Calculate how many periods a quantity takes to double or to halve at a constant growth rate.
| Periods to double | – |
| Periods to half | – |
Use this calculator for constant compound growth or decay per period: population, a balance, or a culture. The exact doubling time is ln(2) / ln(1 + r), where r is the rate as a decimal. The rule of 70 (70 ÷ percent) is a sketch of the same idea.
Periods to half uses ln(0.5) / ln(1 + r). A negative rate (decay) makes doubling time negative (it never doubles) and half-life positive. The rate cannot be 0 or ≤ −100% (which would make 1 + r ≤ 0).
This is discrete compounding per period, not continuous e^(rt). Finance’s rule of 72 is a money-page cousin.
Rate per period (%): Growth each period as a percent. 7 means 7% per period.
Periods to double: How many periods until the quantity doubles.
Periods to half: How many periods until the quantity is halved.
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