See exactly how many years it takes your money to double — or what return you'd need to double it on your timeline.
The Rule of 72 is a mental-math shortcut for estimating how long it takes an investment to double at a given annual rate of return. Divide 72 by the rate, and the answer is roughly the number of years. At a 7% return, for example, 72 ÷ 7 ≈ 10.3 years to double your money.
It works because of how compound interest behaves mathematically — 72 happens to divide evenly by many small, common numbers (2, 3, 4, 6, 8, 9, 12), making it far easier to calculate in your head than the actual logarithmic formula, while staying remarkably close to the real answer across realistic investment returns.
The Rule of 72 is most accurate right around 8% — within a hundredth of a year. It drifts slightly the further you get from that in either direction, but stays close across the range most investors actually see:
| Annual Return | Rule of 72 | Exact Math | Difference |
|---|---|---|---|
| 2% | 36.0 years | 35.0 years | +1.00 yr |
| 4% | 18.0 years | 17.7 years | +0.33 yr |
| 6% | 12.0 years | 11.9 years | +0.10 yr |
| 8% | 9.0 years | 9.01 years | -0.01 yr |
| 10% | 7.2 years | 7.27 years | -0.07 yr |
| 12% | 6.0 years | 6.12 years | -0.12 yr |
| 15% | 4.8 years | 4.96 years | -0.16 yr |
| 20% | 3.6 years | 3.80 years | -0.20 yr |
At low rates it slightly overestimates the time to double; at high rates it slightly underestimates. Either way, the gap stays under a year for nearly every return an actual diversified portfolio would realistically produce.
The same math applies to any balance growing at compound interest — including debt you're not paying down. A credit card at 24% APR would roughly double an unpaid balance in just 72 ÷ 24 = 3 years. It's a fast, sobering way to see why high-interest debt escalates so quickly if left alone.
The same logic extends further: dividing 114 by your rate estimates years to triple your money, and dividing 144 by your rate estimates years to quadruple it. At 7%, that's roughly 16.3 years to triple and 20.6 years to quadruple.