Rule of 72 Calculator
Estimate money-doubling time or required return with the Rule of 72, then compare Rule 70, Rule 69.3, and the exact compound-growth solution.
Rule of 72 Calculator inputs
Change the calculator-specific fields below. The result, checks, and visual update from this calculator’s own formula engine.
Rule of 72 Calculator results
How the double your money calculator works
This page is built for the exact search intent represented by double your money calculator. It exposes the governing assumptions and supporting quantities instead of returning an unexplained headline number.
A mental shortcut with an exact comparison
The Rule of 72 divides 72 by an annual percentage rate to estimate doubling time. It is fast and often close for moderate rates, but it is not an identity. The calculator also solves the exact compound-growth equation and shows the error.
Compounding frequency
The exact doubling time changes with annual, monthly, daily, or continuous compounding. The Rule of 72 does not explicitly contain frequency, so its accuracy varies. The reverse mode solves the nominal annual rate needed to double in a selected time under the chosen compounding convention.
Rule 70 and Rule 69.3
Rule 70 is another simple mental approximation. Rule 69.3 is tied more closely to the natural logarithm of two and often aligns well with continuous compounding. Comparing all three helps show when a shortcut is adequate and when the exact formula matters.
Reliable-use checklist
Confirm that the selected mode, units, signs, dates, rates, and definitions match the original problem. Retain the detailed output and supporting cards so the answer can be reproduced. Test a known example before relying on a custom case, and use current official or professional guidance when taxes, benefits, safety, contracts, or regulated decisions are involved.
The double your money calculator runs locally in the browser. It does not transmit entered values, create an account, or retain a calculation history. Full internal precision is kept during calculation and display rounding is applied only to readable outputs.
Rule of 72 Calculator questions
Does 8% double money in exactly 9 years?
No. Rule 72 gives 9 years; annual compounding gives a slightly different exact time.
Can the rule predict investment performance?
No. It only translates an assumed constant rate into a doubling-time estimate.
Why is continuous compounding different?
Continuous growth uses e^(rt), producing exact doubling time ln(2)/r.