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Comment by ubermonkey | Hacker News Reader

A shockingly useful "quick and dirty" estimate for C to F for temps humans are likely to encounter is 2x + 30. It's not precise at all, but for purposes of "what does that feel like outside" or "should I bring a sweater" it works pretty well.

21C would, by this estimate, be 72F. The true conversion is just shy of 70F, so, again, it's not correct but it's close enough for this kind of conversation.

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I try to remember: 0 = freezing 10 = chilly 20 = comfy 30 = warm 40 = scorching.

30's hot, 20's pleasing, 10 is not, and 0's freezing.

Also fun:

FAHRENHEIT

  0: Very cold    
  100: Very hot

CELSIUS

  0: Moderately cold
  100: Dead

KELVIN

  0: Dead
  100: Dead

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.

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.”

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

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)

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

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