Rule of 72 Calculator (Doubling Time)
Enter an annual return to see how long money takes to double, by the rule of 72 and by the exact math. Add a balance and a horizon to watch the doublings stack up in dollars.
Growth rate per year, compounded annually
To see a concrete doubling in dollars
How many doublings fit in your timeline
Doubles in about
10.3 years
72 ÷ 7 = 10.3
Exact answer
10.2 years
the shortcut is off by 0.0 years at this rate
Your money, doubled forward
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Add a balance and a horizon
Doubling time at common rates
| Annual return | Rule of 72 | Exact |
|---|---|---|
| 2% | 36.0 yrs | 35.0 yrs |
| 4% | 18.0 yrs | 17.7 yrs |
| 6% | 12.0 yrs | 11.9 yrs |
| 8% | 9.0 yrs | 9.0 yrs |
| 10% | 7.2 yrs | 7.3 yrs |
| 12% | 6.0 yrs | 6.1 yrs |
The rule runs slightly long at low rates and slightly short at high ones; it's sharpest in the 6-10% range where most long-run investment math lives.
Where the 72 comes from
Doubling time is really a logarithm problem: money doubles when growth compounds to 2, and the natural log of 2 is about 0.693. That makes 69.3 divided by the rate the "true" shortcut for continuous compounding. Annual compounding runs a touch slower, so a slightly bigger constant fits real-world rates better, and 72 got the job because it's divisible by almost everything: 72 over 6 is 12, over 8 is 9, over 9 is 8. The result is a rule you can run in your head that stays within months of the exact answer through the range where investing actually happens. Our compound interest explainer unpacks the mechanism, and the compound interest calculator lets you watch the same curve with contributions added.
It works in reverse for inflation
Point the rule at inflation and it becomes a countdown instead of a growth story: at 3% inflation, 72 over 3 says prices double in about 24 years, which means every dollar you hold buys half as much by then. That single flip explains most of long-term investing. Cash that merely sits still is quietly halving, so growth assets aren't a luxury, they're the defense. The inflation calculator makes the erosion concrete in dollars, and our guide to inflation and savings covers where to keep money you can't afford to shrink. The same reverse logic applies to fees: a recurring percentage skimmed off your return stretches every doubling, which the investment fee calculator turns into a lifetime dollar figure.
Rules of thumb are for direction, calculators for decisions
The rule of 72 earns its place because it makes compounding legible: three doublings in 27 years at 8% means every dollar a 35-year-old invests is roughly eight dollars at retirement. That's the right tool for gut checks and dinner-table arguments. For actual decisions, use the exact math with your real numbers, contributions included, the way the FIRE calculator does for the biggest question of all. And since every projection starts from where you stand today, it helps to have that number without spreadsheet archaeology: Stoia keeps your net worth current automatically, so the starting balance in calculators like this one is a fact, not a guess.
Frequently asked questions
What is the rule of 72?
A mental shortcut for compound growth: divide 72 by an annual return to get the approximate years for money to double. At 8%, that's 72 divided by 8, about 9 years. It turns compound interest, which is hard to feel intuitively, into arithmetic you can do at a dinner table.
Why 72 and not 69.3?
The mathematically pure constant is about 69.3, which comes from the natural log of 2 and works exactly for continuous compounding. For annual compounding at everyday rates, a slightly larger number fits better, and 72 was the winner because it divides cleanly by 2, 3, 4, 6, 8, 9, and 12.
How accurate is the rule of 72?
Very, in the range that matters. Between roughly 4% and 12% the rule lands within a few months of the exact answer, which the calculator shows side by side. It drifts at the extremes: at 1% the true doubling time is closer to 70 years than 72, and above 20% the rule starts running noticeably short.
Does the rule of 72 work for inflation?
Yes, in reverse. Divide 72 by the inflation rate to see how long prices take to double, which is the same as your purchasing power halving. At 3% inflation, a dollar buys half as much in about 24 years. It's the fastest way to feel why cash can't be a long-term plan.
Can I use it for debt?
Unfortunately, yes. An unpaid balance compounds by the same math: at a 24% APR, a balance you never pay on doubles in about 3 years. The rule cuts both ways, which is a good argument for pointing it at your investments instead of your credit card.
Does this calculator save my numbers?
No. Everything runs in your browser and disappears when you leave. Nothing is uploaded or stored.
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