[mikeash]

I memorize some 10s. 0 is 32, 10 is 50, 20 is 68, 30 is 86. Extend beyond what’s memorized by using the ratio 10/18. I don’t have 40 memorized, but it’s easy to figure that it’s 104. Then use the approximation of 1C=2F to fill in the gaps. High today is 34? That’s 94. Thermostat says 72? That’s 22.

[hyperpape]

If you are ok with being slightly off, you can combined memorizing tens with the approximation that 1C = 2F. Going up or down from the nearest 10C, you’ll never be off by more than 1F.