Rule of 72 calculator: find how long it takes to double your money at a set annual return, or the rate needed to double it by a chosen year. Formula included.
Quick answer
- What it calculates: how long it takes an investment to double at a fixed annual return, or the annual return needed to double by a target year.
- Formula: years to double ≈ 72 ÷ annual rate (%), or rate ≈ 72 ÷ years.
- Best for: fast mental-math estimates on savings, investments, and growth rates — the calculator above also gives the exact compound-interest figure.
What Is the Rule of 72?
The Rule of 72 is a shortcut for estimating how long it takes money to double at a fixed annual rate of return, without working through the full compound-interest formula. Divide 72 by the annual interest rate to get the approximate number of years to double, or divide 72 by a target number of years to find the annual return needed to double your money.
Rule of 72 Formula and Example
The formula is years to double ≈ 72 ÷ r, where r is the annual interest rate as a whole number (use 8 for 8%, not 0.08). For example, at an 8% annual return, an investment doubles in about 72 ÷ 8 = 9 years. To solve for the rate instead, use r ≈ 72 ÷ years: doubling money in 6 years needs an annual return of about 72 ÷ 6 = 12%.
How Long to Double Your Money at Common Interest Rates
| Annual return | Rule of 72 estimate | Exact compound doubling time |
|---|---|---|
| 2% | 36.0 years | 35.0 years |
| 3% | 24.0 years | 23.4 years |
| 4% | 18.0 years | 17.7 years |
| 5% | 14.4 years | 14.2 years |
| 6% | 12.0 years | 11.9 years |
| 7% | 10.3 years | 10.2 years |
| 8% | 9.0 years | 9.0 years |
| 9% | 8.0 years | 8.0 years |
| 10% | 7.2 years | 7.3 years |
| 12% | 6.0 years | 6.1 years |
| 15% | 4.8 years | 5.0 years |
| 20% | 3.6 years | 3.8 years |
Use the calculator above for any rate or year count not shown here, including the exact compound-interest result.
Rule of 72 vs. Exact Compound Interest
The Rule of 72 is an approximation, not the exact compound-interest formula. It is most accurate for annual rates between about 6% and 10%, where the estimate is usually within a few weeks of the exact doubling time. Below 6% the rule slightly overstates the years, and above 10% it slightly understates them — for an exact figure at any rate, use the calculator above, which solves the precise compound-interest doubling time alongside the Rule of 72 estimate.
Rule of 72 vs. Rule of 69 and Rule of 70
Some references use Rule of 70 or Rule of 69.3 instead of 72. Rule of 70 fits slightly better at lower interest rates, and Rule of 69.3 is the mathematically exact constant for continuous compounding. 72 stays the most common version because it divides evenly by more whole numbers — 1, 2, 3, 4, 6, 8, 9, 12 — which makes it easier to use for quick mental math across common interest rates.
When to Use the Rule of 72
The Rule of 72 is useful any time you want a fast, no-calculator estimate involving compound growth, such as:
- Comparing how long money in a savings account or CD takes to double at different interest rates.
- Estimating doubling time for a stock market or retirement portfolio using a long-term average annual return.
- Working out what annual return is needed to double an investment by a specific age or goal date.
- Seeing how inflation cuts purchasing power in half over time — the same formula estimates how long prices take to double, or savings to lose half their real value, at a given inflation rate.
- Comparing debt growth on a credit card or loan, since the same doubling logic applies to any balance growing at a fixed annual rate.
For a full year-by-year breakdown of the same growth, or to project regular deposits over time, try the CAGR calculator or the investment calculator.
Rule of 72 Calculator FAQ
It gives a quick estimate of how many years it takes to double an investment at a fixed annual return. It can also work backwards to estimate the annual interest rate needed to double in a chosen number of years.
Years to double ≈ 72 ÷ annual rate, using the rate as a whole number (8 for 8%, not 0.08). To solve for the rate instead, use rate ≈ 72 ÷ years to double.
It is an approximation, not an exact compound-interest formula. It works best for rates that are not too far from common long-term returns, roughly 6% to 10%, but it is still very useful for fast mental checks outside that range.
At an 8% annual return, the Rule of 72 estimates about 9 years to double (72 ÷ 8 = 9), which is also very close to the exact compound-interest figure.
Rule of 69.3 is the mathematically exact constant for continuous compounding, and Rule of 70 fits slightly better at lower rates. Rule of 72 remains the most common version because it divides evenly by more whole numbers, which makes the mental math easier.
Yes. Any value growing at a roughly fixed annual rate can use the Rule of 72, including long-term stock market averages, real estate appreciation, and inflation eroding purchasing power — not just savings accounts or bonds.
Use it when you want a fast answer to questions like how long to double money, what annual return is needed to double, or how different rates compare over time.
No. The Rule of 72 is a shortcut built on compound growth. It gives a practical estimate, while exact doubling time comes from the full compound-interest calculation.
