The Rule of 72 is a quick mental shortcut for estimating how long it takes a sum of money to double at a fixed annual interest rate.
Years to double = 72 / annual rate (as a percent)
At 4% per year, money doubles in about 18 years (72 ÷ 4). At 8%, it doubles in about 9 years (72 ÷ 8). At 12%, about 6 years.
It works in reverse too: if you want to double your money in 10 years, you need to earn about 7.2% per year.
When it's accurate
The Rule of 72 is most accurate at rates between 6% and 10%. The actual mathematical doubling time uses natural log — the precise formula is:
Exact doubling time = ln(2) / ln(1 + r) ≈ 69.3 / r at low rates
At 8%, the Rule of 72 gives 9.0 years. The exact answer is 9.006 years. Close enough for mental math.
| Rate | Rule of 72 | Exact (monthly compounding) | Error |
|---|---|---|---|
| 2% | 36.0 yr | 35.0 yr | +2.9% |
| 4% | 18.0 yr | 17.4 yr | +3.4% |
| 6% | 12.0 yr | 11.6 yr | +3.4% |
| 8% | 9.0 yr | 8.7 yr | +3.4% |
| 10% | 7.2 yr | 7.0 yr | +2.9% |
| 15% | 4.8 yr | 4.7 yr | +2.1% |
| 20% | 3.6 yr | 3.5 yr | +2.9% |
For low rates (under 4%), some people use the Rule of 70 instead, which is slightly more accurate at low rates because it's closer to the underlying ln(2) ≈ 0.693.
What it's good for
- Quick comparisons — "Is this 5.2% APY meaningfully better than 4.8% APY?" 72/5.2 ≈ 13.8 years vs 72/4.8 = 15.0 years. About a 1-year difference, not huge.
- Rule-of-thumb retirement planning — At a 7% real return, money doubles roughly every 10 years. Someone with 40 years to retirement can roughly expect 4 doublings.
- Loan thinking — If you only pay minimums on a credit card at 24% APR, your unpaid balance compounds. 72/24 = 3 years to double. This is useful for understanding how dangerous high-rate debt is.
What it's not good for
- Variable rates. The Rule of 72 assumes a constant rate. Real savings accounts, stock market returns, and most loans have changing rates over time.
- Regular contributions. The rule only handles a single starting balance. If you're adding monthly contributions, the actual doubling time is much shorter.
- Inflation-adjusted thinking. A 7% nominal return in 3% inflation is really 4% real, so your purchasing power doubles much more slowly than your nominal balance.
- Precise planning. When the answer matters (when to retire, how much to save for college), use a real compound interest calculator instead.
The doubling chain at 7%
Many financial planners use 7% as a long-term real return estimate for diversified stock investments. The Rule of 72 implies money doubles every ~10 years at 7%. Over a working lifetime, that compounds dramatically:
- $10,000 at age 25
- ~$20,000 at age 35
- ~$40,000 at age 45
- ~$80,000 at age 55
- ~$160,000 at age 65
This is why time in the market matters so much. Someone who starts investing at 25 has potentially 4 doublings by retirement; someone who starts at 45 has only 2.
Quick reference: doublings per year
If you can mentally remember "money doubles every 72/X years," you can also estimate how many times money doubles over a fixed period:
- At 6%, money doubles every 12 years → about 2.5 doublings in 30 years → starting balance grows ~5.7×
- At 8%, money doubles every 9 years → about 3.3 doublings in 30 years → starting balance grows ~10×
- At 10%, money doubles every 7.2 years → about 4.2 doublings in 30 years → starting balance grows ~17×
The number-of-doublings framing makes long-term compound growth more intuitive than calculating the exact figure.
For specific doubling-time projections with monthly contributions and your actual APY, use our APY calculator.