You don't need a calculator to figure out roughly when your money will double. You need one number — 72 — and a simple division. This mental shortcut, known as the Rule of 72, is one of the most useful pieces of financial math you'll ever learn. It works for investments, savings accounts, debt, and even inflation.
What Is the Rule of 72?
The Rule of 72 states that the number of years it takes to double your money at a fixed annual rate equals 72 divided by that rate.
It works because of the mathematics of compound interest. When interest compounds, your balance grows exponentially — and 72 happens to be a close approximation to a constant derived from the natural logarithm of 2 (ln 2 ≈ 0.693), scaled for easy mental division at common interest rates.
The result is not exact, but it's accurate within a fraction of a year for typical rates — close enough to make fast, confident estimates.
How to Use It in 5 Seconds
Take your annual interest rate and divide it into 72. That's it.
- Savings account at 4.5% APY: 72 ÷ 4.5 = 16 years to double
- Index fund averaging 7%: 72 ÷ 7 = 10.3 years to double
- Aggressive portfolio at 10%: 72 ÷ 10 = 7.2 years to double
- Credit card at 22% APR: 72 ÷ 22 = 3.3 years for debt to double
You can also use it in reverse: if you want your money to double in 8 years, you need a rate of 72 ÷ 8 = 9% per year.
Doubling Times at Every Common Rate
| Annual Rate | Rule of 72 Result | Exact Calculation | Difference |
|---|---|---|---|
| 1% (cash savings) | 72 years | 69.7 years | 2.3 yrs |
| 2% | 36 years | 35.0 years | 1.0 yr |
| 3% (I-bonds, inflation) | 24 years | 23.4 years | 0.6 yrs |
| 4% (high-yield savings) | 18 years | 17.7 years | 0.3 yrs |
| 6% (conservative portfolio) | 12 years | 11.9 years | 0.1 yr |
| 7% (S&P 500 real return) | 10.3 years | 10.2 years | 0.1 yr |
| 8% | 9 years | 9.0 years | 0.0 yrs |
| 10% (S&P 500 nominal) | 7.2 years | 7.3 years | 0.1 yr |
| 12% | 6 years | 6.1 years | 0.1 yr |
| 18% (some credit cards) | 4 years | 4.2 years | 0.2 yrs |
| 24% (most credit cards) | 3 years | 3.2 years | 0.2 yrs |
The rule is remarkably accurate between 6% and 12% — well under half a year of error. Outside that range it's still directionally useful for quick comparisons.
See Your Exact Doubling Time
Use our Compound Interest Calculator to get a precise projection for any amount and rate.
Applying It to Debt
The Rule of 72 is just as useful — and more alarming — when applied to debt. Any balance that accrues compound interest will double on the same schedule. For credit cards, which often charge 20–28% APR, that schedule is terrifyingly fast.
| Credit Card APR | Years Until Balance Doubles | $5,000 becomes... |
|---|---|---|
| 15% | 4.8 years | $10,000 |
| 20% | 3.6 years | $10,000 |
| 24% | 3.0 years | $10,000 |
| 28% | 2.6 years | $10,000 |
These numbers assume you make no payments, but even minimum payments often barely cover the monthly interest, leaving the balance nearly intact. A $5,000 balance at 24% APR making only minimum payments takes over 20 years to pay off and costs more than $11,000 in interest.
Warning: The math that makes compound interest a wealth-building superpower in investments makes it financially devastating in high-interest debt. Eliminating 20%+ APR debt delivers a guaranteed, risk-free return equivalent to that rate.
Applying It to Inflation
The Rule of 72 also measures how quickly inflation erodes your purchasing power. Prices doubling means your money buys half as much.
| Inflation Rate | Years for Prices to Double | What $100 buys in that time |
|---|---|---|
| 2% (Fed target) | 36 years | $50 worth of today's goods |
| 3% (historical average) | 24 years | $50 worth of today's goods |
| 5% | 14.4 years | $50 worth of today's goods |
| 7% (high inflation) | 10.3 years | $50 worth of today's goods |
This is why cash sitting in a zero-interest account loses purchasing power over time. At 3% inflation, the value of idle cash halves in 24 years. Any investment returning less than the inflation rate is losing ground in real terms.
How Accurate Is It?
The Rule of 72 is an approximation derived from the exact compound interest doubling formula:
The number 72 was chosen (over the mathematically precise 69.3) because it divides evenly by many common interest rates: 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, and 36. This makes mental math easy.
At very low rates (1–2%), the rule slightly overestimates the doubling time. At very high rates (30%+), it slightly underestimates. For practical investing and debt decisions, the approximation is always close enough.
Real-World Examples
Example 1: Comparing Two Investment Accounts
You have $20,000 to invest. Account A offers 5% APY. Account B offers 8% APY.
- Account A: 72 ÷ 5 = 14.4 years to double → $40,000 by year 14.4
- Account B: 72 ÷ 8 = 9 years to double → $40,000 by year 9 — and $80,000 by year 18
The 3% rate difference means Account B doubles your money 5 years faster, and that compounding advantage stacks with each doubling cycle.
Example 2: How Many Times Will Money Double?
If you invest at 7% starting at age 25 and retire at 67, that's 42 years.
- Doubling time: 72 ÷ 7 ≈ 10.3 years
- Number of doublings: 42 ÷ 10.3 ≈ 4 times
- $10,000 × 2 × 2 × 2 × 2 = $160,000
Four doublings turn $10,000 into $160,000 with no additional contributions. Start at 35 instead of 25, and you get only 3 doublings — $80,000. One decade costs you $80,000.
Example 3: Evaluating a "Safe" Savings Account
A savings account offers 2% APY. Is that good enough to keep up with inflation at 3%?
- Money doubles in: 72 ÷ 2 = 36 years
- Inflation halves purchasing power in: 72 ÷ 3 = 24 years
No — your money is growing slower than prices are rising. In real terms you're losing ground. You need returns above the inflation rate just to stay even.
Key Takeaways
- Rule of 72: divide 72 by the annual rate to get years to double
- At 7%, money doubles every 10.3 years — four times by retirement if you start at 25
- Works in reverse: to double in 9 years, you need a 8% rate
- Applied to debt: 24% APR doubles a balance in just 3 years
- Applied to inflation: 3% inflation halves purchasing power in 24 years
- Most accurate between 6% and 12%; still useful directionally outside that range