C to F conversion formula is $C * 9/5 + 32$. But, here is a quick C to F conversion trick: [^1] > Double Celsius, Add 30 i.e., $20 x 2 + 30 = 70$ (real: 68) A table to compare the trick to the actual temp: | :RiCelsiusFill: | Trick | :RiFahrenheitFill: | Delta | | --------------- | ----- | ------------------ | ----- | | 0 | 30 | 32 | -2 | | 10 | 50 | 50 | 0 | | 20 | 70 | 68 | +2 | | 30 | 90 | 86 | +4 | | 40 | 110 | 104 | +6 | | 50 | 130 | 122 | +8 | >[!warning|right|100] Observations: > - After 10c, every 5 degrees, the trick is 1 degree high > - The opposite is true every 5 degrees below 10c A trick to do the *real* formula in your head: > Double Celsius, minus 10%, add 32 Going backwards (F to C) is not as easy, so memorizing some milestone temps might be better overall: | :RiCelsiusFill: | :RiFahrenheitFill: | Description | | --------------- | ------------------ | ----------- | | 0 | 32 | Freezing | | 10 | 50 | Cold! | | 20 | 68 | Cool | | 30 | 86 | Warm | | 40 | 104 | Hot! | [^1]: [source: YSK reddit](https://www.reddit.com/r/YouShouldKnow/comments/1hmrgzp/ysk_an_easy_trick_for_rough_converting_celsius_to/)