Rule of 72 Calculator

How to use Rule of 72 Calculator

1. Enter the annual rate of return.
2. Read the estimated doubling time.
3. Or enter a target number of years to see the rate required.
4. Most accurate near 8%; drifts above about 20%.
5. Assumes a constant annual rate — real returns vary, sometimes a great deal.
6. Works identically on debt, which is where it is most sobering.

Why 72, and how accurate it is

The exact doubling time is ln(2)/ln(1+r), and ln(2) is about 0.693 — so 69.3 would be the mathematically natural numerator. 72 is used instead because it divides cleanly by 2, 3, 4, 6, 8, 9 and 12, which is what makes the rule doable in your head. It is most accurate around 8%: at 8% the rule says 9 years against an exact 9.01. At 2% it overestimates slightly, and above about 20% it drifts noticeably.

Use it in both directions

The reverse is equally useful: 72 divided by the years you have gives the return you would need. To double money in six years requires about 12% a year, which immediately tells you whether a plan is plausible. Applied to inflation it tells you how fast prices double — at 6%, in twelve years — which is the fastest way to show why a long-term nominal figure is misleading.

It also works on debt, unpleasantly

Compound interest works identically against you. A credit card at 36% doubles what you owe in two years if nothing is paid. A personal loan at 18% doubles in four. Applying the rule to debt rather than investments is the clearest way to see why high-interest borrowing should be cleared before almost any investment is started — the guaranteed cost is higher than any reliable return.