At 6% a year, with interest added once a year, 10,000 first reaches double after 12 years, when the balance is 20,122. At 4% it takes 18 years, and at 10% it takes 8.
A quick estimate is the rule of 72: divide 72 by the yearly rate in percent. The table sets that estimate beside the year the balance actually passes double. With interest added once a year, the balance can only pass double at the end of a year, so the actual count is always a whole number of years.
Years to double at each rate
Starting from 10,000:
| Yearly rate | Year it passes double | Balance that year | Rule of 72 estimate (years) |
|---|---|---|---|
| 4% | 18 | 20,258 | 18 |
| 6% | 12 | 20,122 | 12 |
| 8% | 10 | 21,589 | 9 |
| 10% | 8 | 21,436 | 7.2 |
What the example leaves out
Real accounts can add interest more often than once a year, rates change over time, and tax on the interest, where it applies, slows the doubling.