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.