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A friend says a fixed deposit pays 7 percent. Another says a fund gave 12. You do not need a calculator to know how different those are. Divide 72 by each number: about 10 years to double the first, 6 years to double the second. That is the whole rule.
It is called the rule of 72, and bankers have used it for centuries because it turns a scary compound-interest formula into a sum you can do while someone is still talking. Below is why it works, how far you can trust it, and the one place it is most useful for ordinary people: seeing what inflation does to cash.
The rule in one line
Notice it does not matter how much you start with. ₹10,000 or ₹10 crore, the time to double is the same. Only the rate and the time matter, which is exactly why compounding feels so unfair to people who start late.
Why 72, and not some tidier number
The exact answer comes from logarithms: the years to double are ln(2) ÷ ln(1 + r). The number ln(2) is about 0.693, which would suggest a “rule of 69.3”. People use 72 anyway for a practical reason: it divides cleanly by 2, 3, 4, 6, 8, 9 and 12, so the mental maths stays easy at the rates people actually meet.
It also happens to land closer to the true answer at the rates most of us deal with, roughly 6 to 10 percent. At very low or very high rates it drifts a little, which is why some people prefer 70 for low rates and 75 for high ones.
| Yearly return | Rule of 72 | Exact answer | Off by |
|---|---|---|---|
| 3% | 24 years | 23.4 years | 0.6 years |
| 6% | 12 years | 11.9 years | 0.1 years |
| 8% | 9 years | 9.0 years | 0.0 years |
| 12% | 6 years | 6.1 years | 0.1 years |
| 18% | 4 years | 4.2 years | 0.2 years |
| 24% | 3 years | 3.2 years | 0.2 years |
Exact values are ln(2) ÷ ln(1 + rate), rounded to one decimal. For anything between 6 and 12 percent the shortcut is within a few weeks.
Doubling the rate does not just halve the wait in a vague way. It is exactly inverse: twice the return, half the years.
What doubling looks like over thirty years
Doubling once sounds modest. The trick is that it repeats. Put ₹1 lakh away at 12 percent and it doubles about every six years, so in thirty years it has doubled five times: 1, 2, 4, 8, 16, 32. You end up near ₹30 lakh from a single lakh, and you did nothing in between.
Move your finger along the curves. At year 15 the 12 percent line is at ₹5.5 lakh. Between year 25 and year 30 it gains more than it did in the first twenty years put together.
Four percentage points sounds like a small gap between 8 and 12. After thirty years it is the gap between ₹10 lakh and ₹30 lakh. That is not a 50 percent difference. It is three times as much.
The same rule, pointed at inflation
Here is the version that changes how people feel about idle cash. Prices rise, so each rupee buys less. If prices go up 6 percent a year, then 72 ÷ 6 = 12 years for your money to lose half its buying power. The cash did not go anywhere. The things it buys just got more expensive.
- Same number on the paper
- Still “safe”, nothing was lost
- Half the groceries, half the fuel
- Nothing visibly happened, which is the trap
Use the rule backwards to check what your savings need to earn just to stand still.
Where it breaks down
- Taxes and fees. The rule uses the rate you actually keep. A fund that returns 12 percent but costs 1.5 percent in fees is a 10.5 percent investment, which doubles in about 7 years, not 6.
- Returns that are not steady. Shares do not go up 12 percent every year. The rule describes a smooth average, not the bumps. See the CAGR article for why the two differ.
- Very high or very low rates. Below 2 percent or above 30 percent the shortcut gets noticeably loose. Use the exact formula there.
- Regular deposits. The rule is for one lump sum left alone. A monthly SIP is a different shape, because each deposit starts its own clock.
Three quick uses
- Compare two offers. 8 percent versus 10 percent sounds close. 9 years versus 7.2 years to double shows it is not.
- Check a promise. “Double your money in 3 years” means about 24 percent a year. Be suspicious of anything that needs that.
- Size a goal. Need your savings to double in 9 years? You need roughly 8 percent a year. Now you know what rate to look for.
Questions people ask
What is the rule of 72?
A shortcut for how long money takes to double. Divide 72 by the yearly return in percent. At 9 percent, 72 ÷ 9 gives about 8 years.
How accurate is the rule of 72?
Very accurate between about 6 and 12 percent, usually within a few weeks. At very low or very high rates it drifts, and the exact formula, ln(2) ÷ ln(1 + rate), is better.
Why 72 and not 69 or 70?
The exact constant is about 69.3, but 72 divides evenly by many common rates (2, 3, 4, 6, 8, 9, 12), which makes mental maths easier, and it fits the 6 to 10 percent range well.
Can I use the rule of 72 for inflation?
Yes. Divide 72 by the inflation rate to find how many years it takes for money to lose half its buying power. At 6 percent inflation that is about 12 years.
Does the rule work for tripling?
Not directly. The rule of 114 is the usual shortcut for tripling, and 72 is only for doubling. A calculator will give you any multiple exactly.