If someone asked you how long it would take to double your money at 6% interest, could you answer quickly? There’s a mental math trick that makes it effortless. It’s called the Rule of 72, and it’s one of the most useful concepts in personal finance.
Key Takeaways
- Divide 72 by the annual interest rate to get the approximate years to double your money
- Works for investments, savings accounts, and — in reverse — for understanding how fast debt grows
- At 6% return, money doubles in ~12 years. At 10%, ~7.2 years. At 1% savings account, 72 years
- Also applies to inflation: at 3% inflation, prices double in 24 years
- At 22% credit card APR, a balance doubles in just 3.3 years
The Formula
Years to double = 72 ÷ Annual interest rate (%)
That’s it. No calculator needed.
How It Works: Examples
Stock Market Investment
- At 7%: 72 ÷ 7 = 10.3 years to double
- At 10%: 72 ÷ 10 = 7.2 years to double
Invest $10,000 at 7% → ~$20,000 in 10 years.
High-Yield Savings Account (~5%)
72 ÷ 5 = 14.4 years to double
Standard Savings Account (~0.5%)
72 ÷ 0.5 = 144 years to double
This is why keeping large sums in a standard savings account is such a poor long-term strategy.
Inflation (3%)
72 ÷ 3 = 24 years for prices to double
A $50 grocery run in 2026 will cost $100 in 2050 if inflation averages 3%.
Using It for Debt
Credit Card (22% APR)
72 ÷ 22 = 3.3 years to double
A $5,000 balance making minimum payments could grow to $10,000 in just over 3 years.
Student Loan (5%)
72 ÷ 5 = 14.4 years if you never made a payment.
How Accurate Is It?
| Rate | Rule of 72 | Actual |
|---|---|---|
| 4% | 18 years | 17.7 years |
| 6% | 12 years | 11.9 years |
| 8% | 9 years | 9 years |
| 10% | 7.2 years | 7.3 years |
Very accurate in the 6–10% range where most investment returns fall.
The Power of Starting Early
Invest $5,000 at age 25 at 8% annual return:
- Doubles at ~34 → $10,000
- Doubles again at ~43 → $20,000
- Doubles again at ~52 → $40,000
- Doubles again at ~61 → $80,000 by retirement
Wait until 35 to invest the same $5,000:
- $5,000 → ~$40,000 by retirement — half as much from just 10 years’ delay.
Flipping the Rule: What Return Do You Need?
72 ÷ Years = Required annual return
- Double in 6 years? Need 12% return
- Double in 10 years? Need 7.2% return
- Double in 20 years? Need 3.6% return
What the Rule of 72 Helps You Avoid
- Keeping too much in a standard savings account — 144 years is a visceral reminder
- Underestimating credit card debt — doubling in 3.3 years makes payoff urgent
- Waiting to invest — every year delayed is mathematically costly
- Unrealistic investment pitches — a 50% annual return would double money in 1.4 years. If it sounds too good to be true, it is.
The Bottom Line
The Rule of 72 turns complex financial math into 5 seconds of mental arithmetic. Next time you see an interest rate — on a savings account, investment, or loan — divide 72 by it. What you see will guide better decisions.