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📚 Guide 🧮 Mental maths Updated2026-07-10

When does your money double?

⚡
Quick answer

Divide 72 by your annual return. At 8% money doubles in ~9 years; at 12%, ~6 years; at 6%, ~12 years. It's a mental shortcut that lands within a few months of the exact answer, no calculator needed. It works on inflation too: at 6% inflation your cash halves in ~12 years.

One division, decades of insight

Compound growth is hard to feel intuitively. Our brains think in straight lines, but money grows in curves. The Rule of 72 is the bridge: a single division that turns any return rate into a doubling time you can reason about. It's not magic, just a neat consequence of the compounding formula, and it's accurate enough that professional investors use it for back-of-envelope decisions. Here's the shortcut checked against the exact doubling time at each rate:

Annual return Rule of 72 Exact
4% 18.0 yrs 17.7 yrs
6% 12.0 yrs 11.9 yrs
7% 10.3 yrs 10.2 yrs
8% 9.0 yrs 9.0 yrs
10% 7.2 yrs 7.3 yrs
12% 6.0 yrs 6.1 yrs
15% 4.8 yrs 5.0 yrs

The doubling that changes everything

Here's the insight the rule unlocks: doublings stack, and each one is enormous. At an equity-like 12% return, money doubles every ~6 years, so across a 30-year investing life it doubles about 5 times, turning ₹1 lakh into roughly ₹3.2 lakh. The catch that trips everyone up is that the last doubling is bigger than all the earlier growth put together. A portfolio going from ₹40L to ₹80L in its final stretch gains more than it did in its entire first two decades. That's why the strongest lever in investing isn't picking winners: it's time, and why every year you delay starting costs you a doubling at the far end.

And run it the other way for a reality check: at 6% inflation, the rule says your rupee halves in purchasing power in about 12 years. Money earning less than inflation isn't "safe": it's shrinking on a schedule you can now calculate in your head.

❓ FAQ

Common questions.

What is the Rule of 72?
The Rule of 72 is a mental-maths shortcut for how long an investment takes to double: divide 72 by the annual return percentage. At 8% a year, money doubles in about 72÷8 = 9 years; at 12%, about 6 years; at 6%, about 12 years. It works because of the maths of compound growth, and it's accurate enough for quick decisions without a calculator. The estimate is within a few months of the exact answer for typical rates.
How accurate is the Rule of 72?
Surprisingly accurate for everyday rates. Between about 6% and 10% the estimate is within roughly 2-3% of the true doubling time. For example at 8% the rule says 9.0 years and the exact figure is 9.0 years. It drifts a little at very high or very low rates, but for the returns most people deal with (savings, FDs, equity), 72 divided by the rate is close enough to make decisions on.
Why divide by 72 and not another number?
72 is chosen for convenience: it divides cleanly by 2, 3, 4, 6, 8, 9 and 12, so the mental arithmetic is easy for common rates. The mathematically "purest" number is about 69.3 (from the natural logarithm of 2), which is most accurate for continuous compounding. 72 is a slightly rounded-up version that trades a hair of accuracy for arithmetic that you can do in your head, the whole point of the rule.
Can I use the Rule of 72 for inflation too?
Yes, and it's sobering. Run it on inflation to see how fast your money loses half its value. At 6% inflation, prices double (and your cash halves in purchasing power) in about 12 years. At 4% it's 18 years. This is why money sitting in a low-interest account is quietly shrinking: if it earns 3% while inflation runs 6%, the real value is being eroded faster than it grows. The rule works both ways, for compounding wealth and compounding erosion.
How many times will my money double over a lifetime?
That's where the rule gets exciting. At an equity-like 12% return, money doubles about every 6 years, so over a 30-year investing life it doubles roughly 5 times, turning ₹1 into about ₹32. Each doubling is bigger than all the previous growth combined, which is why the last decade of a long investment does the heaviest lifting, and why starting early beats investing more later.
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