The exact equivalent
Subtract 32 first, then divide by 1.8. The order is not optional: dividing before subtracting gives a different and wrong answer.
How to convert it by hand
Temperature is not converted by multiplying by a factor, because each scale puts its zero in a different place. You have to adjust the offset first and then the size of the degree.
A tip that saves mistakes: check the result goes the way you expect. If degrees Celsius are a smaller unit than degrees Fahrenheit, the number has to come out bigger. If it comes out smaller, you divided when you should have multiplied.
Conversion table
| °F | °C |
|---|---|
| 1 | -17.2222 |
| 2 | -16.6667 |
| 5 | -15 |
| 10 | -12.2222 |
| 20 | -6.6667 |
| 50 | 10 |
| 100 | 37.7778 |
Reference points to picture it
| Reference | Equals |
|---|---|
| Freezing point | 32 °F = 0 °C |
| A mild day | 70 °F = 21.1 °C |
| Body temperature | 98.6 °F = 37 °C |
| A hot oven | 425 °F = 218.3 °C |
The trick for doing it in your head
Subtract 30 and halve it. For 70 °F that gives 20 against a true 21.1, and for 90 °F it gives 30 against 32.2. Good enough for the forecast, not for an oven.
It is fine for getting by without a phone, but not for anything that depends on precision: in a recipe, on a building site or in a dose, use the exact conversion.