The United States reports weather, body temperature and oven settings in degrees Fahrenheit. The UK, Canada, the Netherlands and almost every other country use Celsius, although older British recipes and some Canadian ovens still show Fahrenheit. Unlike length or weight, temperature is not converted by a single multiplication, because the two scales start from different zero points. This guide explains why and shows the quickest reliable methods.
Why you cannot just multiply
On the Celsius scale, water freezes at 0 °C and boils at 100 °C. On the Fahrenheit scale, the same points are 32 °F and 212 °F. So:
- the scales are offset by 32 degrees at freezing, and
- 100 Celsius degrees cover the same range as 180 Fahrenheit degrees, so one Celsius degree is 1.8 (9/5) Fahrenheit degrees.
A correct conversion has to handle both the offset and the size of the degree.
The exact formulas
°F = (°C × 9/5) + 32
°C = (°F − 32) × 5/9
Worked example: 25 °C × 1.8 = 45, plus 32 = 77 °F.
Worked example: 98.6 °F − 32 = 66.6, × 5/9 = 37 °C.
Warning: the order matters. For Fahrenheit to Celsius, subtract 32 first, then multiply. Doing it the other way round gives a wrong answer.
The double-and-add-30 shortcut
For weather, an approximation is usually enough:
- Double the Celsius figure.
- Add 30.
For 15 °C: 15 × 2 = 30, plus 30 = 60 °F. The exact answer is 59 °F. The shortcut is exact at 10 °C (50 °F) and drifts by about 1 °F for every 5 °C away from that, so it is good for everyday weather between about 0 and 25 °C.
Reverse it for Fahrenheit to Celsius: subtract 30, then halve. 70 °F becomes 20 °C (exact: 21.1 °C).
Note: do not use the shortcut for body temperature, medicine or baking. Use the exact formula there.
Weather chart
| Celsius | Fahrenheit | Feels like |
|---|---|---|
| −20 °C | −4 °F | Severe cold |
| −10 °C | 14 °F | Very cold |
| 0 °C | 32 °F | Freezing |
| 10 °C | 50 °F | Cool |
| 15 °C | 59 °F | Mild |
| 20 °C | 68 °F | Room temperature |
| 25 °C | 77 °F | Warm |
| 30 °C | 86 °F | Hot |
| 35 °C | 95 °F | Very hot |
| 40 °C | 104 °F | Heatwave |
Body temperature
| Celsius | Fahrenheit |
|---|---|
| 36.5 °C | 97.7 °F |
| 37.0 °C | 98.6 °F |
| 38.0 °C | 100.4 °F |
| 39.0 °C | 102.2 °F |
| 40.0 °C | 104.0 °F |
A temperature of 38 °C (100.4 °F) or above is commonly treated as a fever. This chart is for conversion only and is not medical advice.
Oven temperatures
| Celsius | Fahrenheit | UK gas mark |
|---|---|---|
| 140 °C | 275 °F | 1 |
| 150 °C | 300 °F | 2 |
| 170 °C | 325 °F | 3 |
| 180 °C | 350 °F | 4 |
| 190 °C | 375 °F | 5 |
| 200 °C | 400 °F | 6 |
| 220 °C | 425 °F | 7 |
| 230 °C | 450 °F | 8 |
Recipe temperatures are rounded to convenient numbers, so 180 °C (exactly 356 °F) is written as 350 °F. For a fan (convection) oven, many recipes suggest lowering the temperature by about 20 °C.
The one point where both scales agree
At −40, Celsius and Fahrenheit give the same number: −40 °C = −40 °F. You can check it with the formula: −40 × 1.8 = −72, plus 32 = −40.
Convert any temperature
Our temperature converter handles Celsius, Fahrenheit and Kelvin (K = °C + 273.15) and shows each step of the working.
Frequently asked questions
What is 0 °F in Celsius? (0 − 32) × 5/9 = −17.8 °C.
What is 100 °F in Celsius? (100 − 32) × 5/9 = 37.8 °C.
Why does the US still use Fahrenheit? Habit and cost. The US never completed a switch to metric, so weather services, schools and appliance makers kept Fahrenheit for everyday use, while science and medicine in the US use Celsius.
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