Converting temperatures in your head is a good trick for Americans living abroad.
So here’s the trick. You memorise the following correspondences:
0 °C = 32 °F
10 °C = 50 °F
20 °C = 68 °F
30 °C = 86 °F
Then, to convert any temperature that is near these, approximate 1 °C = 2 °F. This will allow you to convert almost any naturally occurring outdoor temperature in the UK in either direction to within 1° accuracy.
Let’s try it. As I write the current temperature in Edinburgh is 14 °C. This is 10 °C plus 4° extra. From memory convert the 10 °C to 50 °F. Then convert 4 °C extra to 8 °F extra and add it back on. This gives you 14°C = 58°F. This is not exact, but close enough that you know to wear a jumper. The exact formula is
14 * 9 / 5 + 32 = 57 F
Good luck doing that in your head.
from Charles Sutton’s Converting Fahrenheit into Celsius.
A jumper, for Americans, is “A pullover sweater.”