Investing
Rule of 72 Calculator
Quickly estimate how long it takes for your money to double using the Rule of 72, plus see the exact calculation.
Enter an interest or growth rate to see the Rule of 72 estimate and the exact calculation for doubling your money.
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Example
At 7%, your money doubles about every 10.3 years using the Rule of 72. The exact calculation shows 10.24 years—very close!
Results
Rule of 72 estimate
10.3 years
Exact calculation
10.24 years
Doubled amount
$20,000
Projection
Trend view
Milestones
Decision cues
Breakdown
Cost mix
Result reading
The visual trend is moving up, with an approximate change of $27,380 across the period shown.
Planning cue
Use the output as a scenario test rather than a promise. Small input changes can materially change the result.
The Rule of 72 is a great estimate! Use our compound interest calculator for more detailed projections.
How it works
What the result is showing you
These sections explain what the calculator measures, which assumptions matter most, and where the number can be misleading.
What is the Rule of 72?
The Rule of 72 is a quick mental shortcut to estimate how long it takes for money to double. Simply divide 72 by the annual growth rate to get the approximate number of years. It works for interest rates, inflation, or any growth scenario.
Why 72 works so well
72 is chosen because it's easily divisible by many common numbers (2, 3, 4, 6, 8, 9, 12), making mental math easy. It's most accurate around 7-10%, but works reasonably well across a wide range of rates.
The Rule of 72 in reverse
You can also use it in reverse: if you want to double your money in 8 years, what rate do you need? 72 ÷ 8 = 9%. It's great for setting realistic expectations about investment growth.
FAQ
Frequently asked questions
Quick answers to the questions people ask most about this calculator.
How accurate is the Rule of 72?
It's very accurate for rates between 6-10%. For lower rates, you might use 70, and for higher rates, 74 or 76. But 72 is excellent as a general rule of thumb.
Can I use the Rule of 72 for anything else?
Absolutely! It works for inflation (how long until money halves in value), GDP growth, population growth, or anything that compounds. Just remember it's an estimate.
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