The way I do it is count by nines from 32 to get to the temperature I want and then multiply by 5. For example, if I want to know what the corresponding temper

The way I do it is count by nines from 32 to get to the temperature I want and then multiply by 5. For example, if I want to know what the corresponding temperature for 70 °F in Celsius, I would do the following: 0 -> 32 5 -> 41 10 -> 50 15 -> 59 20 -> 68 So I know the temperature in Celsius is just above 20 °C (21 °C give or take). To speed up the process, I can use other starting points I know like 10 °C for 50 °F, 35 °C for 95 °F, -15 °C for 5 °F and, of course, at -40, it doesn't matter ;)