The Rule of 72: How Long Does It Take to Double Your Money?

The Rule of 72 is the fastest piece of financial maths you’ll ever learn. To estimate how many years it takes an investment to double at a fixed annual rate of return:

Years to double ≈ 72 ÷ interest rate (%)

At 8%, money doubles in about 72 ÷ 8 = 9 years. At 6%, about 12 years.

How accurate is it?

Remarkably accurate for everyday rates. Here is the rule compared with the exact answer for annual compounding:

Between about 6% and 10% the rule is within a few weeks of the exact figure. It gets less precise at very low or very high rates. Variants such as the Rule of 69.3 (for continuous compounding) or Rule of 70 are sometimes used for those cases.

Why 72?

The exact doubling time is ln(2) ÷ ln(1 + r). For small rates, ln(1 + r) is close to r, and ln(2) is about 0.693, which gives a “Rule of 69.3”. Seventy-two is used instead because it is slightly more accurate at common interest rates, and because it divides neatly by 2, 3, 4, 6, 8, 9 and 12, which makes mental maths easy.

Five practical uses

1. Compare investment returns

At 4%, money doubles every 18 years. At 9%, every 8 years. Over a 36-year career, that’s two doublings (4×) versus four and a half doublings (about 22×).

2. Understand the cost of fees

A fund returning 7% doubles in about 10.3 years. Take away a 1% annual fee and the net 6% doubles in 12 years. Over a lifetime that fee costs you roughly one full doubling.

3. See what inflation does

At 3% inflation, prices double in about 24 years. A salary that doesn’t rise will buy half as much in a generation.

4. Feel the weight of debt

Credit card debt at 24% doubles in just 3 years if left unpaid. That’s a strong argument for clearing it first; see How to Pay Off a Loan Faster: 7 Strategies That Save Interest.

5. Work backwards

Want to double your money in 6 years? You need about 72 ÷ 6 = 12% a year, a useful reality check for any investment that promises it.

Limits of the rule

• It assumes a constant rate. Real returns fluctuate.
• It ignores contributions and withdrawals.
• It’s an estimate. For exact figures with regular deposits, use the compound interest calculator.

Try it

Set a $10,000 starting balance, no monthly contribution, and a rate of 8% compounded annually in the calculator. Look at the year-by-year table: the balance crosses $20,000 in year 10, just after the 9.01 years the exact formula predicts, and close to the Rule of 72’s estimate of 9.

To see why compounding produces these doublings in the first place, read Compound vs Simple Interest: The Difference, With Examples.