Doubling Time Calculator
Work out how many years it takes for money to double at a given rate of return. Enter the annual rate and how often it compounds to see the exact doubling time, shown next to the quick Rule of 72 estimate.
How to use the doubling time calculator
- Enter the annual rate of return as a percentage.
- Choose how often the balance compounds.
- Compare the exact doubling time with the Rule of 72 estimate.
Examples
6% annual return
rate 6%, compounded annually
Doubles in 11.90 years (Rule of 72 says 12)
10% annual return
rate 10%, compounded annually
Doubles in 7.27 years (Rule of 72 says 7.2)
Frequently asked questions
What is doubling time?
Doubling time is the number of years it takes for an investment to grow to twice its starting value at a constant rate of return. It depends on the rate and how often interest compounds.
What is the doubling time formula?
The exact doubling time is t = ln(2) / (n * ln(1 + r/n)), where r is the annual rate as a decimal and n is the number of compounding periods per year. The result is the years needed for the balance to double.
What is the Rule of 72?
The Rule of 72 is a shortcut that estimates doubling time as 72 divided by the rate as a whole number. At 8% it gives 9 years. It is close to the exact figure for typical rates but drifts at very high or very low rates.
Why does compounding frequency matter?
More frequent compounding adds interest sooner, so the balance reaches double a little faster. Daily compounding doubles money slightly quicker than monthly, which is quicker than annual, for the same stated rate.
Why can a zero or negative rate not be used?
Money only doubles when it grows. At a zero or negative rate the balance never reaches twice its value, so the doubling time is undefined and the calculator asks for a rate above zero.
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