How many years will it take for your savings to double? You don’t need a spreadsheet to find out. Divide 72 by the yearly interest rate and you have a surprisingly good answer in about five seconds. Here is how the rule of 72 works, where it goes wrong, and how to use it on real numbers.
What the rule of 72 actually tells you
The rule of 72 is a shortcut for estimating an investment’s doubling time. You take the number 72 and divide it by the interest rate per year, written as a plain percentage (9, not 0.09). The result is roughly the number of years your money needs to double.
Years to double ≈ 72 ÷ yearly interest rate (%)
Why 72 and not some other number? Mostly because it is easy to divide in your head. 72 splits cleanly by 1, 2, 3, 4, 6, 8, 9 and 12, which covers most of the rates people actually see on deposits and investments. The shortcut is old, too: it already appears in Luca Pacioli’s Summa de arithmetica, printed in Venice in 1494, with the example of money at 6% a year doubling in 12 years.
A real example with ₹1 lakh
Say you put ₹1,00,000 in an investment that earns 9% a year, compounded once a year.
- Rule of 72: 72 ÷ 9 = 8 years to reach ₹2 lakh
- Exact answer: about 8.04 years
The shortcut is off by about two weeks. You can keep going, because each doubling takes the same time. After 16 years the rule says ₹4 lakh (the exact figure is about ₹3.97 lakh), and after 24 years it says ₹8 lakh (exact: about ₹7.91 lakh). The rule gets you in the right neighbourhood, slightly on the generous side.
Try it with a rate you may know. At 7.1% a year, the Public Provident Fund rate for the first quarter of FY 2024-25, 72 ÷ 7.1 gives about 10.1 years. The exact calculation also comes out at about 10.1 years. For rates between roughly 6% and 10%, 72 is a very good fit.
It works for losses too
The same division tells you how fast something shrinks by half. This is where the rule becomes really useful.
Inflation
Divide the rule number by the inflation rate to see how long it takes for your money’s buying power to halve. At 3.5% inflation, using the rule of 70, that is 70 ÷ 3.5 = 20 years. At 6% inflation, 72 ÷ 6 = 12 years: ₹1 lakh kept in cash would then buy only what about ₹50,000 buys today.
Fees
A yearly fee works the same way. A 3% annual fee cuts your account value in half in about 72 ÷ 3 = 24 years, and to a quarter in 48 years, compared with holding the exact same investment without that fee. A small percentage, repeated every year, is not small at all.
Common mistakes with the rule of 72
Using it on simple interest
The rule only works for compound interest, where interest earns interest. With simple interest at 9%, you earn 9% of the original amount each year, so doubling takes 100 ÷ 9, or about 11.1 years, not 8.
Trusting it at high rates
The rule gets less accurate as rates climb. At 24% a year, 72 ÷ 24 gives 3 years, but the real answer is about 3.2 years. For high rates, dividing 78 instead of 72 is closer (78 ÷ 24 = 3.25).
Ignoring how often interest is added
If interest is compounded daily or continuously, 69, 69.3 or 70 give better results than 72. That is because the exact maths is based on ln(2), the natural logarithm of 2, which is about 0.693.
Want the exact number?
The precise doubling time is the natural log of 2 divided by the natural log of (1 + rate ÷ 100). In plain words: it asks how many years of growth at that rate it takes to multiply your money by 2. To find the time to triple your money, just swap the 2 for a 3. At 9% a year, tripling takes about 12.7 years.
For everyday decisions, though, the shortcut is enough. When someone promises your money will “double in five years”, divide 72 by 5. You get 14.4: that promise needs a return of roughly 14% every single year. Now you know what question to ask next.
