Rule of 72: time for an investment to double is roughly 72 / the interest rate [0]. Annual return on small-cap stocks: ~12% Time to double: 72/12 ~= 6 years Number of doubling periods: 99/6 ~= 16 Final investment value: ~2**16 ~= $65k ~= $64,417 Math checks out. 0: https://en.wikipedia.org/wiki/Rule_of_72
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
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