What is the Rule of 72?
The Rule of 72 is a simple shortcut for estimating how many years it will take an investment to double at a fixed annual rate of return. The math couldn't be easier:
Years to double ≈ 72 ÷ annual interest rate (%)
So if your money earns 8% per year, it will roughly double in 72 ÷ 8 = 9 years. Earn 6% instead, and you're looking at about 12 years. That's it — one division you can do in your head.
Why does it work?
The "real" formula for doubling time involves logarithms: it's the natural log of 2 (about 0.693) divided by the growth rate. Because 0.693 expressed as a percentage is roughly 69.3, the mathematically precise number would be the "Rule of 69.3." But 72 is far more convenient — it divides cleanly by 2, 3, 4, 6, 8, 9, and 12 — and it happens to be more accurate for the mid-single-digit interest rates most investors actually see. That trade of a tiny bit of precision for a lot of mental ease is exactly why the rule has stuck around for centuries.
Doubling times at a glance
Here's how long it takes to double your money at common rates of return:
- 2% → about 36 years
- 4% → about 18 years
- 6% → about 12 years
- 8% → about 9 years
- 10% → about 7.2 years
- 12% → about 6 years
Notice how dramatically the timeline shrinks as the rate rises. This is the compounding "snowball" in action — and a clear illustration of why even a one- or two-percent difference in returns matters enormously over a lifetime.
How accurate is it?
The Rule of 72 is most accurate for rates between roughly 6% and 10%. Outside that band, the estimate drifts a little:
- At very low rates (1–3%), the true doubling time is slightly longer than the rule suggests.
- At high rates (15%+), the rule starts to overstate the time.
For continuous compounding, switching to the "Rule of 69.3" is more precise. But for everyday planning, 72 is plenty close — usually within a few months of the exact answer.
Three handy variations
- Rule of 114 — tripling. Divide 114 by your rate to estimate how long it takes to triple your money.
- Rule of 144 — quadrupling. Divide 144 by your rate for a 4× estimate.
- Run it backward. Want to double your money in a set number of years? Divide 72 by that timeline to find the rate you'd need. To double in 10 years, you'd need about 72 ÷ 10 = 7.2% per year.
The flip side: the Rule of 72 and inflation
The rule isn't just for growth — it also shows how fast inflation erodes your purchasing power. At 3% inflation, prices double (and a dollar's value halves) in about 24 years. That's a sobering reminder that money sitting in a no-interest account is quietly losing ground.
From estimate to exact
The Rule of 72 is fantastic for quick gut-checks, but when you want the precise figure — including regular contributions and your specific compounding frequency — run the real numbers. Our free compound interest calculator does the exact math and shows your growth year by year.