The simplest possible way to convert Celsius and Fahrenheit

The simplest possible way to convert Celsius and Fahrenheit

Updated Feb 2022

This is a new way to convert temperatures between Celsius and Fahrenheit. It’s not the most accurate method, but it’s surely the easiest.

If you want a puzzle, here is the system as a cartoon:

conversion as lines

Did you figure it out? Here’s the system in words:

For the numbers 4, 16, and 28, transposing digits switches from Celsius from Fahrenheit.

If you want even more explanation, here’s the system as a diagram:

conversion as transposition

It’s a coincidence that these conversion points exist—I found them by writing a program and searching for all the cases where this happen. It’s an very fortunate coincidence that they divide the range of temperatures in a convenient way.

Range in C Range in F Description
< 4°C < 40°F Cold
4°C - 16°C 40°F - 61°F Cool
16°C - 28°C 61°F - 82°F Warm
> 28°C > 82°F Hot

As an example, suppose you are familiar with Celsius and don’t know how to interpret 71°F. Since this is around halfway between 61°F and 82°F you know it is also about halfway between 16°C and 28°C.


Question: Did you lie a little bit about the numbers?

Answer: Yes, but by less than 1°F.

Question: Don’t I have to remember “4, 16, 28”?

Answer: Yes. But it’s not that hard! You have 4, then 4 + 12, and 4 + 12 + 12.

Question: Isn’t this a bad system for me, smart person who can easily calculate F=(9/5)C + 32 and C=(5/9)(F - 32) in my head?

Answer: Probably yes.

Question: How do I use this system to convert other temperatures?

Answer: You can mentally interpolate: For example, 7°C is ¼ of the way from 4°C to 16°C, so it converts to around 45°F, ¼ of the way from 40°F and 61°F.

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