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Rule of 72 calculator
The Rule of 72 estimates how long money doubles at a given return: years ≈ 72 ÷ rate. Works for stocks, savings, inflation, debt — any compounding rate.
Doubling time by rate
| Annual rate | Rule of 72 estimate | Exact doubling time |
|---|---|---|
| 2% | 36.0 yrs | 35.0 yrs |
| 4% | 18.0 yrs | 17.7 yrs |
| 6% | 12.0 yrs | 11.9 yrs |
| 7% | 10.3 yrs | 10.2 yrs |
| 8% | 9.0 yrs | 9.0 yrs |
| 10% | 7.2 yrs | 7.3 yrs |
| 12% | 6.0 yrs | 6.1 yrs |
| 20% | 3.6 yrs | 3.8 yrs |
Rule of 72 is most accurate near 8% — exact at lower or higher rates drifts slightly.
Practical Rule of 72 uses
- Stocks at 10% nominal: double every ~7.2 years. $10k → $20k → $40k → $80k by year 21.6.
- S&P 500 real (7%): double every ~10.3 years in purchasing power.
- HYSA at 4.5%: double every ~16 years.
- Inflation at 3%: prices double every ~24 years. Your $5 coffee becomes $10 by 2050.
- Credit card debt at 22%: doubles in ~3.3 years if you pay nothing.
Why Rule of 72 works
To double: (1+r)^t = 2
t = ln(2) / ln(1+r)
t ≈ 0.693 / r (for small r)
t ≈ 69.3 / rate%
72 is used instead of 69.3 because it's evenly divisible
by 1, 2, 3, 4, 6, 8, 9, 12 — convenient mental math.Rule of 72 Calculator FAQ
What is the Rule of 72?
A mental-math shortcut for estimating doubling time: years to double ≈ 72 ÷ annual return rate. At 8% return, money doubles every 9 years. At 6%, every 12 years. Works for any compounding rate, in either direction (investments doubling, inflation eroding).
How accurate is the Rule of 72?
Within 1–2% for rates between 4% and 12%, where most real-world investments live. Less accurate at very low (1–2%) and very high (20%+) rates. For more precision: use ln(2) ÷ ln(1+r) = exact years to double.
Can I use the Rule of 72 for monthly compounding?
Use the effective annual rate. A 6% APR compounded monthly = 6.17% APY → ~11.7 years to double. If you use the nominal 6%, you'd get 12 years — close enough for mental math but the APY version is technically correct.
Does the Rule of 72 work for inflation?
Yes — and it's depressing. At 3% inflation, prices double in 24 years. At 4%, in 18 years. Same math, opposite direction: your $100 today buys what $50 buys in 24 years if inflation runs at 3%.
What's the Rule of 70 or Rule of 69?
Alternative approximations. Rule of 69 (or 69.3) is exact for continuous compounding. Rule of 70 is sometimes used for higher precision at moderate rates. Rule of 72 dominates because 72 has many factors — easier to divide mentally.
How do I use the Rule of 72 in real life?
Quick sanity checks. 'My HYSA pays 4.5% — money doubles in 16 years.' 'Stocks should double in 7 years at historical 10%.' 'My 22% credit card balance will double if I ignore it for 3.3 years.' Useful for fast intuition without a calculator.