The Rule of 72: Why Starting Early Beats Investing More
The Rule of 72 is the fastest mental math in finance: divide 72 by your annual return and you get the number of years for money to double. It explains, better than any chart, why time in the market dominates timing and amount.
- At 6% money doubles every ~12 years; at 9% every ~8 years; at 12% every ~6 years.
- Money doubles roughly 4–5 times between age 25 and retirement at typical returns — early dollars get the most doublings.
- The rule is an approximation, accurate within about half a year for rates between 6% and 10%.
The math in one line
Years to double ≈ 72 ÷ annual return (%). At 8%, 72 ÷ 8 = 9 years. The exact answer from the compound growth formula is 9.006 years — the rule is accurate to within days at typical rates.
The rule comes from the natural logarithm of 2 (0.693) scaled to percentage units, nudged from 69 to 72 because 72 divides cleanly by 3, 4, 6, 8, 9 and 12.
Why early money is different money
Consider two investors: Ana invests $5,000 a year from 25 to 35, then stops; Ben invests $5,000 a year from 35 to 65. Ana contributes $50,000, Ben contributes $150,000.
At 8%, Ana's early $50,000 compounds through roughly four doublings and ends near $787,000 at 65. Ben's $150,000, with fewer years per dollar, ends around $610,000. Ana wins by 30% while contributing a third as much. Every doubling period an investment misses is lost forever.
The dark side: debt compounds too
The same math runs in reverse on credit card debt. At 22% APR, 72 ÷ 22 ≈ 3.3 years — a credit card balance doubles every three years if you pay nothing down. Compound growth is indifferent to whose pocket it works for.
This is why the standard advice ordering exists: capture employer retirement matches (instant 100% return), kill high-interest debt (guaranteed 20%+), then invest for the long run.
See it with your own numbers
Our compound interest calculator shows the exact curve for your starting balance, contributions and rate — including the 10-year milestone that makes the doubling effect visible.
Try the calculators
Compound Interest Calculator
Project the future value of savings or investments with compound growth — enter a starting balance, monthly contributions, expected annual return and timeframe.
CAGR Calculator
Compute the compound annual growth rate (CAGR) — the smoothed per-year growth rate that turns a beginning value into an ending value over a number of years.
Frequently asked questions
How accurate is the Rule of 72?
Within roughly half a year for returns between 6% and 10%. For very high or low rates, use 69.3 for precision or the exact formula (1 + r)ⁿ.
Does the rule account for contributions?
No — it describes a single lump sum. Recurring contributions follow the same compounding curve but need the full future-value formula.