Investment Doubling Time Calculator
How it works
Doubling is just the compound growth relationship (1 + rate) ** years = multiple with the multiple fixed at 2. "Years to find a rate" solves it for the exponent using a logarithm: years = ln(2) / ln(1 + rate). "Rate to find a target" solves the same equation the other way, for the base: rate = 2^(1/years) − 1. They are two rearrangements of one identity, so running one answer back through the other returns the number you started with.
Alongside the exact years, the calculator shows the Rule of 72 estimate — 72 divided by the rate as a whole number — the shortcut most people learn instead of the logarithm. It is closest to the exact answer at ordinary rates and drifts further away as the rate climbs, since a fixed constant (72) is standing in for a curve that is not actually a straight line.
Frequently asked questions
Why 72, and not the exact number?
The exact figure needs a logarithm, which is not something most people can do without a calculator. 72 is chosen because it divides evenly by so many common rates (2, 3, 4, 6, 8, 9, 12) that dividing it by the rate is quick mental math, and it happens to sit close to the true curve across the rates people actually earn.
Does this work for a fixed rate only?
Yes. Both figures here assume the same rate applies every single year. Real investment returns vary year to year — some years up, some down — so this is the years or rate a constant return would need, not a forecast of a variable one.
What if my rate is zero or negative?
Then it never doubles, so both calculations reject it rather than returning a number that looks like an answer. A rate of exactly zero would take forever; a negative rate shrinks the value further away from double with every year.