Doubling Time Calculator
At a steady growth rate, how long until the value doubles? Enter the percentage growth per period to get the exact doubling time, alongside the famous Rule of 70 and Rule of 72 shortcuts used for mental math.
Results
Formula
How to use this calculator
- Enter the growth rate per period as a percentage (e.g. 7 for 7% per year).
- Press Calculate.
- Compare the exact answer with the two rule-of-thumb estimates.
Worked example
Growth of 10% per period.
- Exact: ln 2 ÷ ln 1.10 ≈ 7.27 periods.
- Rule of 70: 70 ÷ 10 = 7; Rule of 72: 7.2.
Result: The value doubles in about 7.27 periods.
Frequently asked questions
Rule of 70 or Rule of 72 — which is better?
72 is slightly more accurate for typical investment returns (5–10%) and divisible by more numbers; 70 is closer for small rates and continuous growth. The exact calculation here beats both.
Does doubling time work for decay?
The mirror concept is half-life: how long until half remains. The same math applies with the decay rate, solving (1 − r)^t = 0.5.
Why do the periods not need a unit?
Doubling time inherits whatever period your rate uses — years for an annual return, days for daily bacterial growth, months for a monthly rate.
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