The Rule of 72: How Long Until Your Money Doubles?
Use the Rule of 72 to estimate how long money takes to double, or the return you need, see how accurate it is, and when to use 70 or 69.3 instead.
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Free tool: Rule of 72 CalculatorThe Rule of 72 is a mental shortcut for compound growth: divide 72 by a yearly growth rate to estimate how many years it takes for money to double. At 8% a year, 72 ÷ 8 = 9 years. It works the other way too, and for anything that compounds, from investments to inflation to debt. This guide shows how to use it, how accurate it is, and when to swap in 70 or 69.3, with the free Rule of 72 Calculator.
How the Rule of 72 works
Years to double ≈ 72 ÷ annual rate (in percent)
| Annual return | Rule of 72 | Exact answer |
|---|---|---|
| 2% | 36 years | 35.0 years |
| 4% | 18 years | 17.7 years |
| 6% | 12 years | 11.9 years |
| 8% | 9 years | 9.0 years |
| 12% | 6 years | 6.1 years |
| 20% | 3.6 years | 3.8 years |
The exact answer comes from the compound growth formula: years = ln(2) ÷ ln(1 + rate). The rule is remarkably close for everyday rates, and you can do it in your head.
Finding the return you need
Flip the rule to find the yearly return needed to double money in a set time:
Rate needed ≈ 72 ÷ years
- To double in 9 years, you need about 72 ÷ 9 = 8% a year (exactly 8.006%).
- To double in 6 years, you need about 12% a year (exactly 12.246%).
Doubling again and again
Money that doubles once keeps doubling at the same pace. $10,000 growing at 8% a year becomes about:
- $20,000 after 9 years
- $40,000 after 18 years
- $80,000 after 27 years
- $160,000 after 36 years
That's why starting early matters so much: the last doubling adds more than all the earlier ones put together. The Compound Interest Calculator shows the full growth with regular contributions.
The Rule of 70 and the Rule of 69.3
The number 72 isn't magic; it's chosen because it's close to the exact value and easy to divide by 2, 3, 4, 6, 8, 9 and 12.
- The Rule of 70 is slightly more accurate at low rates, so economists use it for inflation and GDP growth. At 3% inflation, prices double in about 70 ÷ 3 ≈ 23.3 years (exactly 23.4).
- The Rule of 69.3 comes from ln(2) ≈ 0.693 and is exact for continuous compounding.
- The Rule of 72 is most accurate around 8%, the range of many long-term investment returns.
Using the rule for inflation and debt
Compounding works against you too:
- Inflation: at 3% a year, prices double in roughly 24 years, so a dollar buys half as much after that time.
- Debt: an unpaid credit card balance at 24% a year doubles in about 72 ÷ 24 = 3 years.
- Fees: a 1% annual fee slows the doubling noticeably. At a 7% return, money doubles in about 72 ÷ 7 ≈ 10 years; after a 1% fee, at 6%, it takes about 72 ÷ 6 = 12 years.
How to use the Rule of 72 Calculator
- Open the Rule of 72 Calculator.
- Choose your question: How long to double? or What return doubles it in…?
- Enter the yearly return, or the number of years, and a starting amount.
- Read the results: the Rule of 72 estimate, the exact doubling time, and the Rule of 70 and Rule of 69.3 estimates for comparison.
- Explore the charts. One shows your starting amount doubling over time, with each doubling marked; the other shows how far each rule is from the exact answer at different rates. The table lists doubling times from 1% to 25%.
Frequently asked questions
What is the Rule of 72?
A shortcut for compound growth: 72 divided by the yearly rate gives the approximate number of years for an amount to double. At 6%, money doubles in about 12 years.
How accurate is the Rule of 72?
Very close between about 6% and 10%; at 8% it's off by less than a tenth of a percent. At very low or very high rates the error grows, to about a year at 2% and a few months at 20%.
Does the Rule of 72 work with regular contributions?
No. It describes a single amount growing at a steady rate. For savings with monthly deposits, use a compound interest calculator.