Rule of 72 Calculator — How Long to Double Your Money

The Rule of 72 is a simple way to estimate how many years it takes to double your capital at a given annual rate. Divide 72 by the rate: at 8% it is 72 / 8 = 9 years. It also works in reverse — 72 divided by the number of years gives the required annual rate of return.

It approximates compound interest. The exact doubling time is ln(2) / ln(1 + rate). For rates of 6–10% the error is small and the rule works best around 8%. The deviation grows for very low and very high rates.

The Rule of 114 estimates tripling time (114 / rate) and the Rule of 144 estimates quadrupling time (144 / rate). At 6% capital triples in about 114 / 6 = 19 years. The calculator shows doubling and tripling side by side.

You can use it to estimate how fast inflation halves your purchasing power. At 6% inflation, the value of money halves in about 12 years. This shows why holding cash with no interest has a real cost over time.

The Rule of 72 is shorthand for compound interest math. Doubling happens faster as interest earns interest. The higher the rate, the shorter the doubling — even a 5% vs 7% difference meaningfully shortens the time to multiply capital.

At very high rates (above about 20%) it overstates doubling time versus the logarithmic formula. Near zero it is also less precise. In those extremes, compute the exact time from ln(2) / ln(1 + rate).

The number 72 divides evenly by many common rates (2, 3, 4, 6, 8, 9, 12), making mental math easy. The mathematically closest value is about 69.3, but 72 gives a very good approximation across typical investment rates.

Yes, if you use the rate and period in the same unit. Plugging in a monthly rate gives the number of months to double. This calculator assumes an annual rate, so the result is in years. Keep your units consistent.

No. It works on the raw rate of return. Capital gains tax (in Poland 19% — the "Belka tax"), fund fees and commissions lower the effective rate, so the real doubling time is longer. Use an after-tax, after-cost rate for realism.

No. It provides indicative, educational approximations based on a constant annual rate. Actual returns change and are not guaranteed, and investing carries risk. Make decisions after analyzing your situation or consulting a licensed advisor.