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About this tool
Estimate how many years it takes for an investment to double at a given annual return rate, or find the required rate to double in a target number of years.
The Rule of 72 Calculator estimates how long an investment takes to double by dividing 72 by the annual rate of return, and runs the same shortcut in reverse to find the rate needed to double within a target number of years. Alongside the shortcut it computes the exact answer from the compound formula, ln(2) ÷ ln(1 + r), and reports how far the approximation is off in years and as a percentage. It also charts the doubling, tripling, quadrupling and 8× milestones so you can see the whole compounding path rather than a single number.
Open Rule of 72 Calculator on AltFTool — it loads instantly in your browser.
Choose Calculation Mode — Years to Double or Required Rate — then set the Annual Rate of Return (%) or Target Years to Double (yrs), plus the Investment Amount in rupees.
Compare the Years to Double card (72 divided by the rate) with Exact Years from the logarithmic formula, and read Approx Error in years and percent; accuracy is best between 6% and 10%.
Check the 2x, 3x, 4x and 8x Investment Milestones and the Rate Comparison at 4, 6, 8, 10, 12 and 15%, then press Copy Summary or Export CSV to save rule-of-72-calculation.csv.
Puts 72 ÷ rate next to the exact ln(2) ÷ ln(1 + r) result and states the error, so you know when the mental shortcut is safe to use.
Switch between solving for years at a known rate and solving for the rate needed to double within a fixed deadline.
Projects 3×, 4× and 8× milestones and plots the growth curve, since most goals are not exactly a doubling.
Divide 72 by the annual rate of return to get the approximate number of years to double: at 8%, 72 ÷ 8 = 9 years. Reverse it to find a required rate — to double in 6 years you need roughly 72 ÷ 6 = 12% a year.
It is most accurate between about 6% and 10%, where the error is a fraction of a year. At 8% the rule says 9.0 years and the exact compound formula gives 9.01; at 4% the rule says 18 years against an exact 17.7, and at 20% the gap widens further. The calculator shows the exact figure and the error alongside every estimate.
The mathematically exact constant for continuous compounding is 69.3, but 72 is used because it divides cleanly by 2, 3, 4, 6, 8, 9 and 12 — which makes the arithmetic doable in your head. For annually compounded returns in the 6–10% band, 72 also happens to be the closer fit.
No — it uses whatever single rate you enter, so if you want a real doubling time, enter the return after inflation, fees and tax rather than the headline figure. A nominal 10% return with 5% inflation doubles purchasing power in about 14 years, not 7. This is an informational estimate; talk to a licensed financial adviser before making investment decisions.
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