Approximate Conversions Between Celsius and Fahrenheit

Although I'm Americanese, I spend lots of time talking to people in the rest of the world, and sometimes want to talk about the temperature.1 The good news is that it's pretty easy to do approximate conversions between Celsius and Fahrenheit, and the most complicated thing you have to do is multiply by 2.


Exact conversions can be done with C = (F - 32) * (9/5), but this equation is a pain in the ass that we won't use.

Instead, we'll use a set of anchors2:

and for the units place, we'll treat every degree Celsius away from the nearest anchor as 2F.

So, to convert 24C to Fahrenheit, we take 20C + 4C ≈ 68F + 8F = 76F. The exact value is 75.2F.

To convert 17C to Fahrenheit, we take 20C - 3C ≈ 68F - 6F = 62F. The exact value is 62.4F.

With this approximation technique, we'll never be off by more than 1 degree Fahrenheit, because we're treating an increase/decrease of 5C as equivalent to 10F instead of 5 * 9/5 = 9F. Converting to Celsius, it means an error of a half-degree, which Europeans seem to ignore.3

A Simpler Conversion

Stuart Cook pointed out that you can just memorize a single anchor and use the 2F = 1C approximation. This seems fine, though it introduces a little more error. For North Carolina weather, this could leave you thinking you'd never experienced 40C weather.

Where This Breaks Down

Do an exact conversion if you're taking temperatures. The difference between a 102 and 103 degree fever is really important. Use Google for baking, because I'm not memorizing what 170C is.


  1. I believe that Fahrenheit is a decent system for daily temperatures, unlike the rest of the imperial system, which is uniformly bad. Nevertheless, in mixed company, I use Celsius.

    The argument, which I got from John Gruber, is that Fahrenheit has a 100 degree scale that captures the vast majority of temperatures people in many countries will ever experience. I've personally experienced -2F once or twice in Pittsburgh, and pass 100F in North Carolina once or twice in a given summer. In Celsius, the range I've experienced becomes the meaningless -17C to 40.

    Admittedly, some human inhabited places on Earth hit extreme temperatures such as 120F or -50F, but the 0 - 100 range captures "very very cold" to "very very hot" better than Celsius.

  2. If you live somewhere that frequently reaches 50C or -10C, you'll need to memorize another anchor. My condolences.

  3. Horrible, see footnote 1! They'll pry the subtle blatantly obvious difference between 66F and 65F from my cold dead hands.