Centigrade vs Fahrenheit cheat sheet

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Celsius = 5/9 (F - 32)

Fahrenheit = (9/5 C) + 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.

The doubling is off by 10%. 2 is 10/5, but the correct formula is 9/5, so the right factor is nine/tenths below the fudge factor.
Double C, subtract roughly 10% of that product from itself, then add 32 improves the result.
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°

Feelings, whoa-oh-oh-oho, feelings

C feels
0°..9° cold
10°..19° chilly
20°..24° cool
25°..29° warm
30°..34° hot
35°..39° tropical
40°+ unendurable

Fahrenheit vs celsius.jpg