Every once in a while, you may see a temperature in Celsius instead of Fahrenheit. Your car thermometer may revert to the wrong system and you don’t know how to change it, for example. Since almost the whole world uses Celsius except the U.S. and its territories (plus a handful of countries with deep ties to the U.S.), you’re bound to run into this problem eventually. But there’s an easy way to convert the number that doesn’t require math.

Most countries moved to Celsius in 1960 — it’s easier for calculations, because it uses 0 degrees as the freezing point of water and 100 degrees as the boiling point. Fahrenheit, in contrast, has 32 degrees as the freezing point and 212 degrees for the boiling point. It was the first method that successfully and consistently measured temperature.

In the U.S., though, the attempt to convert the public to using Celsius was botched and eventually abandoned. As a result, some very significant errors have been made. For example, in 1999, NASA lost an entire $125 million Mars orbiter because the Lockheed Martin team had used metric units, while the agency’s team stuck with Fahrenheit, causing a navigation communication failure.

**MORE: Try this simple trick to make multiplication easier**

Conversion between the two systems isn’t hard, but it does require some thought, and you won’t always be able to do it in your head. When converting Celsius to Fahrenheit, the formula is: (°C * 1.8) + 32 = °F. You multiply 1.8 to the number of degrees Celsius, then add that number to 32.

There are tricks that will help you convert. One approximation is to double the Celsius temperature and subtract 10%, then add 32 for a ballpark figure.

However, another easy way is to memorize a few key temperatures, then use those as a baseline — no math involved.

Lifehacker suggests studying these numbers, as they can be flipped in order to convert them:

- 4 degrees C is 40 degrees F (cold-ish; your kid is wearing long johns)
- 16 degrees C is 61 degrees F (you could
*almost*wear shorts) - 28 degrees C is 82 degrees F (warm and summery)
- 40 degrees C is 104 degrees F (it’s like July 2023 out there!)

The last number with an extra “1” is, of course, is not perfect, but you get the idea. And just in case you were wondering: -40 degrees F is the same as -40 degrees C, as the two scales intersect at that point mathematically. But let’s hope you’re never in weather that cold.

**MORE: Here are 11 handy math hacks they didn’t teach you in school**