20 °C = 68 °F
30 °C = 86 °F

−40 °C = −40 °F.

A related joke:

> Saul: It's −40 outside.

> Paul: Fahrenheit or Celsius?

> Saul: When it's that cold, it's impossible to tell the difference.

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

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.

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.

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, love it. Thanks for making it!

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.

I really love the design of the site and the well written post.