20 °C = 68 °F
30 °C = 86 °F

## Eric Soroos said:

16 °C = 61 °F. Much more useful summer conversion temperature in Ireland.

## michelpp said:

It doesn't help much for people visiting here in the US, but when I travel outside the country I use this handy rhyme: 30 is hot. 20 is nice. 10 is cold. 0 is ice.

## u801e said:

The way I convert is to remember that 32 °F is 0 °C and then count by "nines" for Fahrenheit to get the desired temperature in Celsius.

32 °F = 0 °C
41 °F = 5 °C
50 °F = 10 °C
59 °F = 15 °C
68 °F = 20 °C
77 °F = 25 °C
86 °F = 30 °C
95 °F = 35 °C

It's also pretty easy to remember that 10 °C = 50 °F and 35 °C = 95 °F if you want to go forward or back from there with the same counting method.

## Sherif said:

Thanks a lot for sharing this neat trick. I just moved to North America and I just suffer as much. Doubling and adding 30 is awesome.

I came to your blog from another post by someone about MathB.in, love it. Thanks for making it!

## Geoff Bailey said:

An alternative approach to the accurate conversions, that may be easier to remember because it is more symmetric, is to first shift the temperatures so that the common value (-40) is the zero point, then scale, then shift back, that is, \begin{align*} f & = (c + 40) \cdot \frac{9}{5} - 40, \\ c & = (f + 40) \cdot \frac{5}{9} - 40. \end{align*} The formula "looks" the same for each direction, just with inverted scaling factors.