← Blog · May 15, 2026 · finance, investing
The Rule of 72: How Long to Double Your Money
There's a mental shortcut every investor should know: divide 72 by your annual return percentage. The result is roughly how many years it takes to double your money. At 8% you double in 9 years. At 6% you double in 12. At 3% you double in 24. It's the most useful piece of finance math you can carry in your head.
Why 72?
The exact answer comes from logarithms: years to double = ln(2) / ln(1 + r). For r = 7%, that's 0.693 / 0.0677 = 10.24 years. Rule of 72 says 72/7 ≈ 10.3. Close enough.
72 happens to be a convenient number because it's divisible by 1, 2, 3, 4, 6, 8, 9, 12 — every common interest rate. The approximation is most accurate at 6-10%. Below 4% use 70. Above 15% use 76. Most people use 72 for everything.
What this means for your money
Stocks have returned about 7% real (after inflation) per year historically. That's a doubling every 10 years in inflation-adjusted dollars. Some scenarios at 7%:
- $10,000 today → $20,000 in 10 years
- $10,000 → $40,000 in 20 years
- $10,000 → $80,000 in 30 years
- $10,000 → $160,000 in 40 years
- $10,000 → $1,067,000 in 67 years (after 6.7 doublings)
This is the engine of generational wealth. A single $10k deposit, untouched at 7%, approximately becomes a million dollars in a 30-year-old's lifetime.
The other side: how debt doubles too
Credit cards charge 22-29% APR. At 24% your debt doubles every 3 years. That's the same compound math working against you. A $5,000 balance left untouched at 24% becomes:
- $10,000 in 3 years
- $20,000 in 6 years
- $40,000 in 9 years
This is why financial advisors insist on killing credit card debt before investing. A guaranteed 24% return (by paying it off) beats almost any investment.
The hidden cost of waiting
Compound interest has a cruel asymmetry: the early years matter most. Consider two savers, both retiring at 65 with 7% returns:
- Saver A: Saves $5k/year from age 25 to 35 (10 years, $50k contributed), then stops.
- Saver B: Starts at 35 and saves $5k/year until 65 (30 years, $150k contributed).
Who wins?
- Saver A retires with about $602,000 (despite contributing 1/3 as much)
- Saver B retires with about $510,000
Saver A wins by contributing $100k less. Time in the market trounces money in the market when you're talking about decades. This is why the cliché "start saving early" is actually one of the most important pieces of financial advice ever given.
Inflation: the doubling that costs you
Inflation averages about 3% historically (2% Fed target). That's a halving of purchasing power every 24 years. A million dollars in 1990 had the buying power of about $2.4 million today. Your nominal returns need to beat inflation just to stand still.
This is why cash under the mattress is a slow-motion loss, and why bonds (currently ~4-5%) barely keep pace with inflation. Real returns matter.
The math, demystified
Compound interest formula: FV = PV × (1 + r)n
- FV = future value
- PV = present value (what you start with)
- r = rate per period (decimal, so 7% = 0.07)
- n = number of periods
For $10k at 7% over 30 years: 10000 × 1.0730 = 10000 × 7.612 = $76,123.
Run your numbers
- Compound Interest Calculator — see your future value, year by year
- Retirement Calculator — including employer matches and regular contributions
- Savings Goal Calculator — work backwards from a target
- Inflation Calculator — what your money will be worth in the future
- Exponent Calculator — for the underlying math