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

The Rule of 72 is the most useful mental math shortcut in all of investing. It tells you approximately how many years it takes for an investment to double at a given annual return — no calculator needed. Divide 72 by the annual return percentage and you get the doubling time. Simple, surprisingly accurate, and immensely practical.

How It Works

Years to Double ≈ 72 ÷ Annual Return (%)

An investment earning 8% annually doubles in roughly 9 years (72 ÷ 8 = 9). At 12%, it doubles in 6 years. At 6%, it takes 12 years. At 3%, it takes 24 years. The formula is an approximation — the exact math involves logarithms — but it's remarkably accurate for returns between 4% and 15%.

Why the Rule of 72 Matters for Investors

It Makes Compounding Tangible

Compounding is abstract until you see the numbers. The Rule of 72 makes it concrete. A portfolio returning 10% annually doubles roughly every 7.2 years. Starting with $100,000: after 7 years you have $200,000, after 14 years $400,000, after 21 years $800,000, after 28 years $1.6 million. Each doubling adds more absolute wealth than all prior doublings combined.

This progression helps you understand why starting early is so powerful. A 25-year-old who invests has roughly six doublings before retirement at 67. A 40-year-old has roughly four. Those two extra doublings represent a 4× difference in ending wealth — all from starting 15 years earlier.

It Shows Why Return Differences Matter

The difference between 8% and 12% annual returns might seem modest — just 4 percentage points. But through the Rule of 72, you can see the compounding impact: at 8%, money doubles every 9 years; at 12%, every 6 years. Over 36 years, the 8% investor's money has doubled 4 times (16× total). The 12% investor has doubled 6 times (64× total). The 4% annual return difference produced a 4× difference in ending wealth.

This is why quality investing matters so intensely. A portfolio of high-ROIC compounders earning 12-15% annually will dramatically outpace a portfolio of average businesses earning 7-8% — not by a little, but by multiples. The Rule of 72 makes this differential viscerally clear.

It Reveals the Cost of Fees and Inflation

The Rule of 72 works in reverse for costs. If inflation runs at 3%, your money's purchasing power halves every 24 years (72 ÷ 3). If a fund charges 2% in annual fees, those fees halve your potential wealth every 36 years. Seeing fees and inflation as "negative doublings" makes their long-term cost unmistakable.

A 1% annual fee difference between two otherwise identical funds cuts your ending wealth by roughly 25% over 30 years. The Rule of 72 makes this concrete: the fee costs you nearly a full doubling over that period.

Using the Rule of 72 in Practice

When evaluating a stock, estimate the expected annual return (roughly ROIC for a reinvesting business, or earnings yield for a mature one) and apply the rule. A company compounding intrinsic value at 15% annually will double its value roughly every 4.8 years. If you buy at a 20% discount, the combination of compounding plus valuation normalization gets you to a double even faster.

When comparing investment options — a stock versus a bond, a growth company versus a value company — use the Rule of 72 to translate return differences into doubling time differences. "Fund A returns 9% and Fund B returns 6%" is abstract. "Fund A doubles every 8 years while Fund B doubles every 12 years" is immediately meaningful.

And when someone asks you why you invest in stocks rather than keeping money in savings, the Rule of 72 provides the answer in seconds. At a 1% savings rate, your money doubles in 72 years. At a 10% stock market return, it doubles in 7.2 years. The difference over a lifetime is staggering — and the Rule of 72 makes it undeniable.