How to Calculate Investment Doubling Time
The precise formula for doubling time is ln(2) ÷ ln(1 + r), where r is the annual return rate as a decimal. Even a small difference in return rate has a dramatic effect over the long run. At 5%, it takes 14.2 years to double; at 7%, just 10.2 years.
Doubling Time by Annual Return
| Annual Return | Exact Time | Rule of 72 |
|---|---|---|
| 3% | 23.45 years | 24.00 years |
| 5% | 14.21 years | 14.40 years |
| 7% | 10.24 years | 10.29 years |
| 10% | 7.27 years | 7.20 years |
| 15% | 4.96 years | 4.80 years |
The Rule of 72 is most accurate in the 6–10% range. Outside that range, the exact logarithm formula gives a more reliable result. For long-term planning, always use the exact formula and include your total return — capital gains plus dividends reinvested.
Frequently Asked Questions
For long-term planning, subtract expected inflation from your nominal return to get the real rate. If you expect 7% returns and 2.5% inflation, enter 4.5% to see how long it takes to actually double your purchasing power.
Yes. Enter the APY (Annual Percentage Yield) of your savings account. Just note that APY already accounts for compounding frequency, so no further adjustment is needed.