Rule of 72 Calculator
Estimate how long it takes your money to double.
How to use
- 1 Choose whether to solve for years to double or the rate to double.
- 2 Enter your annual interest rate, or the number of years.
- 3 Optionally switch the divisor between 72, 70 and 69.3.
- 4 Read the estimate, compare it to the exact figure, and copy the summary.
About Rule of 72 Calculator
The Rule of 72 is a famous mental-math shortcut for compound growth.
Divide 72 by an annual interest rate and you get a surprisingly good estimate of how many years it takes for money to double.
At 8% a year, 72 divided by 8 is 9, so an investment roughly doubles every nine years; the true compound-interest answer is about 9.01 years, so the rule is accurate enough for a quick gut check without a calculator.
This tool runs that calculation both ways.
In "years to double" mode you enter an expected annual return and instantly see the estimated doubling time alongside the exact figure from the compound-interest formula, so you can judge how close the approximation is.
In "rate to double" mode you enter a target number of years and the tool tells you the annual return you would need to hit it.
The rule works best for rates in the 6–10% range; outside that band 70 (better for low rates) or 69.3 (the value for continuous compounding) can be more accurate, so you can switch the divisor to compare.
Everything is computed locally in your browser — no figures are uploaded, logged or stored — so it is private and keeps working offline.
Copy the plain-language summary whenever you want to drop it into a note or message.
FAQ
Why 72 and not another number?
72 has many small divisors (2, 3, 4, 6, 8, 9, 12) which makes the mental arithmetic easy, and it is accurate for the mid-single-digit rates most investments earn.
How accurate is the Rule of 72?
For rates between about 6% and 10% it is within a fraction of a year of the exact compound-interest answer, which is shown alongside so you can see the difference.
When should I use 70 or 69.3 instead?
Use 70 for lower rates and 69.3 for continuously compounded returns; both are shown as divisor options so you can compare estimates.