How do you memorize how to covert Fareheit to Celcius, and Celsius to Fahrenheit?

32F is 0C, from there every 9F is 5C. Increase the digit on the left, decrease the digit on the right for every 5°C. So 41F=5C, 50F=10C, 59F=15C, etc. Then I approximate each 2 degrees F as 1 degree C, so for example if it is 13°C outside, than is close to 15°C, which is 59°C, and I subtract 4, which gives me 55°F. This is just how I do it, not sure if it helps.

Someone here suggested multiplying by 2, add 32, and someone else said multiply by 2, add 30. I don't know if this complicates it more, but what you can do is multiply by 2, add 32, and subtract 2 times the first digit, assuming that the number is 2 digits. So for example, let's say the temperature is -21°C. Multiply by 2, and you get -42, add 32, that is -10, and then subtract 2(-2), and that's -6°F.

For converting from F to C, you could adapt this method. Subtract 32, divide by 2, and add the first digit of the answer rounded to the closest tens place. So if we have 83°F, we subtract 32 to get 51, divide by 2 and that's 25.5, add 3 and that's 28.5°C.

Also sometimes some numbers are easy to convert, such as -40°, and there is also that 3.9°C=39°F, although these probably aren't that useful.