Convert Celsius to Fahrenheit Quickly

Problem

The exact formula (C × 9/5) + 32 is difficult to calculate mentally.

Solution

Double the Celsius, subtract 10%, then add 32.

Benefit

Estimates Fahrenheit within ±1 degree using simple mental math.

Example

20°C to °F:
20 × 2 = 40
40 - 4 = 36
36 + 32 = 68°F
(Exact: 68°F)

30°C to °F:
30 × 2 = 60
60 - 6 = 54
54 + 32 = 86°F
(Exact: 86°F)