[bryant]

for what it's worth: add 9 degrees F (or 18 if it's easier to remember) for every 5 degrees C (10 C, easier), and peg 32F to 0C. You get:

-40F=-40C

-22F=-30C

-4F =-20C

14F =-10C

32F = 0C

50F = 10C

68F = 20C

86F = 30C

104F= 40C

and then approximate in between from there. It's quick enough for me now that I skip the 2x+30.

[abduhl]

In this HN subthread: users slowly converge on the conversion formula for Celsius to Fahrenheit (32+9C/5) in greater and greater precision while calling it an “approximation.”

[freedomben]

Always interesting what's easier for some people. Personally 9/5x + 32 is much easier for me to remember and calculate

[noslenwerdna]

These points make it easy to remember for me, adding ~5C for 10 F.

40 F = 4 C (forty is four)

50 F = 10 C

60 F = 16 C (sixty is sixteen)

[euroderf]

10c is 50f (easy to remember) 27c is 81f (it's all threes!)