How to Convert Celsius to Fahrenheit Without Doing Math

I consider myself reasonably well versed in metric units. I know that 20 kilograms is 44 pounds (double and add 10%) and 5 kilometers is 3 miles (I have run many 5 kilometers). But Celsius has always been a mystery to meā€”until now.

Not only are degrees Celsius and Fahrenheit different sizes (each degree Celsius covers most of the scale), they have different baselines. Zero degrees Fahrenheit is a very cold day in the dead of winter. Zero degrees Celsius means that you are right on the cusp of the moment when the predicted snow flurry could turn into a downpour or vice versa.

Because of these two factors, conversion is traditionally a two-step process: you need to multiply and add (or subtract and divide). This was still the case last time we looked at how to “fast” convert from one system to another – the speed in this example came from the suggestion to use 2 and 30 as the appropriate conversion factors instead of 1.8 and 32.

But there is a better way and all you have to do is memorize the four numbers and flip them over to convert. (Or memorize one of them and know that you can add or subtract 12 to get each of the others.)

Here’s the trick:

  • 04 degrees Celsius is 40 degrees Fahrenheit (cold!)
  • 16 degrees Celsius is 61 degrees Fahrenheit (hood weather)
  • 28 degrees Celsius is 82 degrees Fahrenheit (delicious!)
  • 40 degrees Celsius is 104 degrees Fahrenheit (Stay inside and turn on the A/C)

For these specific numbers, you just swap the numbers: 16 becomes 61, you know? The only odd thing is 40, but now that you’ve read this you get the idea. Four degrees Celsius is 40 Fahrenheit, and 40 Celsius is 104 Fahrenheit.

Now, if you’re on vacation and they tell you it’s going to be 32 today, you know it’s more than 28 (82 Fahrenheit) but thankfully less than 40 Celsius (104 Fahrenheit). Weather in a T-shirt and shorts.

On the other hand, if it’s 10 degrees, you know it will be colder than 61 Fahrenheit (10 less than 16) but warmer than 40 (10 more than 4). Take a jacket and you’ll be fine.

And a fun bonus fact in case you ever vacation in the Arctic: -40 equals -40 on both scales.

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