Convert Fahrenheit and Celsius: The Easy Way

thoughtbot’s a global company; I’m primarily based in the United States.

So when I chat with teammates about the weather, or I plan events and consider…the weather, I frequently need to convert temperatures between Fahrenheit and Celsius.

I’m tired of relying on conversion calculators, and the oft-circulated conversion formula is tricky to memorize, let alone to use with mental math.

Traditional formula

Convert Fahrenheit to Celsius: ºC = (ºF - 32)(0.5556)
Convert Celsius to Fahrenheit: ºF = (ºC * 1.8) + 32

I thought I might write a /math bot for Slack, but then I’d still be counting on an external device, and I’m curious if so many decimals are even necessary.

Luckily, there’s a much easier way.

Super-easy formula

  • Fahrenheit to Celsius, subtract 30 then divide by 2.
  • Celsius to Fahrenheit, multiply by 2 then add 30.
Convert Fahrenheit to Celsius: ºC = (ºF - 30) / 2 
Convert Celsius to Fahrenheit: ºF = (ºC * 2) + 30

When facing a problem, it’s important to define success.

Although results with this formula are not as accurate as other methods, it is easy to remember, quick and convenient to calculate, and gives me the answers I need within an acceptable margin of error. If, like me, all you ever want to know is if you’ll need a jacket or will burst into flames, this easy conversion formula fits the bill.

Thank you to fellow thoughtbotter, Joël Quenneville, proud Canadian and deft navigator of metric and imperial units of measurement, for introducing me to this formula.