The Rule of 72 is the most useful mental math trick in all of investing. It takes 2 seconds and no calculator — and instantly tells you how long it will take to double any investment at any interest rate. Here's everything you need to know.
What Is the Rule of 72?
The Rule of 72 states: divide 72 by your annual interest rate to get the approximate number of years it takes to double your money.
Years to Double = 72 ÷ Annual Interest Rate (%)- At 6%: 72 ÷ 6 = 12 years
- At 8%: 72 ÷ 8 = 9 years
- At 10%: 72 ÷ 10 = 7.2 years
- At 12%: 72 ÷ 12 = 6 years
How Accurate Is It?
| Rate | Rule of 72 Estimate | Exact Calculation | Difference |
|---|---|---|---|
| 2% | 36.0 years | 35.0 years | +1 year |
| 6% | 12.0 years | 11.9 years | +0.1 yr |
| 8% | 9.0 years | 8.99 years | +0.01 yr |
| 10% | 7.2 years | 7.27 years | +0.07 yr |
| 20% | 3.6 years | 3.80 years | +0.2 yr |
The Rule of 72 is remarkably accurate for rates between 6-10% — the most common investment return range. For 8% returns, it's essentially perfect (off by just 0.01 years).
Beyond Investing: Other Uses of the Rule of 72
- Inflation: At 3% inflation, purchasing power halves in 24 years (72÷3=24)
- Debt growth: At 18% credit card rate, balance doubles in 4 years (72÷18=4)
- Population growth: A country growing at 2% annually doubles its population in 36 years
- GDP growth: An economy growing at 4% doubles in 18 years
The Reverse Rule of 72
You can also use the Rule of 72 in reverse: if you know how many years until you need to double your money, what rate do you need?
Required Rate = 72 ÷ Years to Double- Need to double in 10 years: 72 ÷ 10 = 7.2% required
- Need to double in 5 years: 72 ÷ 5 = 14.4% required
- Need to double in 20 years: 72 ÷ 20 = 3.6% required
Related Rules: 69, 114, and 144
- Rule of 69.3: More precise for continuous compounding. 69.3 ÷ rate = doubling years.
- Rule of 114: Estimates when money triples. 114 ÷ rate = years to triple.
- Rule of 144: Estimates when money quadruples. 144 ÷ rate = years to 4×.
Apply the Rule of 72 to Your Investments
Use the Rule of 72 to quickly compare investment options mentally. A 5% HYSA doubles in ~14 years. A 10% index fund doubles in ~7.2 years. Over 30 years, the index fund doubles ~4 times (16×) vs ~2 times (4×) for the savings account — dramatically different outcomes from the same money.
For precise calculations with contributions, use our compound interest calculator to see exact projections year by year.
Get Exact Doubling Projections
The Rule of 72 gives a quick estimate — our calculator gives you the precise year-by-year breakdown.
Calculate Exact ProjectionFrequently Asked Questions
A mental math shortcut: 72 ÷ annual interest rate = years to double money. At 8%: 9 years. At 6%: 12 years. Extremely accurate for rates between 6-10%.
Yes! At 3% inflation: purchasing power halves in 24 years. At 6% inflation: halves in 12 years. The rule works for any exponential growth or decay.
Very accurate for 6-10% rates — common investment returns. At 8%, it's essentially perfect (off by 0.01 years). At extreme rates (20%+), it becomes less precise.
Rule of 114: your money triples. Divide 114 by your rate. At 8%: 114÷8=14.25 years to triple. Rule of 144: money quadruples. At 8%: 144÷8=18 years.