The Rule of 72: How Fast Does Your Money Double?
Divide 72 by your annual return and you get the years it takes to double your money. Move the slider to see it for any rate — or flip it around to find the return you need.
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What is the Rule of 72?
The Rule of 72 is a mental-math shortcut for figuring out how long it takes an investment to double. You take the number 72 and divide it by your annual rate of return. The answer is roughly the number of years your money needs to double at that rate. No calculator, no spreadsheet, no compound-interest formula — just one division you can do in your head.
At an 8% return, money doubles in about 72 ÷ 8 = 9 years. At 6%, it takes 72 ÷ 6 = 12 years. At 12%, just 72 ÷ 12 = 6 years. That is the entire rule. Its power is that it turns an abstract percentage into something you can actually feel: the difference between a fund that doubles your money every 7 years and one that takes 18.
The formula (and why it works)
The exact time to double at a compounded rate is ln(2) ÷ ln(1 + r). That is not something anyone does in their head. Because ln(2) is about 0.693, and because of how the math behaves at typical investment returns, multiplying by 100 and rounding to a conveniently divisible number lands you near 72. It is an approximation, but a remarkably good one in the range that matters most to investors.
The rule is most accurate between roughly 5% and 12% annual returns, where it is usually within a couple tenths of a year of the true answer. At very low or very high rates it drifts a little — which is why some people reach for 69.3 or 70 for lower, continuously compounded rates. For estimating in your head, 72 wins because it divides evenly by 2, 3, 4, 6, 8, 9, and 12.
Rule of 72 for common ETF returns
Plug an ETF's expected long-run return into the rule and you get a rough doubling time. These are illustrative long-run figures, not forecasts:
| Investment type | Rough annual return | Years to double |
|---|---|---|
| Cash / high-yield savings | ~4.5% | ~16 years |
| Bond ETF (e.g. BND) | ~4% | ~18 years |
| S&P 500 ETF (e.g. VOO) | ~10% | ~7.2 years |
| Nasdaq-100 ETF (e.g. QQQ) | ~13% | ~5.5 years |
This is exactly why a broad, low-cost stock index fund is the workhorse of most long-term portfolios: at its historical return, it has doubled invested money roughly every seven to ten years. Want the precise, dollar-by-dollar version instead of the shortcut? Use our investment calculator.
The fee angle most people miss
The Rule of 72 also shows why expense ratios matter more than they look. A fee comes straight off your return, so it slows how fast your money doubles. Turn a 10% return into 9.25% with a 0.75% fee and your doubling time stretches from about 7.2 years to about 7.8 — and over a lifetime of doublings, that gap is enormous. Run your own funds through the ETF fee calculator to see the dollar cost.
The same shortcut runs in reverse for inflation: divide 72 by the inflation rate to see how fast your purchasing power halves. At 3% inflation, a dollar loses half its value in about 24 years. Growth doubles your money; inflation and fees quietly halve it. The Rule of 72 lets you size up both sides in seconds.
Frequently asked questions
What is the Rule of 72?
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Educational purposes only. The Rule of 72 is an approximation for illustrating compound growth, not a forecast. This tool does not constitute financial advice, investment recommendations, or personalized guidance. Return figures shown are hypothetical long-run illustrations; actual returns vary and past performance does not indicate future results. ETF BFF is not a registered investment advisor. Always do your own research and consult a qualified financial professional before making investment decisions.