The Rule of 72: How to Calculate When Any Investment Doubles
The Rule of 72 is the most useful mental math shortcut in personal finance. It estimates how long any investment takes to double in value — without a calculator. The formula takes three seconds. The insight it produces can change how you think about every financial decision you make.
The Rule of 72 Formula
Doubling time (years) = 72 / Annual interest rate
Example: At 6% annual return, your money doubles in 72 / 6 = 12 years. At 9%, it doubles in 72 / 9 = 8 years. At 12%, in 72 / 12 = 6 years.
The rule works in reverse too: if you want to double your money in 10 years, you need 72 / 10 = 7.2% annual return.
Rule of 72 for Common US Rates (2026)
Here is how the Rule of 72 applies to current financial products:
- High-yield savings account at 4.5% APY: doubles in 16 years
- US Treasury bonds (10-year) at 4.4%: doubles in 16.4 years
- S&P 500 index fund historical average at 10%: doubles in 7.2 years
- Credit card debt at 21.47% APR: your debt doubles in 3.4 years
- Personal loan at 11.14% APR: debt doubles in 6.5 years
- Auto loan at 7% APR: debt doubles in 10.3 years
The last three are the most important. The Rule of 72 applied to debt shows how fast an unpaid balance grows if you only make minimum payments. Credit card debt at 21.47% doubles in 3.4 years — meaning a $5,000 balance becomes $10,000 in just over three years of minimum-only payments.
Why the Rule of 72 Works
The rule is an approximation based on the natural logarithm of 2 (ln 2 ≈ 0.693), which is the mathematical constant underlying compound doubling. Dividing 69.3 by the rate gives the exact answer; using 72 instead introduces a small error (typically 1-3%) but produces round, easy-to-compute numbers. For rates between 5% and 15%, the error is negligible for planning purposes.
Rule of 72 Examples — Investments vs Debt
The cost of credit card debt
$6,000 credit card balance at 20% APR. Rule of 72: 72 / 20 = 3.6 years to double. In 7.2 years, the balance reaches $24,000 if you make no payments. Minimum payments slow but rarely stop this growth. The Rule of 72 makes the urgency of high-interest debt viscerally clear in a way that interest rate percentages alone do not.
The power of starting early
$10,000 invested at 10% annual return. Doubles in 7.2 years. After 36 years, the original $10,000 has doubled five times: $10k → $20k → $40k → $80k → $160k → $320k. The same $10,000 invested 10 years later would only double four times, ending at $160k — half as much despite the same rate and a 10-year difference in start time.
Limitations of the Rule of 72
- It applies to constant annual rates — investments with variable returns (stocks, real estate) use the average rate, which introduces uncertainty
- It assumes continuous compounding — for annual compounding, the rule slightly overestimates doubling time
- It does not account for taxes or inflation — after-tax, inflation-adjusted doubling time is longer than the rule suggests
- It is an approximation — for rates above 20%, use the exact formula: ln(2) / ln(1 + r) for precise results
Use the free compound interest calculator to verify your Rule of 72 estimates and see the year-by-year growth curve for any investment scenario. For tracking investment performance, use the ROI investment calculator to get both simple and annualized return figures.
Frequently Asked Questions
Q: How accurate is the Rule of 72?
The Rule of 72 is accurate to within 1-3% for interest rates between 5% and 15%. At 6%, the exact doubling time is 11.9 years; the rule gives 12 years. At 10%, the exact time is 7.27 years; the rule gives 7.2 years. For planning purposes, this level of accuracy is perfectly adequate.
Q: What is the Rule of 114 and Rule of 144?
The Rule of 114 estimates when money triples (114 / rate = years to triple). The Rule of 144 estimates when money quadruples (144 / rate = years to quadruple). At 8% return: doubles in 9 years (Rule of 72), triples in 14.25 years (Rule of 114), quadruples in 18 years (Rule of 144).
Q: Can I use the Rule of 72 for inflation?
Yes. The Rule of 72 applies to any growth rate including inflation. At the Fed’s 2% target inflation rate, the purchasing power of cash halves in 36 years. At the 2022 peak inflation rate of 9.1%, purchasing power halved in approximately 8 years. This is why holding large amounts of cash without interest for extended periods erodes wealth even without apparent risk.