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Comment by kchhina | Hacker News Reader

I have family/friends that visit often from Celsius using countries and every time the weather comes up, they ask me what xx F is in C. Tired of trying to answer them, I figured out a quick, ballpark conversion --

50 deg F = 10 deg C

For every 10 def F, there's roughly a 6 deg C change.

So (approximately)

40 deg F = 4 deg C

60 deg F = 16 deg C

etc.

I find that it works reasonably well for frequent requests from said Celsius understanding visitors.

*Edited for line breaks

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Very close over human ranges:

    C = (F-30) / 2
    F = 2 * C + 30

Exact conversions:

    C = (F-32) * 5 / 9
    F = C / 5 * 9 + 32

Table:

    C     F
  -40   -40
    0    32 (Approx water freezing point)
   10    50
   20    68 ( ~ 70)
  ~21    70
   30    86
   37    98.6 (body temperature)
   40   104
   50   122
  ...   ...
  100   212 (Approx water boiling point)

The easiest ballpark conversion I know is:

F -> C: subtract 32, divide by 2. So 76F = 44/2 ~= 22C

C -> F: multiply by 2, add 32. So 15C = 30+32 ~= 62F

The factor of 2 isn’t precise (the actual ratio is closer to 5/9). But it only really diverges at hotter temps: so 95F is actually 35C, whereas this formula will give you 31.5C, which is more like 89F; but 50F will give you 9C (48F), when the correct value is 10C. For most weather purposes it’s close enough - at least to decide what to wear - and it’s easy for most people to calculate.

Took me a while to figure that out until I remembered that our zeroes are not the same.

-40 is the same in both though, as far as I recall

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