The short answer
°F = (°C × 9/5) + 32
°C = (°F − 32) × 5/920 °C × 1.8 = 36, + 32 = 68 °F.
Two mental methods
Double and add 30. 22 °C → 44 + 30 = 74 °F (true 71.6). Fast, off by ~2–3° in the middle of the weather range.
Double, subtract 10%, add 32. 22 °C → 44 − 4.4 = 39.6, + 32 = 71.6 °F — exact, and still doable in your head. This is the method to learn.
Reverse: subtract 32, halve, add 10% of the result. 80 °F → 48 → 24 → 26.4 °C (true 26.7).
The −40 crossover
The two scales cross at −40°: −40 °C = −40 °F. It is the only point where they read the same, and a useful sanity check — if your conversion of a very cold value does not converge toward −40, the formula is inverted.
Weather chart
| °C | °F | Feels like |
|---|---|---|
| −20 | −4 | Severe cold |
| −10 | 14 | Hard freeze |
| 0 | 32 | Water freezes |
| 10 | 50 | Cool jacket |
| 20 | 68 | Room temperature |
| 25 | 77 | Warm |
| 30 | 86 | Hot |
| 37 | 98.6 | Body temperature |
| 40 | 104 | Heatwave |
| 100 | 212 | Water boils at sea level |
Oven chart
| °C | °F | Gas mark | Use |
|---|---|---|---|
| 140 | 275 | 1 | Slow roast, meringue |
| 160 | 325 | 3 | Rich fruit cake |
| 180 | 350 | 4 | Standard baking |
| 200 | 400 | 6 | Roasting, pastry |
| 220 | 425 | 7 | Bread, pizza |
| 240 | 475 | 9 | Very hot |
Fan (convection) ovens run about 20 °C / 35 °F hotter than the dial suggests — reduce the set temperature by 20 °C when a recipe specifies a conventional oven.
Differences vs temperatures
A temperature difference converts with the ratio only, no offset:
Δ°F = Δ°C × 1.8A 5 °C rise is a 9 °F rise, not 41 °F. Applying the +32 offset to a delta is the single most common temperature error in engineering spreadsheets.
Kelvin and Rankine
- K = °C + 273.15 (SI absolute scale)
- °R = °F + 459.67 (absolute Fahrenheit, used in some US thermodynamics)
Kelvin has no degree symbol, and absolute zero is 0 K = −273.15 °C = −459.67 °F.
Key takeaways
- °F = °C × 1.8 + 32; the scales meet at −40
- "Double, minus 10%, plus 32" is exact and mental
- Temperature differences skip the +32 offset
- Reduce fan-oven temperatures by 20 °C versus conventional recipes