The Rule of 72: How Long Until Your Money Doubles?

The Rule of 72 is one of the handiest shortcuts in all of personal finance — a quick way to estimate how many years it takes for your money to double, using nothing but mental math.

How it works

Divide 72 by your annual interest rate (as a whole number), and you get the approximate number of years for your money to double:

Years to double ≈ 72 ÷ interest rate

At an 8% return, your money doubles in about 72 ÷ 8 = 9 years. At 6%, it takes 12 years. At 4%, a full 18. You can watch this play out precisely on the compound interest calculator.

Why it's so useful

The Rule of 72 makes the power of compounding tangible. It instantly shows why a couple of extra percentage points of return — or a couple of percent of fees — matter enormously over a lifetime. A fund charging 2% in fees isn't taking 2% of your money; it's quietly stealing one of your doublings.

It works for inflation too

Flip it around and the rule shows how fast prices double — or how fast cash loses half its value. At 3% inflation, prices double in about 24 years. That's why money sitting in a low-interest account quietly shrinks in real terms; see the inflation calculator.

The limits

The Rule of 72 is an approximation. It's most accurate for rates between about 5% and 10%; at very high or very low rates it drifts a little. It also assumes a steady return, while real markets bounce around year to year. For planning, it's a brilliant back-of-the-envelope tool — just don't treat it as a guarantee.

See it in action

Frequently asked questions