A shortcut that has survived for a reason
Before calculators were in every pocket, the Rule of 72 let investors estimate doubling time with simple division. It still holds up remarkably well for everyday rates — good enough to compare investments or sanity-check a projection at a glance, without needing to work through logarithms.
When to reach for the exact number
For rates far from the 6-10% range the approximation drifts a bit more, which is why this tool shows both. For a full projection with contributions rather than a single doubling point, see the Compound Interest Calculator, or the CAGR Calculator to find a rate from real start and end values.
Frequently asked questions
What is the Rule of 72?
A quick mental-math shortcut for estimating how long it takes an investment to double at a given annual compound rate: divide 72 by the interest rate. At 8% a year, money doubles in roughly 72 ÷ 8 = 9 years.
How accurate is the Rule of 72?
It is a close approximation for typical rates (around 6-10%), usually within a few months of the exact answer. It gets less accurate at very low or very high rates, which is why this calculator also shows the precise, exact figure alongside it.
What is the exact formula?
Years to double = ln(2) ÷ ln(1 + rate). The Rule of 72 approximates this without needing logarithms — handy for quick mental estimates, even though the exact formula is what this calculator actually plots against it.
What can I use this for?
A fast sanity check on any investment, savings account or debt with a compounding rate — "how long until this doubles (or, for debt, how long until it doubles against me)?" It works the same way whether you're thinking about money growing or a balance compounding upward.
Last updated: June 2026