Calculate how long it takes to double your money with the Rule of 72. Solve by rate or by years, compare scenarios, and see real investment presets.
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The Rule of 72 is a simple mental math shortcut: divide 72 by your annual interest rate to find how many years it takes to double your money. For example, at 8% per year your money doubles in 72 รท 8 = 9 years. Our calculator goes further โ you can also solve backwards (what rate do I need to double in 10 years?), compare Rule of 72 vs. Rule of 69 vs. the exact logarithmic formula, and explore a rate comparison table.
The Rule of 72 is a quick estimation formula used in finance and investing to approximate the doubling time of an investment. Divide the constant 72 by the annual percentage rate of return, and the result is roughly how many years it takes to double your principal. The rule works because ln(2) โ 0.693, and 72 is close to 69.3 while being more divisible โ making mental arithmetic easier. Rule of 69 is slightly more accurate for continuous compounding; Rule of 115 estimates tripling time.
Core Formula
Quickly evaluate whether an investment meets your goals without complex spreadsheets โ just divide and you have an answer in seconds.
Most tools only solve one direction. This calculator lets you solve for required rate given a target timeline, which is essential for retirement and savings planning.
See exactly how far the rule deviates from the precise logarithmic formula at your interest rate, so you know when to rely on it and when to use the exact calculation.
Rule of 72, 69, and 115 each serve different purposes. The calculator explains when each is most appropriate and lets you compare results side by side.
Estimate how many doubling periods your 401k or IRA will go through before retirement, giving you a quick gut-check on whether you are on track.
Flip the rule around: at 3% inflation, your purchasing power halves in about 24 years โ a sobering reminder of why investing matters.
A HYSA at 4.5% doubles in ~16 years vs. a traditional savings account at 0.5% that takes 144 years. The rule makes the difference viscerally clear.
If you want to double your portfolio in 7 years, the rule tells you that you need roughly a 10.3% annual return โ immediately framing your asset allocation decision.
Divide 72 by your annual return percentage to get the approximate number of years it takes to double your money. At 6% it takes 12 years; at 9% it takes 8 years.
Very accurate for rates between 6% and 10%, with errors under 1%. Accuracy decreases at very low (<3%) or very high (>25%) rates. Our calculator always shows the exact value alongside for comparison.
Rule of 69 (sometimes 69.3) is slightly more accurate for continuously compounded interest. Rule of 72 is more commonly used because 72 has more integer divisors, making mental arithmetic easier.
The Rule of 115 estimates how long it takes for money to triple (not double). Divide 115 by the annual rate to get approximate tripling time. At 10%, your money triples in about 11.5 years.
Yes โ the Rule of 72 specifically applies to compound interest. For simple interest, the math is linear and you do not need the rule.
Divide 72 by your target number of years. If you want to double in 8 years, you need a 72 รท 8 = 9% annual return. Switch to 'Solve for Rate' mode in this calculator to do exactly that.