askchapo

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.