What the Rule of 72 tells you
The Rule of 72 is the single most useful piece of mental maths in investing. Take your expected annual return, divide 72 by it, and you get the approximate number of years for your money to double. At 6% that is 12 years; at 9%, just 8. It turns an abstract percentage into something you can actually feel.
What makes it powerful is what it reveals about small differences in return. Earning 9% instead of 6% doesn’t sound dramatic — but it cuts the doubling time from 12 years to 8. Over a lifetime, that gap means several extra doublings, which is the difference between a comfortable outcome and a life-changing one. This is exactly why fees and a couple of percentage points of return matter so much.
The rule works both ways. At 3% inflation, the purchasing power of idle cash halves in about 24 years. On high-interest debt, the balance can double in just a few. Compounding is relentless — the Rule of 72 just makes its speed visible. To see the full picture with contributions and reinvested dividends, use the compound interest calculator.
Frequently asked questions
What is the Rule of 72?
The Rule of 72 is a quick mental shortcut for estimating how long an investment takes to double. You divide 72 by the annual rate of return, and the result is the approximate number of years to double your money. At 8% a year, for example, money doubles in about 9 years (72 ÷ 8).
Why does the Rule of 72 work?
It is a simplification of the compound-growth formula. Doubling means growing by 100%, which mathematically takes ln(2) ÷ ln(1+r) years. For the range of returns investors usually see (roughly 4–12%), the number 72 approximates that relationship closely — and 72 is easy to divide by many numbers, which is why it stuck.
How accurate is the Rule of 72?
Very accurate for typical returns. It is most precise around 8%. For lower rates, 69.3 or 70 is slightly more exact; for higher rates, 72 or even 73 fits better. For everyday estimation, 72 is more than good enough — usually within a few months of the true answer.
Can I use the Rule of 72 for inflation or debt?
Yes — and it is sobering. At 3% inflation, prices double in about 24 years (72 ÷ 3), so cash quietly loses half its purchasing power. On a credit card charging 20%, a balance can double in under 4 years if unpaid. The rule works for anything that compounds, for you or against you.