A mental math heuristic to convert between Fahrenheit and Celsius

I sometimes have to convert between Fahrenheit and Celsius. The actual formula is hard to do in my head, but someone once told me a useful approximation:

• To convert from Celsius to Fahrenheit, double it and add 30.
• To convert from Fahrenheit to Celsius, subtract 30 and halve it (the reverse).

For example, if it’s 12ºC, this heuristic would return 54ºF. (12 × 2) + 30 = 54. The actual amount is not far off: 53.6ºF.

To convert the other way: 68ºF becomes 19ºC. (68 − 30) ÷ 2 = 19. Again, this is close to the actual answer of 18ºC.

These are pretty close because the numbers we’re using (2 and 30) are pretty close to their counterparts in the real formula (1.8 and 32).

This isn’t exact, of course. But it’s come in handy! Now if we could only get the US to use the metric system…