Divide 72 by your interest rate, and you get roughly how many years it takes your money to double. 6% → 12 years. 8% → 9 years. It's mental math that replaces a spreadsheet.
How it works
Years to double ≈ 72 ÷ interest rate.
| Rate | Years to double |
|---|---|
| 3% | 24 years |
| 6% | 12 years |
| 7.2% | 10 years |
| 9% | 8 years |
| 12% | 6 years |
It also works in reverse: want to double your money in 10 years? You need 72 ÷ 10 = 7.2% annual return.
Why 72 and not 70 or 75?
It's an approximation of the real doubling formula (ln(2) ÷ ln(1+r)), and 72 just happens to have the most divisors — it divides cleanly by 2, 3, 4, 6, 8, 9, and 12, which covers the rates people actually use. The error is under a year for rates between 4% and 12%.
Tip: Use it on debt too — and shudder. A 24% APR credit card balance doubles in 72 ÷ 24 = 3 years if you only pay... well, if you barely pay. That's the rule working against you.
A real example
You're 30 with $20,000 invested at 7% average annual return. Doubling time: 72 ÷ 7 ≈ 10.3 years.
- Age 40: ~$40,000
- Age 50: ~$80,000
- Age 60: ~$160,000
No new contributions. That's the whole pitch for starting early — each doubling needs zero effort from you.
Don't take our word for it
Plug 7% and 30 years into the calculator and watch the doublings happen.
Open the interest calculator →Quick answers
Is the Rule of 72 exact?
No — it's an estimate, accurate within about a year for rates between 4% and 12%. For precision, use A = P(1+r)^t and solve for t.
Does it work for simple interest?
No. The rule assumes compounding. With simple interest, doubling time is just 100 ÷ rate (e.g., 5% simple = 20 years).
What's the Rule of 114 / Rule of 144?
Same family: 114 ÷ rate ≈ years to triple, 144 ÷ rate ≈ years to quadruple. Less famous because doubling is the question people actually ask.
General information only, not financial advice. Past returns don't predict future results.