The formula I learned in elementary school was °C * 9/5 + 32, which is not particularly amenable to calculating in your head. Here's how to figure that in your head without multiplying.

Let's say the temperature is 7°C.

- Calculate 1/10th (10%) of that.
*Ten percent of a number is simply that number with the decimal place shifted left one spot*. So ten percent of 7 is 0.7 - Subtract 0.7 (1/10th) from 7 = 6.3
- Double 6.3 = 12.6
- Add 32 = 44.6
__7°C is 44.6°F__

#### How does this work?

In the original formula 9/5 = 1.8. Half of 1.8 is 0.9

If you subtract 1/10th of the original number from itself, you get 0.9 * °C. Then when you double that, you get 1.8 * °C.

After that, you merely need to add 32 to get the Fahrenheit equivalent. Going the other way (°F to °C) is a little more complicated, and I will get to that in a subsequent post.