Rule of 72 Calculator: Formula and Examples
Estimate how long a constant positive annual return takes to double an investment.
How the calculation works
The Rule of 72 is an approximation. Its accuracy varies with the rate; the logarithmic calculation assumes a constant annual compounded return.
Rule Of 72 formula
Approximate doubling years = 72 ÷ annual percentage rate. Exact years = ln(2) ÷ ln(1 + annual rate as a decimal).
Worked example
At 8%, the approximation is nine years. Exact annual compounding doubles value in approximately 9.0065 years.
Inputs, assumptions, and interpretation
Amounts and rates apply only to the defined scenario. For projections, returns remain constant and timing follows the selected input. Taxes, market variability, and unentered costs are excluded.
Displayed values are rounded, while calculations use unrounded numbers. The charts show modeled values rather than historical performance. For background, see Investor.gov introduction to investment returns.
Frequently asked questions
Are the default rates guaranteed?
No. Default inputs illustrate how the calculation works. Enter rates and contribution amounts relevant to your scenario, and review the applicable account or scheme terms.
Why might an actual statement differ?
Actual payment dates, compounding conventions, changing returns, fees, tax treatment, and account rules can differ from a simplified projection.
How should I interpret a negative result?
Some comparisons or value changes can be negative, including investment losses or a gap between alternative scenarios. Read the result labels to distinguish a balance from a change in value.