What Is the Rule of 72?
The Rule of 72 is a quick mental shortcut to estimate how long an investment takes to double: divide 72 by the annual rate of return. At 12% a year, money roughly doubles every six years (72 ÷ 12). It is an approximation, but a remarkably handy one for judging compounding.
Why the Rule of 72 matters
The Rule of 72 makes compounding intuitive without a calculator. It instantly shows why a small difference in return matters so much: money doubles every 6 years at 12% but takes 9 years at 8% — a gap that compounds over a lifetime.
It also reframes inflation: at 6% inflation, prices double in about 12 years, so your money must grow faster than that just to hold its value.
How the Rule of 72 works
Divide 72 by the annual percentage return to estimate the years to double. It works because of the mathematics of exponential growth, and it is most accurate for rates between about 6% and 15%. You can run it in reverse too: to double your money in 6 years, you need roughly a 12% return (72 ÷ 6). The same trick applied to an inflation rate tells you how fast prices double.
A real Indian example of the Rule of 72
You have ₹5,00,000 to invest. In a fixed deposit at about 7%, the Rule of 72 says it doubles in roughly 10 years (72 ÷ 7). In an equity SIP assuming a long-run 12%, it doubles in about 6 years (72 ÷ 12) — and doubles again in another 6. Over 24 years, the 12% money doubles four times while the 7% money barely triples. Small rate differences, compounded, create huge gaps.
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Frequently asked questions about the Rule of 72
How does the Rule of 72 work?
Divide 72 by the annual rate of return to estimate the years for money to double. At 9%, that is 8 years (72 ÷ 9). It is an approximation based on exponential growth, most accurate for rates around 6–15%.
Is the Rule of 72 accurate?
It is close for typical rates but not exact. For very low or very high rates the estimate drifts, and it ignores taxes, fees and irregular returns. Use it for quick comparisons, not precise planning.
Can I use the Rule of 72 for inflation?
Yes. Divide 72 by the inflation rate to estimate how long prices take to double. At 6% inflation, prices roughly double in 12 years — a stark reminder to grow your money faster than that.
What return do I need to double money in 5 years?
About 14.4% a year (72 ÷ 5). That is well above safe fixed-income returns, so doubling in five years generally means taking equity-style market risk, with no guarantee.
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How BudgetAnalyzer uses this
BudgetAnalyzer helps you put the Rule of 72 to work: track spending to invest more each month, then watch your invested balances grow inside your net worth as compounding doubles them over time.
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