At a glance
12 full years
At a constant 6% annually, $1,000 → about $2,012
The Rule of 72 puts the answer at about 12 years: divide 72 by the annual return of 6%. To see how close that estimate is, start with $1,000 and calculate the balance at the end of each year.
When does $1,000 become $2,000?
After the first year, the balance is $1,060. In year two, the 6% return applies to that full amount, giving $1,123.60. Reinvesting the gains is what makes the growth compound.
Balance after n years = $1,000 × 1.06n
| Point in time | Balance |
|---|---|
| Start | $1,000.00 |
| End of year 11 | $1,898.30 |
| End of year 12 | $2,012.20 |
At the end of year 11, the balance is still below $2,000. It passes that mark at the end of year 12, reaching $2,012.20. The calculation assumes a constant 6% return, with no deposits or withdrawals along the way.
Try the $1,000 to $2,000 calculation →
Why does the exact calculation give 11.90 years?
Solving the equation gives ln(2) ÷ ln(1.06), or about 11.90. That is a fractional-year answer. This example credits growth once a year and checks the balance at each year-end, so the calculator reports 12 complete periods.
What changes at a different rate?
The estimate is not always as close to a whole year. Here is the same calculation at four assumed annual returns.
| Assumed return | Rule of 72 | First year-end at 2× |
|---|---|---|
| 3% | About 24 years | Year 24 |
| 6% | About 12 years | Year 12 |
| 9% | About 8 years | Year 9 |
| 12% | About 6 years | Year 7 |
At 9%, the Rule of 72 suggests eight years. But $1,000 grows to about $1,992.56 after eight full years—just short of double. It first passes $2,000 at the end of year nine. The estimate is useful for a quick answer; the yearly balances show exactly when the target is reached.
Use the same time unit for the rate and period
For this example, one period is one year, so enter 6 as the return per period. If you instead treat a period as a month, that same input means 6% every month. It no longer represents a 6% annual return.
To try a different starting amount, open the example above and change both the starting and target amounts. Set the target to twice the starting amount if you want to compare doubling times.
These rates are assumptions, not forecasts or promised returns. The calculation excludes fees, taxes and inflation. Regular contributions, withdrawals or changing returns will produce different results.
For more on how compounding works, see Investor.gov’s explanation. To include regular deposits, use its compound interest calculator.