Free tool · Growth
How fast does money grow, and when does it double?
Enter a starting amount, a monthly saving and a rate. See the balance year by year, how much of it is growth, and how the Rule of 72 compares with the exact doubling time.
Balance after 25 years
$252,548
You would put in $100,000. Growth adds $152,548, which is 60% of the final balance.
- Rule of 72 says it doubles in
- 12.0 years
- Exact doubling time
- 11.9 years
The math, in the open
How this is calculated
No black box. Every formula the tool uses is written out here, so you can check it yourself.
The balance compounds monthly: each month it earns one twelfth of the yearly rate, then that month’s saving is added.
The Rule of 72 estimates doubling time as 72 divided by the yearly rate in percent. The exact doubling time with yearly compounding is ln 2 ÷ ln(1 + rate).
The rate you enter is held steady every year, which real returns never are. Taxes, fees and inflation are not included. Use it to see how time and steady saving work together, not to predict what any account will earn.
What is the Rule of 72?
Divide 72 by a yearly rate of return to estimate how many years it takes money to double. At 8%, 72 ÷ 8 = 9 years. The shortcut is close for rates between about 4% and 12%; the tool shows the exact figure beside it.
What does compounding monthly mean?
Interest is added every month, and next month’s interest is earned on the new, larger balance. Over long periods that growth on growth makes up most of the final balance.
Why does starting earlier matter so much?
Each dollar saved early has more years to compound. Saving the same total later usually ends with a smaller balance, because the later dollars have less time to grow.
Keep going
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