Sunday, January 10, 2016

LPT: How to Convert °C to °F in Your Head

These are quick math tricks to make certain day-to-day calculations much easier.  Sometimes you need to convert temperatures in Celsius (centigrade) into Fahrenheit.

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.

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