Divide seventy-two by your annual return and you get roughly how many years until your money doubles. That is the whole trick, and it is worth five minutes because it converts vague feelings about rates into numbers you can argue about, including on the side where rates are being charged to you.
The calculation
A savings account paying two percent doubles in about thirty-six years. Seventy-two divided by two. An investment averaging seven percent doubles in a little over ten years. At ten percent, about seven years. Twenty-four percent, three years, which is why carrying a credit-card balance at that rate turns a four-thousand-dollar phone purchase into eight thousand dollars of debt while you still have the receipt, and at forty-eight percent, payday borrowing doubles in eighteen months. Nobody experiences this as an emergency, because each month’s charge looks small in isolation.
Run it backwards when someone makes a promise. Need to turn one thousand into four thousand? That is two doublings. Divide seventy-two by however many years they’re promising and you get the annual rate required. If the pitch implies thirty percent yearly sustained, you’ve just identified that it requires outperforming what large professional investors have achieved over long stretches, which is information you needed before transferring anything, not after.
Two doublings deserves emphasis because beginners routinely read growth linearly. Ten percent annually for twenty years is not doubling twice, it’s approximately sixfold, and the difference between those two intuitions explains why people accept modest-sounding fees and modest-sounding returns without asking what decades actually do. Compound Interest: The Eighth Wonder of the World lays that out properly, and everything here is the shortcut version of it.
What the number quietly assumes
The rule presumes a steady rate applied every single year through compounding. Real portfolios don’t behave that way; a decade averaging seven percent arrives through seasons ranging from heavy losses to huge gains, sometimes sequentially arranged in exactly the wrong order relative to when you contribute or withdraw, which matters more than most people expect and is covered in What Is Volatility? Riding the Roller Coaster. Mathematically the approximation also loosens at extreme rates, since continuous compounding would suggest sixty-nine rather than seventy-two at very high figures. It stays comfortably accurate through the realistic range, which is the range where you’ll use it, so nobody bothers correcting it.
Three further cautions keep the tool honest. First, gross versus net: subtracting what you actually pay changes conclusions sharply. An assumption of seven percent becomes six percent the moment a one-percent annual fund charge appears, shifting doubling time from roughly ten years to twelve, and across a lifetime that gap compounds into an amount larger than most people’s first car. The Hidden Costs of Trading: Slippage, Spreads, and Market Impact itemizes costs presented as free, and expense ratios work identically inside funds. Second, nominal versus real: if prices rise three percent annually, a nominal seven percent gives you purchasing-power growth nearer four percent, halving the usefulness of the headline figure, with fixed-income consequences walked through in Bonds: The Other Side of the Portfolio. Third, nothing in the arithmetic forecasts markets; it converts whatever rate you assume into time, remaining silent on whether the assumption survives contact with reality.
For comparison purposes it works across any growth quantity, including things outside finance like subscriber counts or follower graphs, though same caveats apply whenever someone quotes percentages without stating the period.
Try this today, five minutes
Write down every place your money currently sits, checking account, savings, any app balance, brokerage positions, plus debts in reverse column. Assign each line a realistic annual percentage: probably under one percent for cash parking, actual contract rates for borrowed money, hypothetical-but-honest expectations for investments. Apply seventy-two divided by each and write years beside the numbers.
Then ask which column contains the shortest doubling times. Usually the answer is a liability, meaning the highest-return action available involves no market prediction whatsoever, just eliminating expensive interest before debating which fund to buy. Only then compare the investment figures against horizons from Long-Term vs Short-Term Investing, because a fourteen-year doubling projection means nothing for money needed next spring, and mismatching those timelines converts ordinary fluctuations into forced selling at the worst possible moment. For the default answer once debts are clean, see Index Funds: Set It and Forget It.
Educational content, not financial advice. All rates used above are illustrations, not projections, and investing involves losing money.