Ad placeholder — leaderboard
Rule of 72 Calculator
Estimate how long it takes to double your investment using the Rule of 72 — and compare it to the exact compound formula.
Ad placeholder — in-article
Years to Double (Rule of 72)
Exact Years to Double
Amount After Doubling
Calculated in your browser. We never see your numbers.
How to Use This Calculator
Enter your expected annual rate of return and optionally a starting investment amount. Click Calculate to see how many years it takes for your money to double, both using the quick Rule of 72 estimate and the mathematically exact compound interest formula. If you enter a starting amount, the calculator also shows what your investment will be worth after doubling.
Rule of 72 Formula
The Rule of 72 formula is: Years to Double = 72 ÷ Annual Rate (%). For example, at 8% annual return: 72 ÷ 8 = 9 years. The exact formula uses logarithms: Years = ln(2) ÷ ln(1 + r), where r is the decimal rate. At 8%: ln(2) ÷ ln(1.08) ≈ 9.006 years. The two formulas agree remarkably well in the 6–10% range — making the Rule of 72 one of the most useful mental shortcuts in personal finance.
Quick Doubling Time Reference
Use this as a quick reference: 2% → 36 years. 3% → 24 years. 4% → 18 years. 6% → 12 years. 8% → 9 years. 9% → 8 years. 10% → 7.2 years. 12% → 6 years. 24% → 3 years. These benchmarks help you quickly assess and compare different investment options or understand the real cost of inflation and debt.
Ad placeholder — rectangle
Frequently Asked Questions
What is the Rule of 72?
The Rule of 72 is a simple mental math shortcut to estimate how long it takes for an investment to double in value at a fixed annual rate of return. You simply divide 72 by the annual interest rate. For example, at 6% annual return, your investment doubles in approximately 72 ÷ 6 = 12 years. It works equally well for understanding how quickly debt or inflation doubles costs.
How accurate is the Rule of 72?
The Rule of 72 is a close approximation, not an exact calculation. It is most accurate for interest rates between 6% and 10%. The exact doubling time uses the formula: ln(2) / ln(1 + r), where r is the decimal rate. For example, at 6% the rule gives 12 years while the exact answer is about 11.9 years — a difference of less than 1%. At higher rates (20%+) or lower rates (1–2%), the approximation diverges more noticeably.
Can you give examples of the Rule of 72?
Here are common examples: At 4% (high-yield savings), money doubles in 18 years. At 7% (historical S&P 500 inflation-adjusted), money doubles in about 10 years. At 10% (S&P 500 nominal average), money doubles in about 7.2 years. At 3% (inflation rate), prices double every 24 years. At 2% (typical CD rate), savings double in 36 years. These quick estimates help compare investment options without a calculator.
What are the limitations of the Rule of 72?
The Rule of 72 assumes a constant annual rate of return compounded annually, which rarely happens in real investing. It does not account for taxes, fees, or variable returns. It is less accurate for very high rates (above 20%) or very low rates (below 2%). It also does not factor in regular contributions — it only models a lump sum investment. For precise planning, use a compound interest calculator with your specific inputs.
When should I use the exact formula instead of the Rule of 72?
Use the exact formula (ln(2) / ln(1 + r)) when precision matters — such as when comparing investment options with close return rates, or when rates are outside the 6–10% sweet spot. This calculator shows both: the Rule of 72 estimate for quick mental math and the exact compound formula for accuracy. For everyday financial discussions and quick comparisons, the Rule of 72 is perfectly adequate.
Ad placeholder — leaderboard