Investing & Retirement

Doubling Time Calculator

Rule of 72, but exact.

By TheCalcTools Updated Formula checked and tested

Your numbers

%

Your results

Periods to reach the multiple
Periods to reach the multiple
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Rule-of-72 estimate (doubling)
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Exact doubling time
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Doubling Time Calculator by TheCalcTools

How the Doubling Time Calculator works

Enter the growth rate per period (year, month, day).

Enter the target multiple: 2 for doubling, 3 for tripling, 10 for tenfold.

The tool solves (1 + r)^t = multiple.

The formula

t = ln(multiple) ÷ ln(1 + r)Rule of 72: t ≈ 72 ÷ r (r in %)

Worked example

Using the default values shown in the calculator:

Growth rate per period7 %
Target multiple2
  1. Time to double at 7% = ln 2 ÷ ln 1.07 = 10.24 periods.
  2. Time to reach 2× is 10.24; Rule of 72 gives 10.29.

Result:

Periods to reach the multiple10.24
Rule-of-72 estimate (doubling)10.29
Exact doubling time10.24

Frequently asked questions

How accurate is the Rule of 72?

It is accurate to within a few percent for rates between about 5% and 10% and gets less accurate at very low or very high rates.

Does it work for shrinking values?

Yes. A negative rate gives a negative time unless the multiple is below 1; use a halving multiple below 1 for decay.

Sources and further reading

Rule of 72The Rule of 72 is a quick mental shortcut for doubling time Compound Interest CalculatorSee what a starting balance and regular monthly deposits can grow into when interest is… CAGR CalculatorWhat steady annual rate turns your start value into your end value? Dollar-Cost Averaging CalculatorSee how regular buying smooths out price swings Tax-Equivalent Yield CalculatorIs the tax-free bond really better than the taxable one?

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This calculator gives estimates for information only. It is not financial advice, and your lender's or provider's figures will govern. Rates, fees and rules vary. See our disclaimer.