Worth GlossaryBeginner4 min read

The Rule of 72: doubling time in your head

A back-of-the-envelope trick that tells you how long money takes to double at any rate — and how fast inflation or debt halves it. The math, the limits, and where it misleads.

The Rule of 72 is the most useful piece of mental math in personal finance: divide 72 by an annual rate of return, and you get the approximate number of years for money to double. No calculator, no spreadsheet — just a quick sense of how fast growth (or decay) actually moves.

How it works

72 ÷ rate = years to double. At 8%, money doubles in about 9 years (72 ÷ 8). At 6%, about 12 years. At 4%, about 18 years. Run it backward and it estimates the rate you'd need: to double in 6 years, you need about 12% (72 ÷ 6). It's an approximation that leans on the way compounding works, and it's accurate enough for real decisions in the 5-12% range where most long-term returns live.

The same rule, three jobs

Growth: $25,000 at 7% doubles to $50,000 in about 10 years (72 ÷ 7 ≈ 10.3), then to $100,000 in another 10. Inflation: at 3% inflation, prices double — and your cash halves in buying power — in about 24 years (72 ÷ 3). Debt: a balance at 24% APR left unpaid would double in just 3 years (72 ÷ 24). One rule reveals why investing compounds, cash quietly erodes, and high-interest debt explodes.

Where it's handy

Comparing investments — a fund expected to return 9% doubles money in 8 years; one at 6% takes 12. That gap is easy to feel once it's in years.
Understanding fees — a 2% annual fee doesn't just cost 2%; it drags your doubling time noticeably longer, which is where the real damage hides.
Sizing inflation — knowing prices double every ~24 years at 3% reframes why retirement plans must keep growing, not just preserve cash.
Reality-checking hype — anything promising to double your money in a year implies a ~72% return, a figure that should trigger skepticism, not excitement.

It's an approximation, not a law

The Rule of 72 assumes a single steady rate and annual compounding. Real returns are volatile and lumpy, so it estimates the long-run average, not any specific year. It also drifts at extreme rates (for very low rates, 69.3 is more accurate; for very high ones it overshoots). Use it for intuition, not for a mortgage payoff schedule.

Rates and their doubling times

Annual rate	Years to double	Common example
2%	~36 years	A sluggish savings account
4%	~18 years	A high-yield savings account or short bonds
7%	~10 years	Long-run stock returns after inflation
10%	~7 years	Long-run stock returns before inflation
24%	~3 years	An unpaid credit card balance

72 ÷ rate ≈ years to double

The bottom line

The Rule of 72 turns abstract percentages into something you can feel: years. It won't replace a real projection, but it will let you judge an investment, an inflation figure, or a debt rate in the time it takes to do one division. Memorize it, and you'll never again mistake a 3% drift for something harmless or a 24% rate for something survivable.

Check your understanding

1 of 2

Using the Rule of 72, roughly how long does money take to double at an 8% annual return?

Not quite — try again.

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