Enter an interest rate to find how long it takes to double your money — or enter a time goal to find the required rate.
| Annual rate | Rule of 72 (years) | Exact years | $10,000 becomes | Real-world example |
|---|---|---|---|---|
| 2% | 36.0 yrs | 35.0 yrs | $20,000 | High-yield savings (low rate env) |
| 3% | 24.0 yrs | 23.4 yrs | $20,000 | I-bonds / short-term Treasuries |
| 4% | 18.0 yrs | 17.7 yrs | $20,000 | Conservative bond portfolio |
| 6% | 12.0 yrs | 11.9 yrs | $20,000 | Balanced 60/40 portfolio (approx) |
| 7% | 10.3 yrs | 10.2 yrs | $20,000 | S&P 500 inflation-adjusted avg (approx) |
| 8% | 9.0 yrs | 9.0 yrs | $20,000 | Equity portfolio (nominal, approx) |
| 10% | 7.2 yrs | 7.3 yrs | $20,000 | S&P 500 nominal long-term avg (approx) |
| 12% | 6.0 yrs | 6.1 yrs | $20,000 | Aggressive growth target |
| 18% | 4.0 yrs | 4.2 yrs | $20,000 | Credit card debt rate (avoid) |
| 24% | 3.0 yrs | 3.2 yrs | $20,000 | High-interest debt (pay first) |
How the Rule of 72 works: Divide 72 by your annual interest rate to estimate how many years it takes to double your money. It's an approximation — the exact formula is ln(2) ÷ ln(1 + r) — but the Rule of 72 is accurate to within 1–2% for rates between 3% and 12%, making it a useful mental math shortcut. The rule also works in reverse: if you want to double your money in 9 years, you need a return rate of approximately 72 ÷ 9 = 8%.
The debt insight: The Rule of 72 applies to debt too — at 18% credit card interest, your balance doubles in 4 years if unpaid. This is why paying off high-interest debt is often the highest guaranteed "return" available.
The Rule of 72 is the fastest way to estimate compound growth in your head. Divide 72 by the annual interest rate and you get the approximate number of years to double your money. At 6%, money doubles in 12 years. At 8%, in 9 years. At 12%, in 6 years. The simplicity makes it one of the most useful financial shortcuts in existence — and it works for debt as well as investments.
Use the compound interest calculator to model specific investment scenarios with contributions over time, or the inflation-adjusted returns calculator to see real purchasing power growth.
Each doubling doesn't just add the same amount — it compounds. $10,000 doubling four times doesn't become $40,000. It becomes $160,000. This is the core of compound growth that the Rule of 72 makes tangible.
The Rule of 72 is a mental math approximation, not an exact formula. The precise calculation uses natural logarithms: years = ln(2) ÷ ln(1 + r). At a 7% rate, the exact answer is 10.24 years; Rule of 72 gives 10.29 — a difference of 0.05 years, or about 18 days. That level of precision is more than sufficient for financial planning.
The rule is most accurate between 6% and 10% — the range that covers most realistic investment returns. Below 3% or above 15%, the approximation drifts more meaningfully. A more precise version uses 69.3 (the natural log of 2 × 100) instead of 72, but 72 is preferred because it divides evenly into more integers, making mental math easier.
The rule works in both directions. At 18% credit card interest, an unpaid balance doubles in 4 years. At 24%, in 3 years. This is why high-interest debt is often described as "wealth destruction" — the same compounding that builds investment wealth works against you when you're the borrower. Paying off a 20% credit card balance is mathematically equivalent to earning a guaranteed 20% return — a rate no investment reliably delivers.
The mathematically precise version is the Rule of 69.3, because ln(2) × 100 ≈ 69.3. However, 69.3 doesn't divide cleanly into many common numbers, making mental arithmetic harder. The Rule of 72 sacrifices a tiny amount of precision for the practical benefit of easier division — 72 is divisible by 2, 3, 4, 6, 8, 9, and 12, covering most common interest rates. For quick mental math, 72 is the better tool. For exact calculations, use the compound interest calculator.
No — the Rule of 72 works on nominal rates, not real (inflation-adjusted) rates. If your investment returns 8% but inflation is 3%, your real return is approximately 5% — and your purchasing power doubles in roughly 14 years (72 ÷ 5), not the 9 years the nominal rate suggests. For real purchasing power doubling, subtract the inflation rate from your return rate first. Use the inflation-adjusted returns calculator to model real returns.
The Rule of 72 assumes annual compounding. For more frequent compounding (monthly, daily), the time to double is slightly shorter because interest compounds on interest more often. For most long-term investment planning, the difference is small enough that the annual approximation is useful. Savings accounts and CDs that compound monthly will double slightly faster than the Rule of 72 suggests — typically by a few months over a 10-year period.
Common planning assumptions: 6–7% for inflation-adjusted long-term equity returns, 9–10% for nominal equity returns, 3–4% for balanced portfolios, 1–2% for bonds or cash in lower-rate environments. These are historical approximations — actual future returns are uncertain. Financial planners often use 6–7% as a conservative real-return assumption for retirement modeling. Verify which rate is appropriate for your specific asset mix and time horizon with a qualified financial advisor.