Why is 70 used to calculate doubling time?
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Because, roughly, 1.01^70 = 2. So if you have 1 percent increase per year, it takes you 70 years to double. Everything else is a slightly farther estimate, but it works well enough
Usually you hear about the ‘rule of 72’. At x percent, how long does it take to double — divide 72 by x. Convenient, since 72 is divisible by 2, 3, 4, 6, 8, 9, 12,… Not exactly correct, but close enough for most purposes.
https://en.wikipedia.org/wiki/Rule_of_72
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