Rule of 72
Explore roughly how long an investment could take to double.
All amounts NZD · Examples are editable · Entries stay in this browser session and are not saved
Constant illustrative return; no tax, fees or cash movements.
72 ÷ annual return (%) ≈ years to double.
| Annual return | Approximate doubling time |
|---|---|
| 3% | 24.0 years |
| 5% | 14.4 years |
| 7% | 10.3 years |
| 9% | 8.0 years |
| 12% | 6.0 years |
So what does this actually mean?
At a steady 7% return, money left invested could roughly double in 10.3 years. The exact constant-return formula gives 10.2 years.
Time and reinvesting returns drive compounding. Higher assumed returns shorten the time, but usually mean greater risk. Actual returns vary, and tax, fees and inflation reduce the benefit. Compare a lower return before relying on this shortcut.
Understand this tool
What this calculator is showing you
72 divided by an annual percentage return gives an approximate doubling time for money left invested. It is a shortcut, not an exact forecast.
How to use it
- Enter a positive annual return and press Calculate.
- Compare the example returns to see how the times change.
What the result means
Shorter doubling times require higher sustained returns. For an exact constant-return comparison, the formula is log(2) ÷ log(1 + return).
Things this calculator does not know
Tax, fees, inflation, changing returns, deposits and withdrawals are excluded. A higher assumed return usually comes with more risk; it is not an offer or promise.
Try changing this
Compare 3% with 7%, then consider whether the higher risk suits your timeframe. Negative or zero returns do not double your money.
Scenario prepared 30/10/2026. Assumptions and results are illustrative.