Mentally Converting Fahrenheit to Celsius (better)

Many people know the trick to convert from F to C mentally:

(F-32) ÷ 2

However, the accurate formula being multiplying by 5/9 instead of 1/2 (or 5/10),

(F-32) × 5/9

we introduce a relative error of 10% (we arrive at 10% below our real figure).

To compensate, we could multiply by 10/9 (1.111…), but dividing by 9 isn’t so convenient. We can however appoximate this to 11/10 (which is 1.1, vs. 1.1111…).

So really all you have to do, is after your division by 2, add 10% of the result to itself. We then end up at 99% of our real figure, undershooting by 1% only. Nice improvement over our 10%-error estimation.

(F-32) ÷ 2 * (1 + 1/10)

So if we have 72°F:

72 - 32         = 40
40 ÷ 2          = 20
10% × 20        = 2
20 + (10% × 20) = 22

About 22°C, the real figure being 22.2°C (much better than our initial 20°C estimation!)