Rule of 72 Calculator
A quick mental-math approximation for how long it takes money to double at a given annual rate of return.
This is an approximation, not an exact calculation. It's a rule of thumb for quick estimates, not a projection.
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Approximate doubling time (72 ÷ rate)
10.3 years
Exact doubling time
10.2 years
Formula: Years ≈ 72 ÷ rate. It's a widely used approximation because it's easy to compute without a calculator — the exact answer (ln(2) ÷ ln(1+rate)) is very close for typical rates but diverges more at very high or very low rates.
Sources & further reading