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.

▲ bryant 3 days ago | parent | next [-]

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 3 days ago \| parent \| next [-]
		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 2 days ago \| parent \| prev \| next [-]
		Always interesting what's easier for some people. Personally 9/5x + 32 is much easier for me to remember and calculate
	▲	noslenwerdna 2 days ago \| parent \| prev \| next [-]
		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 2 days ago \| parent \| prev [-]
		10c is 50f (easy to remember) 27c is 81f (it's all threes!)

▲ iambateman 2 days ago | parent | prev [-]

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

▲ patrickmay 2 days ago | parent [-]

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

	▲	ubermonkey 2 days ago \| parent [-]
		Also fun: FAHRENHEIT `0: Very cold 100: Very hot` CELSIUS `0: Moderately cold 100: Dead` KELVIN `0: Dead 100: Dead`