[marsten]

I've always found it amusing that mathematically it should be the rule of 70, but it's commonly rounded to 72 because the latter has more convenient divisors.

70 is divisible by 1, 2, 5, 7, 10, 14, 35, 70

72 is divisible by 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, 72

[MarkusQ]

It's also because in the interest rate range we're normally dealing with (5%-10% APR) the approximation of 1+x/100 as 1.01^x in the derivation of the rule is off a bit, and something like 1.0097^x works somewhat better. ln(2)/ln(1.0097) is about 71.8, thus the use of 72 and it's convenient divisors.

[trod1234]

Interesting, thanks for sharing this.

[Balgair]

Oh gosh, 72 is way more anti-prime, I've always just used the rule of 70 in my head though.

if I have a calculator handy, I'll use [(69.3/rate) + 0.3] as that's really close to the actual numbers for rates under about 10 or so.

I'm curious as to what other methods people use.

Honestly, I should probably just memorize the 12 or so numbers.

For those interested, here's a mnemonic that I cooked up quick with chatGPT:

70 bears ate 35 lions who ate 23 tigers who ate 18 wolves who ate 14 dogs who ate 12 cats who quickly ate 10, 9, 8, 7 mice who ate 6.6 grasshoppers who ate 6.1 barleycorns

Look, I know this is not the best, but hey, it's Friday. Please help me out and make up a better one.

70, 35, 23, 18, 14, 12, 10, 9, 8, 7, 6.6, 6.1