Rule of 72: how long to double your money
A mental shortcut to estimate compound interest without a calculator.
The rule of 72 says that to find out how many years it takes for money to double, divide 72 by the annual interest rate (in %).
Examples
- At 6% per year: 72 ÷ 6 = 12 years
- At 12% per year: 72 ÷ 12 = 6 years
It's an approximation: it works well for rates between 4% and 15% per year.
Check it in the calculator
Enter an initial amount, set the monthly contribution to zero and see how many years it takes to double.
Compound interest calculator
Final balance
R$125,302.74
- Total invested
- R$61,000.00
- R$1,000.00 upfront + 120 contributions of R$500.00
- Total interest
- R$64,302.74
- 51.32% of final balance
Equivalent monthly rate: 1.08%
Anyone who opens the link sees this same simulation. Nothing is stored.
Show year-by-year table
| Year | Total invested | Total interest | Final balance |
|---|---|---|---|
| 1 | 7,000 | 507 | 7,507 |
| 2 | 13,000 | 1,908 | 14,908 |
| 3 | 19,000 | 4,328 | 23,328 |
| 4 | 25,000 | 7,904 | 32,904 |
| 5 | 31,000 | 12,798 | 43,798 |
| 6 | 37,000 | 19,190 | 56,190 |
| 7 | 43,000 | 27,285 | 70,285 |
| 8 | 49,000 | 37,319 | 86,319 |
| 9 | 55,000 | 49,557 | 104,557 |
| 10 | 61,000 | 64,303 | 125,303 |
Contributions at the end of each month (the first one at the end of month 1). Yearly rate converted to the equivalent monthly rate. Taxes, fees and inflation are not considered: to compare investments after tax, use the fixed income calculator.