If you’re wanting to convert Fahrenheit to Celsius, the real formula looks like this:
(°F – 32) / 1.8 = °C
Or going from Celsius to Fahrenheit:
(°C * 1.8) + 32 = °F
That’s accurate, but hard to do in your head. Here are two quick ways to do it instead; just memorize a few numbers (from whichever method you prefer) and you’re all set.
30 and half
This is the method I generally use. It takes the formula from above, but simplifies it a lot. It’s not precise, but good enough for normal use.
If you have a number in Fahrenheit, just subtract 30 and then divide it in half. For example if it’s 70° Fahrenheit right now, subtract 30 (so you have 40) and divide by 2 = 20°. The real number is 21.1, so you’re certainly close enough.
To go the other way, double it and then add 30. If someone said it was a cool 10° Celsius right now, just double it (so you have 20) and then add 30 = 50°. In this case, that 50° is exactly accurate.
Remember four numbers
A new way I saw to do this is to just remember four numbers: 04 16 28 40
In most any case, just take any of those as Celsius and reverse the numbers for Fahrenheit.
- 04° C = 40° F
- 16° C = 61° F
- 28° C = 82° F
- 40° C = 104° F
Those numbers are all nearly perfect, but they obviously leave some gaps. Still, in most cases you’ll have a number somewhere near one of those so you can estimate.
Really, the goal with any of these is just a quick estimate — we all have phones in our pockets that can give us the exact number in a matter of seconds. This is simply so if someone says “It’s 20° C outside”, you can know it’s a very pleasant day (20 x 2 = 40, then + 30 = 70) without having to look it up!