Centigrade vs Fahrenheit cheat sheet

Celsius = 5/9 (F - 32)

Fahrenheit = (9/5 C) + 32

Fast, overstating approximation: double C and add 32.

The doubling is off by 10%. 2 is 10/5; the correct formula is 9/5, so the correct factor is nine/tenths of the fudge factor.

double C, subtract 10% of that product from itself, then add 32

C	F	How does it feel?
0	32	cold
5	41
10	50	chilly
15	59
20	68	cool
25	77	warm
30	86	hot
35	95	tropical
40	104	melting point

(2 × C) + 32

Double C and add 32 becomes less accurate as the temperatures rise. It is exactly right at 0 C, off by 8 at 40 C, and off by 20 at 100 C.

C	F	2*C + 32
0	32	32
5	41	42
10	50	52
15	59	62
20	68	72
25	77	82
30	86	92
35	95	102
40	104	112