DollarWise

Free Rule of 72 Calculator — Doubling Time for Investments

November 3, 2026 · 3 min read · By the DollarWise team

How it works

Enter either an annual rate of return to see how long your money takes to double, or a target number of years to see what return you would need. The calculator shows the quick Rule of 72 estimate and the mathematically exact doubling time, plus a comparison table of common returns and their doubling periods.

Rule of 72 Calculator

How fast does your money double? Find out in seconds.

100% Free Forever No Sign-up No Credit Card Private — runs in your browser Unlimited Use

1 Pick a direction

At 8%, money roughly doubles every 9 years (72 ÷ 8).

2 How the rule works

Rule of 72: divide 72 by the annual % return to get the approximate years to double. It is a mental shortcut; the exact formula uses logarithms and is shown alongside it.

3 See the doubling time

📈 Doubling time

🎉 Free forever. No sign-up. No credit card. No limits.

Test as many rates and time horizons as you like — everything runs in your browser.

## How the math works The Rule of 72 is a shortcut: **years to double ≈ 72 ÷ annual % return**. It works because the exact doubling time for annually compounded money is **ln(2) ÷ ln(1 + r)**, and 72 is close to 100 × ln(2) ≈ 69.3 while staying easily divisible by common rates (6, 8, 9, 12). In reverse, the required rate to double in y years is approximately **72 ÷ y**, with the exact value **(2^(1/y) − 1) × 100%**. The calculator shows both the quick estimate and the exact figure. ## Everyday uses - **Goal setting:** decide what return you actually need to hit a target, instead of guessing. - **Inflation reality check:** divide 72 by the inflation rate to see how fast purchasing power halves (at 4% inflation, money halves in ~18 years). - **Fee impact:** compare a 1% fee difference — over decades it can cost you a full doubling of your balance. ## Frequently asked questions ### Why 72 and not 69.3 (the mathematically exact number)? Because 72 divides cleanly by the most common returns (2, 3, 4, 6, 8, 9, 12), making mental math easy. The error versus the exact formula is small at typical rates (under ~10%), which is why the rule has survived for centuries. ### Does the Rule of 72 work for fees and inflation too? Yes. Anything growing or shrinking at a steady annual percentage rate obeys the same math: divide 72 by the rate to get the halving or doubling period. For inflation it tells you how fast your cash loses half its purchasing power. ### Is the result before or after inflation and taxes? The calculator uses the nominal annual return you enter. To find the inflation-adjusted (real) doubling time, subtract the inflation rate from your return first. Taxes will also reduce your effective return in taxable accounts.