Rule of 72 Calculator
Estimate how many years it takes to double your money at any rate of return.
The rule of 72
Divide 72 by the annual return to estimate the years needed to double your money. At 8% it takes about nine years. It works for debt too: at 24% interest, a balance doubles in about three years.
How accurate is it?
It is a good estimate for rates between about 4% and 15%. We also show the exact figure from the logarithm formula.
Worked example
Last reviewed November 2026. Formulas are standard and shown in the sections above; results are estimates.
Tips and common mistakes
- Use it to size up any offer quickly.
- Subtract inflation to see real doubling time.
- Use it on fees too, a 2% fee halves growth faster than it looks.
Learn more
Frequently asked questions
Why 72?
72 has many divisors and approximates 100 × ln(2) well for typical rates.
Does it work for inflation?
Yes, it shows how fast prices double.
Is it exact?
No, but it is close for common rates.
Money guides for this topic
All guides →Investing for Beginners: A Simple Plan with the Numbers
A plain-English starter plan for investing: low-cost index funds, regular contributions and why fees matter.
Read guide →The Rule of 72: How Fast Will Your Money Double?
A one-line mental shortcut for doubling time, with examples, and how it relates to CAGR and compound interest.
Read guide →Compound Interest Explained (and Why Starting Early Wins)
How compound interest works, the rule of 72, and a simple example showing why time matters most.
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Open calculator →Results are estimates and not financial advice.