The Rule of 72: How Long to Double Your Money

Last updated: June 2025 · 7 min read

The Rule of 72 is a beautifully simple mental shortcut that tells you approximately how long it takes to double your money at a given interest rate. Divide 72 by the annual interest rate and you get the number of years to double. No calculator needed — just basic division you can do in your head.

This rule has been used by investors, financial planners, and economics students for centuries. It's attributed to the Italian mathematician Luca Pacioli, who described it in 1494. Despite its age, it remains one of the most practical tools in personal finance because it makes the abstract concept of compound interest instantly tangible.

How the Rule Works

At 6% interest: 72 ÷ 6 = 12 years to double your money. At 4%: 72 ÷ 4 = 18 years. At 10%: 72 ÷ 10 = 7.2 years. The higher the rate, the faster your money doubles. Simple as that.

You can also flip the formula: if you want to double your money in a specific number of years, divide 72 by that number to find the required rate. Need to double in 8 years? You need roughly 72 ÷ 8 = 9% annual returns.

How Accurate Is It?

The Rule of 72 is an approximation, but a remarkably good one. Let's compare it against exact calculations:

The rule is most accurate around 6-10% — conveniently the range where most investment returns fall. At very low or very high rates, the approximation drifts slightly. For rates above 20%, the Rule of 69.3 is more precise, though 72 remains popular because it's divisible by more numbers (2, 3, 4, 6, 8, 9, 12).

Practical Applications

Evaluating savings accounts: Your easy-access savings account pays 4.5%. How long to double? 72 ÷ 4.5 = 16 years. Is that fast enough for your goals? If not, you might consider a fixed-rate bond or investment account.

Retirement planning: If you're 30 with £50,000 in a pension growing at 6%, the Rule of 72 tells you it doubles every 12 years. By 42 it's £100,000, by 54 it's £200,000, by 66 it's £400,000. Three doublings turn £50,000 into £400,000. This is why starting a pension early matters so much — see our retirement planning guide for more.

Understanding inflation: The rule works for inflation too. At 3% inflation, the purchasing power of your money halves every 72 ÷ 3 = 24 years. That £100 in your pocket today will only buy £50 worth of goods in 2049. This is why leaving cash under the mattress is a guaranteed way to lose wealth.

Comparing investments: Fund A returns 5%, Fund B returns 8%. With the Rule of 72: Fund A doubles in 14.4 years, Fund B in 9 years. Over 36 years, Fund A doubles twice and a half (roughly 4.5× growth), while Fund B doubles four times (16× growth). A small difference in rate creates a massive difference in outcome — and you figured that out with mental arithmetic.

Limitations and Edge Cases

The Rule of 72 assumes a constant interest rate over the entire period. Real-world returns fluctuate year to year, especially for equity investments. It also doesn't account for taxes, fees, or additional contributions. For precise projections, use our compound interest calculator which handles all these variables.

The rule applies to compound interest, not simple interest. With simple interest at 6%, doubling takes 16.7 years (100 ÷ 6), not 12. The difference highlights why compounding is so powerful.

Frequently Asked Questions

This guide is for informational purposes only and does not constitute professional financial advice.