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%

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

C	F
0	32
5	41
10	50
15	59
20	68
25	77
30	86
35	95
40	104

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