The Rule of 72 is a piece of mental math that tells you, in seconds, how long it takes money to double. Just divide 72 by the annual growth rate, and the answer is the number of years. At 8% a year, money doubles in about 72 ÷ 8 = 9 years. No calculator, no spreadsheet — a back-of-the-envelope estimate you can do in your head.
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How the Rule of 72 works
The formula could not be simpler:
Plug in the rate (as a whole number, not a decimal) and you get the doubling time. A few examples:
| Annual return | Years to double |
|---|---|
| 4% | 18 years |
| 6% | 12 years |
| 8% | 9 years |
| 9% | 8 years |
| 12% | 6 years |
So $10,000 earning 8% a year becomes roughly $20,000 in nine years, about $40,000 in eighteen, and $80,000 in twenty-seven — each doubling taking the same nine years. That repeated doubling is the essence of compound growth, and the Rule of 72 makes it visible without any heavy math.
Why 72?
The "true" doubling time comes from a logarithm, and the exact constant is closer to 69.3. But 72 is used instead because it is remarkably convenient: it divides cleanly by 2, 3, 4, 6, 8, 9 and 12 — the rates people actually care about — and it gives very accurate answers in the 6–10% range where most long-term investment returns sit. The small loss of precision is worth the ease of doing it in your head.
Using it in reverse
The rule works both ways. If you know how fast you want your money to double, divide 72 by the number of years to find the return you would need. Want to double your money in 10 years? You would need about 72 ÷ 10 = 7.2% a year. This is a quick sanity check on whether a savings goal is realistic with a given investment.
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It's not just for investments
Anything that grows or shrinks at a steady percentage obeys the same rule:
- Inflation. At 3% inflation, prices double — and your cash loses half its purchasing power — in about 24 years (72 ÷ 3). It is a sobering way to see why money sitting idle quietly erodes.
- Debt. A balance growing at 18% interest with no payments would double in just four years. The rule cuts both ways.
- Fees. Even the drag of fees can be framed this way — though here you want the number as small as possible.
The limitations
The Rule of 72 is an estimate, not an exact answer. It is most accurate for rates between about 5% and 10%; at very low or very high rates it drifts. For low rates, some people use 70 or 69.3 for a closer figure. It also assumes a single steady rate — real markets jump around year to year, so treat the result as a useful approximation rather than a precise forecast. When you need the real number, including regular contributions and a specific time horizon, the Compound Interest Calculator does the exact math.
The bottom line
Divide 72 by your rate to estimate doubling time; divide it by your time horizon to find the rate you need. It is the fastest way to build intuition for how compound growth — and inflation — works, and it fits on the back of a napkin. For the precise figures behind any real decision, reach for the full calculator.
See the exact growth, not just the estimate
Free and private — model any rate, contribution and time horizon.
Frequently asked questions
Is the Rule of 72 accurate?
It is a close approximation, most accurate for annual rates between about 5% and 10%. At those rates it is usually within a few months of the exact answer. For very low or very high rates it drifts a little, so treat it as a quick estimate rather than a precise figure.
Why is it 72 and not 70?
The mathematically exact constant is about 69.3, but 72 is used because it divides evenly by many common rates (2, 3, 4, 6, 8, 9, 12), making the mental math effortless. Some people use 70 or 69.3 for more accuracy at low interest rates.
Can I use the Rule of 72 for inflation?
Yes. Divide 72 by the inflation rate to estimate how long until prices double and your money's purchasing power halves. At 3% inflation, that is about 24 years — a useful reminder of why idle cash loses value over time.
What return should I assume for investments?
Use a figure that reflects your investments, and lean conservative. Broad stock-market averages have historically been in the high single digits before inflation, but no return is guaranteed. The Rule of 72 just turns whatever rate you choose into an easy doubling-time estimate.