The Rule of 72: The Only Compound-Interest Math You Need in Your Head

Here's the thing about money: almost all of it runs on one piece of math nobody can feel in their gut — exponential growth. The Rule of 72 is the shortcut that finally makes it click. No spreadsheet, no calculator, done in your head at a red light.

The rule

Divide 72 by an annual growth rate and you get, approximately, the number of years it takes for money to double.

• At 4% (a high-yield savings account): 72 ÷ 4 = money doubles every 18 years.
• At 7–8% (the stock market's long-run after-inflation average): about every 9 to 10 years.
• At 24% (a typical credit card APR): what you OWE doubles roughly every 3 years.

The rule works because 72 sits close to the mathematically exact answer for common rates, and it divides cleanly by almost everything — which is why this centuries-old trick has outlived every fancier tool.

Count your doublings

Here's where the rule becomes a life decision instead of trivia. Take your years until retirement and divide by your doubling time. A 30-year-old investing at 8% has until 66 to get about four doublings. A 45-year-old has two.

Why does that matter so much? Because each doubling equals all previous growth combined. The fourth doubling adds more money than the first three put together. When you delay starting by a decade, you don't lose the *first* doubling — you lose the *last* one, the biggest one.

Time in the market isn't a platitude. It's the arithmetic of which doubling you get to keep.

The rule works against you too

Every mechanism that builds wealth compounds — and so does every mechanism that destroys it. Inflation at 3% halves the buying power of idle cash every 24 years (72 ÷ 3). A credit card at 24% doubles a neglected balance every 3 years. Investment fees of 1% quietly consume a shocking share of a lifetime's growth by slowing your doubling clock.

Which is why the sharpest question in personal finance is also the simplest: for each dollar in your life, which side of the compounding is it on?

Make it concrete

Ten seconds of mental math is the appetizer. Run your actual numbers — your savings rate, your expected return, your years — and count your real doublings in the calculator below. Then, if you want the full system for getting more dollars onto the right side of the curve, that's Module 5 of the Wealth System: compounding, index funds, and your first automated portfolio, explained so it finally clicks.