How to use the calculator
- Enter the annual return or interest rate, for example the yield on a savings account or the return you expect from an index fund.
- Add a starting amount if you want to see it in money; leave it at zero to see the growth as a multiple.
- To work backwards, enter the number of years you want to double your money in. The calculator shows the rate you would need.
What is the Rule of 72?
The Rule of 72 is a quick way to estimate how long an investment takes to double: divide 72 by the annual rate of return. At 8% a year, money doubles in about 72 ÷ 8 = 9 years. It also works the other way: to double in 6 years you need about 72 ÷ 6 = 12% a year.
The exact doubling time for a rate r compounded once a year is:
years = ln(2) / ln(1 + r)The same idea gives the Rule of 114 for tripling (114 ÷ rate) and the Rule of 144 for quadrupling (144 ÷ rate). 72 is used because it is close to the exact answer for everyday rates and divides evenly by 2, 3, 4, 6, 8, 9 and 12, which makes mental math easy.
How accurate is it?
| Rate | Rule of 72 | Exact |
|---|---|---|
| 2% | 36.0 years | 35.0 years |
| 4% | 18.0 years | 17.7 years |
| 6% | 12.0 years | 11.9 years |
| 8% | 9.0 years | 9.0 years |
| 10% | 7.2 years | 7.3 years |
| 12% | 6.0 years | 6.1 years |
| 20% | 3.6 years | 3.8 years |
| 25% | 2.9 years | 3.1 years |
The rule is almost exact around 8% and stays within a few months between about 4% and 15%. At very high rates it underestimates the time; at very low rates it overestimates it slightly.
Other uses
- Inflation: divide 72 by the inflation rate to see how fast prices double. At 3% inflation, prices double in about 24 years, so money kept in cash loses half its buying power in that time. The inflation calculator shows this in detail.
- Fees: a 1% yearly fee stretches the doubling time. At 7% money doubles in about 10.2 years; at 6% (7% minus a 1% fee) it takes 11.9 years.
- Debt: a credit card balance at 24% interest doubles in about 3 years if nothing is paid.
Common mistakes
- Using an average of volatile returns. The rule assumes a steady rate. A fund that averaged 10% with big ups and downs grows more slowly than one that earned 10% every year.
- Forgetting taxes and fees. Use the return you actually keep. For a full projection with monthly savings, fees and tax drag, use the compound interest calculator.
Frequently asked questions
What is the Rule of 72?
A shortcut for estimating how long an investment takes to double: divide 72 by the annual rate of return. At 6% a year, money doubles in about 72 ÷ 6 = 12 years.
How accurate is the Rule of 72?
Very close for rates between about 4% and 15%: at 8% it gives 9 years and the exact answer is 9.01 years. It drifts at very high rates; at 25% it says 2.9 years against an exact 3.1 years.
What is the exact formula for doubling time?
Years to double = ln(2) ÷ ln(1 + r), where r is the annual rate as a decimal. For tripling use ln(3), for quadrupling ln(4).
What are the Rules of 114 and 144?
The same shortcut for tripling and quadrupling your money: divide 114 or 144 by the annual rate. At 8%, money triples in about 14 years and quadruples in about 18.
How do I find the rate needed to double my money?
Divide 72 by the number of years. To double in 10 years you need about 7.2% a year; the exact figure is 7.18%.
Does the Rule of 72 work for inflation?
Yes. Divide 72 by the inflation rate to see how long prices take to double, which is also how long it takes cash to lose half of its buying power. At 3% inflation that is about 24 years.
Can I put this calculator on my website?
Yes. It is free for any site. Use the embed code generator at the bottom of this page, copy the code and paste it into your page. Please keep the credit line with the two links under the calculator.
Related calculators
- Compound Interest CalculatorHow a starting amount and monthly savings grow over time, after fees, tax drag and inflation.
- Inflation CalculatorWhat things will cost in the future and how much of its value your money loses.
- Millionaire CalculatorThe month your savings reach $1 million, and what $1 million will be worth by then.
- Investment Return Calculator (CAGR & IRR)Your annual return from start and end values, or from dated deposits and withdrawals (XIRR).
See all free investing calculators, or add live stock quotes to your site with the free stock widgets.
Add this calculator to your website
The calculator is free to embed on any website, personal or commercial, and works on WordPress, Blogger, Wix, Squarespace, Webflow, Ghost and plain HTML. Choose the options, copy the code and paste it into your page.
Preview: this is how the calculator and the credit line will look on your page
Free for personal and commercial websites. Please keep the “by Quotelinks” credit line under the calculator visible and unchanged, and don’t modify the calculator or present it as your own. The <script> line is optional: it sizes the frame to fit the calculator; if your platform removes scripts, the calculator still works at the fixed height. Results assume a steady rate of return compounded once a year. They are for illustration only, not financial advice.