How to convert Fahrenheit to Celsius without complicated calculations

Do you know the exact temperature? Maybe not. But you will definitely know whether it is worth taking a jacket with you. Lifehacker

There are not many people in the world who understand metric units. They know that 20 kilograms is 44 pounds (double that and add 10%) and 5 kilometers is 3 miles. But the temperature in Celsius, even for them, remains a mystery.

Not only do Celsius and Fahrenheit have different sizes, they have different inputs. Zero degrees Fahrenheit is a very cold day in the dead of winter. Zero degrees Celsius means that a 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). But there is a better way. All you have to do is memorize the four numbers and flip them over to convert.

Here's the focus:

04 degrees Celsius is 40 degrees Fahrenheit (cold!)
16 degrees Celsius is 61 degrees Fahrenheit (bad weather)
28 degrees Celsius is 82 degrees Fahrenheit (nice!)
40 degrees Celsius is 104 degrees Fahrenheit (stay inside and turn on the air conditioning).

For these particular numbers, you just swap the digits: 16 becomes 61. The only weird case is 40. But now that you've read it, you get the idea. Four degrees Celsius is 40 degrees Fahrenheit, and 40 degrees Celsius is 104 degrees Fahrenheit.

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If you're told on vacation that it's going to be 32 degrees today, you know it's more than 28 (82 Fahrenheit). But luckily below 40 Celsius (104 Fahrenheit). This is a T-shirt and shorts according to the weather.

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

And a fun bonus fact, in case you've ever vacationed in the Arctic: -40 is -40 on both scales.