The Rule of 72: How to Estimate When Your Money Will Double

The Rule of 72 is the simplest formula in personal finance: divide 72 by your annual return rate to find how many years it takes for your money to double. At 8% return: 72 / 8 = 9 years to double. At 10%: 7.2 years. At 6%: 12 years. At 4%: 18 years.

Applied to real scenarios. $10,000 in an S&P 500 index fund at 10% average return: doubles to $20,000 in 7.2 years, $40,000 in 14.4 years, $80,000 in 21.6 years, $160,000 in 28.8 years, $320,000 in 36 years. Starting at age 25, that $10,000 becomes $320,000 by age 61 — through pure compounding with zero additional contributions.

The Rule of 72 also works in reverse — for understanding how fast inflation erodes your money. At 3% inflation: 72 / 3 = 24 years for your money to lose half its purchasing power. $100,000 in a checking account earning 0% is worth only $50,000 in real purchasing power after 24 years. This is why keeping large sums in non-interest-bearing accounts is a guaranteed loss.

For debt, the rule shows how fast debt doubles against you. Credit card debt at 24% APR: 72 / 24 = 3 years to double. A $5,000 balance left unpaid at 24% becomes $10,000 in 3 years, $20,000 in 6 years, $40,000 in 9 years. This is why credit card debt is a financial emergency.

Limitations: the rule is an approximation that works best for rates between 4% and 12%. At very low or very high rates it becomes less accurate. For exact calculations, use the [compound interest calculator](/calculators/compound-interest). For retirement projections, the [retirement gap calculator](/calculators/retirement-gap) applies compound growth to your specific savings rate. For more on compound interest, read our guide on [how $200/month becomes $500K](/blog/compound-interest-200-per-month).

Frequently Asked Questions:

Is the Rule of 72 exact? No, it is an approximation. The exact doubling time for 8% is 9.006 years versus the Rule of 72 estimate of 9 years. Close enough for planning.

Does it work for monthly compounding? The Rule of 72 assumes annual compounding. With monthly compounding, doubling is slightly faster — but the difference is minimal for estimation purposes.