Short answer: years to double ≈ 72 ÷ annual return %. At 8%, money doubles in about 9 years; at 10%, about 7.2 years; at 6%, about 12 years. It is a mental-math shortcut for compound growth — accurate within months for returns between 6% and 10%.
Where the 72 comes from
Doubling time solves 2 = (1+r)^t, so t = ln(2)/ln(1+r). For small r, ln(1+r) ≈ r, giving t ≈ 0.693/r — the “rule of 69.3.” Bankers rounded 69.3 up to 72 because 72 divides cleanly by 2, 3, 4, 6, 8, 9, and 12, making the mental math easy. The rounding costs almost nothing in accuracy across normal investing returns.
Verified examples
| Annual return | Rule of 72 | Exact doubling time |
|---|---|---|
| 6% | 12.0 years | 11.9 years |
| 8% | 9.0 years | 9.0 years |
| 10% | 7.2 years | 7.3 years |
(The “exact” column is ln(2)/ln(1+r).) The shortcut is off by a month or two — plenty accurate for planning.
What it means for real money
$10,000 invested at an 8% average annual return:
- ~9 years → $20,000
- ~18 years → $40,000
- ~27 years → $80,000
Each doubling takes the same 9 years, but each adds twice as many dollars — that acceleration is compounding. Test your own scenario with our Compound Interest Calculator, which shows the year-by-year growth behind the shortcut.
The rule also exposes weak returns: at 2% (a typical savings account in many years), doubling takes 36 years. At 0.5%, 144 years. This is why long-term money usually belongs in investments, not cash — a point the SEC makes in its free compound-interest resources at investor.gov.
The rule works in reverse, too
Halving by inflation: at 3% inflation, your cash’s purchasing power halves every 24 years (72 ÷ 3). Money “earning” 0% in a drawer is shrinking 3% a year in real terms.
Debt doubling: a credit card balance at 24% APR doubles every 3 years if you pay nothing (72 ÷ 24). That mirror image is worth remembering the next time only minimums feel affordable — run it through our Credit Card Payoff Calculator instead of guessing.
Using it to compare two investments
The rule shines when comparing options. A fund averaging 10% doubles money in 7.2 years; one averaging 7% takes 10.3 years. Over a 30-year horizon, that gap compounds brutally:
- 10%: $10,000 → roughly $174,000 (about 4.2 doublings)
- 7%: $10,000 → roughly $76,000 (about 2.9 doublings)
A mere 3-point return gap more than doubles the final amount. This is the real cost of high-fee funds and conservative allocations held too long — and the reason comparing long-run average returns matters more than picking last year’s winner. Our VOO Calculator lets you project S&P 500-style growth with your own contribution schedule.
Where the shortcut breaks down
- Above ~15% returns, the approximation drifts (at 20%, the rule says 3.6 years; exact is 3.8).
- It assumes a constant return. Real markets bounce around; use long-run averages (the S&P 500’s ~10% historical nominal return is the classic input — try projecting it with our VOO Calculator).
- It ignores contributions. Adding $500/month changes everything — the rule only describes a lump sum compounding untouched.
- It ignores taxes and fees, which shave the effective return.
Frequently asked questions
Is it 72, 70, or 69.3?
All three are the same idea. 69.3 is mathematically exact for continuous compounding; 72 is the popular version because it divides neatly. Use 72.
Can I use it for monthly compounding?
Yes — just use the annual rate. Compounding frequency barely moves doubling time (monthly vs. annual compounding differs by days, not months).
Does the Rule of 72 work for the Rule of 114 (tripling)?
Same family: 114 ÷ rate ≈ years to triple. At 8%, money triples in about 14.3 years. And 144 ÷ rate gives quadrupling time.
Why does doubling take the same time regardless of the starting amount?
Because growth is proportional: 10% of $10,000 and 10% of $1,000,000 both take the same time to double. Only the rate matters — which is exactly why fees and fund expenses deserve your attention.