When to use this calculator
Approximate how long a sum takes to double at a constant annual return. An approximation that is most useful around moderate positive rates. It is not a guaranteed return.
Explore the savings & interest calculators for other questions in this topic. For a related calculation, use the Savings Contribution Calculator: Find the monthly contribution needed to reach a savings target without interest.
PUTTING IT INTO PRACTICE
Rule of 72: a worked example
The values below are an illustrative example, not a recommendation. Replace them with measurements or assumptions that match your question.
| Input | Example value |
|---|---|
| Annual return (%) | 6 |
| Result | 12 years |
- Check the input labels and enter the example values above.
- Apply the relationship: Approximate doubling time = 72 ÷ annual return (%).
- The result is 12 years, rounded for display.
How to interpret your result
An approximation that is most useful around moderate positive rates. It is not a guaranteed return.
Match the quoted rate to its compounding convention. A nominal rate, effective annual rate and continuous rate are not interchangeable. Contribution timing also matters when interest is included in a future-value model.
Frequently asked questions
Is a future savings value a forecast?
It is a scenario based on your entered rate and contributions. Changing rates, fees, tax or interrupted payments can change the actual outcome.
How do I use the rule of 72 calculator?
Enter annual return (%). The result updates automatically. Use the units shown beside each input; the example values illustrate the method.
Which formula does this calculator use?
Approximate doubling time = 72 ÷ annual return (%).
What should I check before using the result?
An approximation that is most useful around moderate positive rates. It is not a guaranteed return.
How are results rounded?
Scalar numbers display up to six decimal places, with scientific notation for very large or small values. Text, fraction, date and schedule outputs use the format described on the tool. Intermediate calculations use JavaScript floating-point arithmetic. Keep sensible precision for your measurements.
Further reading
For background on this topic, see OpenStax: compound interest ↗. This is a topic reference, not an external review or endorsement of this tool.
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