C to F: Multiply the temperature by 2 and then add 30
(actual formula is multiply by 1.8, add 32)
ex: 22 C is roughly equal to 74 degrees using my method, actual conversion is 71.6
F to C: Now just work it backwards, subtract 30 from the temp. and then divide by 2
(again, actual formula constants are 1.8 and 32)

===================== Reddit user quaxon

If you live in one of the few countries that use the Fahrenheit temperature scale (that would be the United States, the Bahamas, the Cayman Islands, Liberia, Palau, the Federated States of Micronesia and the Marshall Islands), you may not be used to dealing in Celsius. But that doesn't mean that it doesn't pop up from time to time, like in recipes from other parts of the world or on a trip to Canada...or literally anywhere else not listed above.

You may have learned how to convert Fahrenheit to Celsius (and vice versa) during your elementary school math class, but there's a good chance you don't remember how to do that today. Thankfully, Reddit user quaxon shared a dead-simple method for converting between Fahrenheit and Celsius in your head with no need for a calculator.