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Rule of 72 Calculator

The Rule of 72 is a simple mental-math shortcut that estimates how many years it takes an investment to double at a fixed annual interest rate — or what rate is needed to double money in a given number of years.

Enter the annual percentage return (e.g., 7 for 7%).

Enter a value above to see your result.

Quick Reference

RateYears to Double
2%36.0 yrs
3%24.0 yrs
4%18.0 yrs
5%14.4 yrs
6%12.0 yrs
7%10.3 yrs
8%9.0 yrs
9%8.0 yrs
10%7.2 yrs
12%6.0 yrs

Quick answer

The Rule of 72 states that dividing 72 by an annual interest rate gives the approximate number of years needed to double an investment. For example, at 6% annual return, 72 ÷ 6 = 12 years to double. Conversely, to find the required rate to double in N years, divide 72 by N. The rule works because 72 is close to 100 × ln(2) ≈ 69.3, but 72 has more integer factors, making mental division easier.

Formula & method

Years to Double = 72 / Annual Interest Rate (%)
  • 72 — The constant approximation of 100 × ln(2)
  • Annual Interest Rate (%) — The fixed annual growth or interest rate as a percentage (e.g., 6 for 6%)

Divide 72 by the annual percentage rate to get the approximate doubling time in years.

Required Rate (%) = 72 / Years to Double
  • 72 — The constant used in the rule
  • Years to Double — The target number of years in which you want the investment to double

Divide 72 by the desired number of years to find the annual rate needed to double the investment.

Examples

Example 1: Doubling at 6% annual return
Input
Annual rate = 6%
Result
12 years to double
Why
72 ÷ 6 = 12. An investment growing at 6% per year will approximately double in 12 years. For instance, export const batch4Tools: ToolContent[] = 0,000 becomes roughly $20,000 after 12 years at 6% compounded annually.
Example 2: Doubling at 8% annual return
Input
Annual rate = 8%
Result
9 years to double
Why
72 ÷ 8 = 9. At an 8% annual return, a $5,000 investment would grow to approximately export const batch4Tools: ToolContent[] = 0,000 in 9 years. This is a common benchmark for stock market index fund returns.
Example 3: Required rate to double in 10 years
Input
Target years = 10
Result
7.2% annual rate required
Why
72 ÷ 10 = 7.2. To double an investment within 10 years, you need an average annual return of approximately 7.2%. This is close to the long-run historical real return of diversified equity portfolios.
Example 4: Doubling at 4% (e.g., high-yield savings)
Input
Annual rate = 4%
Result
18 years to double
Why
72 ÷ 4 = 18. At a 4% annual interest rate — typical of some high-yield savings accounts — it takes 18 years to double your money. This illustrates why even modest rate differences matter over long horizons.

Frequently asked questions

How accurate is the Rule of 72?

The Rule of 72 is most accurate for interest rates between 6% and 10%. At rates below 3% or above 25%, the error grows. For example, at 2% the exact doubling time is about 35 years while the rule gives 36 — close but slightly off. For very high or very low rates, use the exact formula: ln(2) / ln(1 + r).

Can the Rule of 72 work for inflation?

Yes. You can apply the same rule to inflation to see how long it takes for prices to double. At 3% inflation, 72 ÷ 3 = 24 years for the price level to double — meaning your purchasing power halves in 24 years if your savings earn nothing.

Why 72 and not 69.3 (which is more mathematically precise)?

The natural log of 2 is approximately 0.693, so the exact constant is 69.3. However, 72 is preferred because it divides evenly by more integers (1, 2, 3, 4, 6, 8, 9, 12, 24, 36) making mental arithmetic much easier, and the error introduced is negligible for everyday planning.

Does the Rule of 72 apply to compound or simple interest?

The Rule of 72 is designed for compound interest, where returns are reinvested each period. For simple interest (no reinvestment), the doubling time is simply 100 ÷ rate. The compounding effect is what makes the rule approximate rather than exact.

Can I use the Rule of 72 to calculate tripling or halving time?

For tripling time, use the Rule of 114 (114 ÷ rate). For quadrupling, apply the doubling rule twice. For halving (e.g., inflation eroding value or a declining asset), the same Rule of 72 applies: 72 ÷ rate gives the time to lose half the value.

What is the Rule of 72 used for in practice?

Financial advisors use it to quickly compare investment options, illustrate the power of compound interest to clients, and estimate the impact of fees or inflation. It is also commonly used in personal finance education to show why starting to invest early dramatically increases long-term wealth.

Sources & references

External references open in a new tab. We are independent and not affiliated with these organizations.

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Provided “as is” for general information only — results may be inaccurate, so verify before you rely on them. No warranty; use at your own risk.

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