Rule of 72

The Rule of 72 estimates doubling time by dividing 72 by the annual return or rate.

Why investors care

It is the fastest mental math for compounding and debt.

Where people go wrong

It is an approximation, less precise at very high or low rates.

In the real market

A 25-year-old sees two options: a savings account paying 1%, where money doubles every 72 years — essentially never in a working life — and a diversified stock fund historically returning 8%, where money doubles every 9 years. Over 45 years the difference is the entire difference between retiring comfortably and retiring broke. The rule makes it visceral: a 1% return and an 8% return are not 7 percentage points apart; they are 63 years apart on the doubling clock.

Using it in practice

Use the rule to make the power of compounding concrete. It also exposes the trap of fees: a 1% higher cost on a long horizon can delay your doubling by years, which is why expense ratios matter so much.

Example in numbers

Key takeaway

The Rule of 72 is instant mental math for compounding: divide 72 by the annual rate to get doubling time. An 8% return doubles money every 9 years; a 1% return takes 72 — the difference between retiring comfortably and retiring broke. The rule works in reverse for debt, showing how fast unpaid balances double, and it exposes the trap of fees on a long horizon. It is an approximation, but for quick arithmetic it is unbeatable and unforgettable.

Definitions reviewed by the Investing Glossary editorial team.