Estimate how long it takes an investment to double with the Rule of 72, or how many months it takes to reach a specific savings target.
The Rule of 72 divides 72 by the annual rate to estimate doubling time. The goal-based calculator instead simulates the balance month by month, applying the monthly-equivalent of the annual return and then adding the contribution, until the balance crosses the target. Both approaches assume a constant rate, which real markets never actually provide.
Starting with $1,000, contributing $100 every month, at 12% annual return (1% monthly): after month one the balance is 1000 × 1.01 + 100 = 1,110, and after month two it is 1,110 × 1.01 + 100 = 1,221.10. This matches the closed-form future-value-of-annuity formula FV = PV(1+r)^n + PMT × (((1+r)^n − 1) / r), which for the same inputs also gives $1,221.10 after two months.
| Annual rate | Years to double (Rule of 72) |
|---|---|
| 2% | 36.0 |
| 4% | 18.0 |
| 6% | 12.0 |
| 8% | 9.0 |
| 10% | 7.2 |
| 12% | 6.0 |
Both tools assume a constant, guaranteed rate of return, which no real investment provides year to year. They also ignore taxes, fees, and inflation. Treat the results as rough planning estimates rather than a forecast of actual outcomes.
This calculator is for informational purposes only and is not financial advice. Actual investment returns vary and are never guaranteed. Talk to a qualified financial advisor before making investment decisions.