How Long to Double Your Money? Rule of 72, Tested
Divide 72 by your annual return: that's your doubling time in years. At 8%, money doubles every ~9 years; at 4%, every ~18. The rule is startlingly accurate — we test it against exact math below — and its real lesson is how many doubles your timeline still contains.
Rule of 72 accuracy: the rule vs. exact math
The Rule of 72 says doubling time ≈ 72 ÷ annual return. Here’s how it performs against the exact formula (ln 2 ÷ ln(1 + r)):
| Return | Rule of 72 says | Exact answer | Typical vehicle |
|---|---|---|---|
| 2% | 36.0 yrs | 35.0 yrs | Ordinary savings, short CDs |
| 4% | 18.0 yrs | 17.7 yrs | High-yield savings, bonds |
| 6% | 12.0 yrs | 11.9 yrs | Balanced portfolios |
| 8% | 9.0 yrs | 9.0 yrs | Stock-heavy portfolios |
| 10% | 7.2 yrs | 7.3 yrs | Long-run US stock average (nominal) |
| 12% | 6.0 yrs | 6.1 yrs | Optimistic — plan with less |
A 500-year-old mental shortcut that lands within months of logarithms. Use it in conversations; use the compound interest calculator when dollars are on the line — it reports your exact doubling time in the results card.
Count remaining doubles, not dollars
The rule’s real power is reframing your timeline as a number of remaining doubles. $50,000 at age 35, growing at 7% (one double per decade), with no further contributions:
- Age 45 → $100,000
- Age 55 → $200,000
- Age 65 → $400,000
Three doubles left. Now see what waiting costs: start at 45 instead and you get two doubles — $200,000 at 65. The last double you gave up was worth as much as all the previous growth combined, because the final double is always the biggest. This is the entire “start early” argument compressed into one mechanic — and why each decade of delay roughly doubles the required monthly saving.
Doubling times by account type
“4.5% vs 0.4%” sounds like a small decision. “16 years vs 173 years to double” does not:
- Big-bank savings at 0.40% → ~173 years per double
- High-yield savings at 4.5% → ~15.7 years per double
- Balanced 6% portfolio → ~12 years
- 7% planning assumption for stocks → ~10.2 years
Same dollars, same effort — the only variable is where the money sits. (Why the big-bank number is even worse than it looks: inflation is compounding against you at ~3% the whole time.)
How fast debt doubles at credit card rates
Debt compounds by the same arithmetic. At 24% APR, an untouched card balance doubles in about 3 years; at 27% (typical store cards), about 2.9. Anyone simultaneously carrying a 24% balance and investing at 7% is doubling their debt every 3 years to double their assets every 10. The debt payoff calculator shows how fast the escalator stops once you attack the rate order properly.
Two honest caveats
Returns arrive in lumps. A “10% average” decade can contain a −20% year and a +30% year; the Rule of 72 describes the average path, not the ride. And inflation doubles too — at 3%, prices double every ~24 years, so a nominal double over 24 years is running in place. For purchasing power, use real returns (nominal minus inflation): 7% nominal ≈ 4% real ≈ an 18-year real double. Plan with that number; brag with the other one.
Frequently asked questions
How long does it take to double money in the stock market?
At the market's rough long-run average of 10% before inflation, about 7.3 years; at a conservative planning assumption of 7%, about 10.2 years. Real markets don't deliver returns smoothly, so treat any doubling time as an average across decades, not a promise for one.
How accurate is the Rule of 72?
Within a few months for rates between 4% and 12% — at 8% it's essentially exact (9.0 vs 9.006 years). It drifts at the extremes: at 2% the rule says 36 years versus a true 35; at 20% it says 3.6 versus a true 3.8.
How long does money take to double in a savings account?
At a 4.5% high-yield APY, about 15.7 years. At the 0.40% many big banks pay, about 173 years — which is the entire argument for moving cash to a high-yield account in one sentence.
Does the Rule of 72 work for debt too?
Yes, grimly. A debt at 24% APR doubles in about 3 years if nothing is paid — the rule is symmetric, which is why carrying card debt while investing at 7% is mathematically running down an up escalator.