quantities, the Unit converter has a Temperature category and shows all the equivalents at once. The tool calculates the exact value, rounded to 8 significant digits, and does not accept temperatures below absolute zero.
Frequently asked questions
What is the formula to convert Fahrenheit to Celsius?
°C = (°F − 32) × 5/9. Subtract 32 first, then multiply by 5 and divide by 9. For example, 77 °F is (77 − 32) × 5/9 = 25 °C.
How many degrees Fahrenheit is 20 degrees Celsius?
20 °C is 68 °F: 20 × 1.8 = 36 and 36 + 32 = 68. It is a typical room temperature.
What is 350 °F in Celsius for the oven?
350 °F is about 177 °C: (350 − 32) × 5/9 = 176.7. Recipes in Celsius round this to 180 °C, which strictly is 356 °F.
Why is −40 the same in Celsius and Fahrenheit?
It is the only point where the two scales meet: (−40 − 32) × 5/9 = −72 × 5/9 = −40.
Is the subtract 30 and halve trick reliable?
Only as an estimate and for weather temperatures, where the error is usually 1 to 3 degrees. For cooking, medicine or science, use the exact formula or the tool.