The formulas
Celsius to Fahrenheit: °F = (°C × 1.8) + 32
Fahrenheit to Celsius: °C = (°F − 32) ÷ 1.8
The 1.8 is the same as 9/5, and dividing by 1.8 is the same as multiplying by 5/9.
Worked examples
25°C to Fahrenheit
- 25 × 1.8 = 45
- 45 + 32 = 77°F
98.6°F to Celsius
- 98.6 − 32 = 66.6
- 66.6 ÷ 1.8 = 37°C
−10°C to Fahrenheit
- −10 × 1.8 = −18
- −18 + 32 = 14°F
Order matters. For Celsius to Fahrenheit, multiply first, then add. For Fahrenheit to Celsius, subtract first, then divide.
The quick mental trick
Celsius to Fahrenheit: double it and add 30.
Fahrenheit to Celsius: subtract 30 and halve it.
| °C | Trick | Exact |
|---|---|---|
| 0 | 30°F | 32°F |
| 10 | 50°F | 50°F |
| 20 | 70°F | 68°F |
| 30 | 90°F | 86°F |
| 40 | 110°F | 104°F |
It is exact at 10°C and within a few degrees across everyday weather temperatures, so it is fine for deciding whether to wear a coat. It drifts at extremes, so do not use it for cooking, medicine or science.
A rhyme for the key points
Thirty is hot, twenty is nice, ten is chilly, and zero is ice.
Weather chart
| °C | °F | Feels like |
|---|---|---|
| −20 | −4 | Bitterly cold |
| −10 | 14 | Very cold |
| 0 | 32 | Freezing point of water |
| 5 | 41 | Cold |
| 10 | 50 | Cool |
| 15 | 59 | Mild |
| 20 | 68 | Pleasant |
| 25 | 77 | Warm |
| 30 | 86 | Hot |
| 35 | 95 | Very hot |
| 40 | 104 | Extreme heat |
| 45 | 113 | Dangerous heat |
Body temperature chart
| °C | °F | What it usually means in adults |
|---|---|---|
| 35.0 | 95.0 | Low; below this is hypothermia |
| 36.1 to 37.2 | 97.0 to 99.0 | Typical normal range |
| 37.0 | 98.6 | The traditional average |
| 38.0 | 100.4 | Fever |
| 39.0 | 102.2 | High fever |
| 40.0 | 104.0 | Very high fever; seek medical advice |
For what counts as a fever at different ages, and when to see a doctor, see what temperature is a fever.
Cooking
For oven temperatures, including gas marks, see our cooking measurement conversions. Common ones: 180°C = 356°F (recipes round to 350°F), 200°C = 392°F (recipes round to 400°F), and 220°C = 428°F (recipes round to 425°F).
Why there are two scales
- Celsius sets the freezing point of water at 0° and the boiling point at 100° at sea level. It is used in nearly every country and in science.
- Fahrenheit sets freezing at 32° and boiling at 212°, so there are 180 Fahrenheit degrees between them, against 100 Celsius degrees. That ratio, 180 ÷ 100 = 1.8, is where the formula comes from.
- Fahrenheit is used mainly in the United States and a few other countries and territories, including the Bahamas, the Cayman Islands and Liberia.
A third scale, Kelvin, is used in science. It starts at absolute zero: K = °C + 273.15, so 0°C is 273.15 K.
Key takeaways
- °F = °C × 1.8 + 32; °C = (°F − 32) ÷ 1.8.
- Quick estimate: double the Celsius and add 30.
- −40 is the same on both scales.
- 37°C is 98.6°F, and 38°C (100.4°F) is a fever.