The Rule of 72: How Fast Will Your Money Double?

Here is the most useful piece of mental math in all of investing: divide 72 by your annual return, and you get the number of years it takes your money to double.

• At 6%, money doubles every 12 years.
• At 8%, every 9 years.
• At 10%, about every 7 years.
• At the S&P 500's historical 10.7%, roughly every 6.7 years.

No spreadsheet, no calculator. Luca Pacioli — the monk who invented double-entry bookkeeping — described the trick in 1494, and it remains accurate enough that professionals still use it as a sanity check.

Why it works

Doubling your money means solving (1 + r)^t = 2. Take the natural log of both sides and you get t = 0.693 ÷ ln(1 + r), which for realistic returns simplifies to roughly t ≈ 69.3 ÷ r. So the "honest" rule would be the Rule of 69.3 — but 72 is used instead because it's nearly as accurate in the 6–10% range where real portfolios live, and it divides cleanly by 2, 3, 4, 6, 8, 9, and 12. The error at typical stock-market returns is a few months.

The insight most people miss: count the doublings, not the dollars

The rule's real power appears when you stack doublings. At 10%, a 40-year investing life contains about six doublings: 2, 4, 8, 16, 32, 64× your original money.

Now look at where the wealth arrives. After three doublings (about 21 years) you have 8×. The final two doublings alone take you from 16× to 64× — three-quarters of your lifetime gain lands in the last third of the journey. This is why starting at 25 instead of 35 doesn't make you "a bit" richer; it hands you one extra doubling, which is more than everything you accumulated in the first two decades combined. Our S&P 500 pillar article shows the extreme version: 68 years produced roughly ten doublings, turning $10,000 into $8.9 million.

The rule also works in reverse — on your enemies

• Inflation: at 3%, divide 72 by 3 — the purchasing power of cash halves every 24 years. A retirement that lasts 30 years will watch uninvested dollars lose more than half their real value.
• Fees: a portfolio earning 8% in a fund charging 1% compounds at 7% — pushing each doubling from 9 years to about 10.3. Over a working lifetime, that "small" fee quietly deletes an entire doubling. Divide 72 by any fee and you get the years until that fee has consumed half your money's potential.

When the rule breaks

The approximation degrades outside roughly 4%–15%. At very high rates (crypto-bro territory) it overestimates the time; at near-zero savings-account rates, use 69.3 for accuracy — though at 0.5%, the honest answer of "139 years to double" is the real lesson.

The takeaway

The Rule of 72 turns abstract percentages into something your gut understands: time. A 10% return isn't "10%" — it's "my money doubles every 7 years." A 1% fee isn't "1%" — it's "one of my doublings, gone." Run your own numbers through it once, and you'll never look at a return, a fee, or a decade the same way.