The post about the math mistake in temperature conversation reminded me of a formula that a friend told me about (thanks DSD!). She was traveling abroad, and the guide she was with said that to convert Celsius to Fahrenheit people should use the formula:

**Double the Celsius, and subtract from it the amount obtained by moving the decimal place one unit to the left. Then add 32 to get the corresponding Fahrenheit.**

For example, with a temperature like 50°C, you’d double 50 to get 100, then from that subtract 10.0 to get 90. Finally, you’d add 32 to 90 to get 122°F.

This is equivalent to the formula

Temp in °F = (9/5) (Temp in °C) + 32.

In particular, if C is the temperature in Celsius, the description to double and the subtract that amount with the decimal place moved describes 2C – 0.1(2C), which is 1.8C, or 9/5C.

It does seem to me to be quicker to compute 9/5C by doubling C and subtracting a tenth of the result than to multiple by 9 and divide by 5 in some order. The conversion isn’t as quick as “Double and add 30″*, perhaps, but unlike that estimation it has the advantage of being exact.

*a formula that always brings to mind the movie *Strange Brew*

*The thermometer is by Bernard Gagnon – Own work, CC BY-SA 3.0. It has Centigrade rather than Celsius at the top, which I found interesting since I remember learning both terms in school*.