**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:

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:

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.

# Questions

**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.