Rule of 72 Calculator
Use our Rule of 72 calculator to estimate how long it may take to double an investment or what interest rate may be required to achieve that growth. This calculator allows you to choose whether to enter a number of years or an interest rate.
Enter either the time period or the interest rate below to see the estimated result using the Rule of 72.
Enter either the time period or the interest rate below to see the estimated result using the Rule of 72.
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Disclaimer: This calculator provides estimates only and is for general informational use. It does not constitute financial advice. Last checked/updated February 2026.
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How the Rule of 72 Calculator Works
The Rule of 72 is a simple formula used to estimate how long it may take for an investment to double, or the interest rate required for that growth. It works by dividing 72 by either the interest rate or the number of years.
This calculator can be used in two ways:
This calculator can be used in two ways:
The calculator then displays the corresponding estimated result.
Understanding the Rule of 72
The Rule of 72 is a famous financial shortcut created centuries ago to quickly estimate the effects of compound interest without doing complex math. You simply divide the number 72 by your expected annual interest rate, and the result is the approximate number of years it will take for your money to double.
While it is a fantastic tool for quick mental math, it is important to remember that it is just an approximation. It works best for interest rates between 5% and 10%. If you are dealing with very high interest rates (like a 25% credit card) or very low rates (like a 1% savings account), the Rule of 72 becomes less accurate. For more precise forecasting, you should use our dedicated Compound Interest Calculator.
Example 1: Estimating Time to Double (Using an Interest Rate)
If you invest £10,000 in a stock market index fund that historically returns an average of 8% per year, how long will it take to double to £20,000?
The Calculation: 72 ÷ 8 = 9.
The Result: It will take approximately 9 years for your investment to double, assuming the return stays consistent.
Example 2: Estimating Required Interest (Using a Timeframe)
If you have £5,000 saved and you want it to double to £10,000 in exactly 6 years to put down a house deposit, what interest rate do you need to find?
The Calculation: 72 ÷ 6 = 12.
The Result: You would need to find an investment that yields an average annual return of 12% to reach your goal. Because 12% is a very high target, this tells you that you will likely need to take on higher risk or increase your monthly contributions instead of relying purely on interest.
While it is a fantastic tool for quick mental math, it is important to remember that it is just an approximation. It works best for interest rates between 5% and 10%. If you are dealing with very high interest rates (like a 25% credit card) or very low rates (like a 1% savings account), the Rule of 72 becomes less accurate. For more precise forecasting, you should use our dedicated Compound Interest Calculator.
Example 1: Estimating Time to Double (Using an Interest Rate)
If you invest £10,000 in a stock market index fund that historically returns an average of 8% per year, how long will it take to double to £20,000?
The Calculation: 72 ÷ 8 = 9.
The Result: It will take approximately 9 years for your investment to double, assuming the return stays consistent.
Example 2: Estimating Required Interest (Using a Timeframe)
If you have £5,000 saved and you want it to double to £10,000 in exactly 6 years to put down a house deposit, what interest rate do you need to find?
The Calculation: 72 ÷ 6 = 12.
The Result: You would need to find an investment that yields an average annual return of 12% to reach your goal. Because 12% is a very high target, this tells you that you will likely need to take on higher risk or increase your monthly contributions instead of relying purely on interest.
When Should You Use the Rule of 72 Calculator?
This calculator is useful when you want to:
It provides a simple way to explore growth assumptions before using more detailed calculators.
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About This Rule of 72 Calculator
This calculator applies the widely used Rule of 72 formula to provide quick growth estimates. It is designed for educational and planning purposes rather than precise financial forecasting.
Results should be treated as approximations and not relied upon for detailed investment decisions.
Find out more about how we build our calculators here and if you have an idea for a calculator, click the link at the bottom of the page.
Please note: All investments carry risk, and historical returns do not guarantee future performance. For impartial UK financial guidance, visit MoneyHelper.
Results should be treated as approximations and not relied upon for detailed investment decisions.
Find out more about how we build our calculators here and if you have an idea for a calculator, click the link at the bottom of the page.
Please note: All investments carry risk, and historical returns do not guarantee future performance. For impartial UK financial guidance, visit MoneyHelper.
Frequently Asked Questions
How accurate is the Rule of 72?
It is surprisingly accurate for moderate interest rates (between 5% and 10%). For example, at an 8% interest rate, the Rule of 72 estimates it will take 9 years to double. If you use a precise compound interest formula, the true answer is 9.006 years. However, for rates outside this moderate window, the estimate begins to drift away from reality.
Does this calculator use compound interest?
It is based on an approximation of compound growth, not a full compound interest formula.
Why do results differ from compound interest calculators?
The Rule of 72 simplifies calculations and does not account for precise compounding effects.
Can I use this for any interest rate?
It works best for interest rates between roughly 5% and 10%.
Does this calculator account for inflation?
No. The Rule of 72 can tell you how long it will take for the number in your bank account to double, but it does not account for inflation eating away at the purchasing power of that money. In reality, while your money may double in 10 years, it will likely buy you less than it would today.
Disclaimer
This calculator provides estimates only and should not be relied upon as financial advice.
Last checked/updated February 2026.
Last checked/updated February 2026.