One number lets you do compound-interest maths in your head, and every investor should carry it: 72. Divide 72 by an annual return and you get the rough number of years for money to double. It is the fastest sanity check in finance — and it works just as brutally in reverse, against your money, when the rate is inflation.
The rule, in one line
- 72
- a fixed constant that happens to work well for everyday rates
- annual return
- the yearly growth rate as a whole number — 12, not 0.12
Example: At 12% a year: 72 ÷ 12 = 6 years to double your money.
Elders always had a rough answer to “iska paisa kab dugna hoga?” — a shortcut passed down, not a calculator. The Rule of 72 is that shortcut written down: no app, no formula sheet, just one division.
A fixed deposit at 7% doubles your money in about ten years (72 ÷ 7). Equity compounding at 12% doubles it in about six. The rule makes that difference something you can feel, not just read.
Doubling at different rates
| Annual return | Years to double (72 ÷ rate) | Typical Indian example |
|---|---|---|
| 4% | ~18 years | A savings account — barely keeping up |
| 7% | ~10 years | A bank fixed deposit |
| 8% | ~9 years | PPF, roughly |
| 12% | ~6 years | Long-run equity, assumed |
| 15% | ~4.8 years | An optimistic equity run |
Watch how many years each rate needs to double a corpus, and how the doublings stack up over a long horizon. The Rule of 72 is the mental shortcut for exactly this curve.
The same rule, running against you
Inflation is a return too — a negative one, on the purchasing power of cash. Feed the inflation rate into the same rule and it tells you how fast prices double and your money halves in real terms.
- 12% equity doubles money in about six years
- Two doublings — four times your money — in about twelve
- A higher rate shortens the doubling dramatically
- This is why both time and rate matter
- 6% inflation halves purchasing power in about twelve years
- Cash left idle quietly loses
- A return equal to inflation preserves nothing
- This is why “safe” cash is not risk-free
A scheme promises to double your money in four years. What annual return is it implying, and is that realistic?
Purane log seedha bata dete the — “iska paisa dus saal mein dugna”. Formula yahi hai: 72 ko return se bhaag do. 12% pe paisa 6 saal mein double, 7% FD pe 10 saal mein. Aur ulta bhi chalta hai — 6% mehngai pe aapke paise ki keemat 12 saal mein aadhi. Isiliye bank mein pada cash bhi chup-chaap ghat raha hota hai.
- Divide 72 by the annual return to estimate the years to double your money.
- Run it in reverse: 72 ÷ years gives the return you would need.
- At 12% money doubles in about six years; at 7% in about ten.
- Applied to inflation, it shows how fast idle cash loses half its value.
- It is a mental shortcut, accurate for 6–10% — not a substitute for a real plan.
Mark it done to track your progress through the curriculum.
Common questions
Short, direct answers to what people ask about this topic.
- rule of 72 formula and how to use it
- The Rule of 72 estimates how many years an investment takes to double: divide 72 by the annual return rate. At 12% a year money doubles in about 72 ÷ 12 = 6 years; at 8% it takes about 9 years. It is an approximation, most accurate for returns between roughly 6% and 10%, and it works in reverse too — 72 divided by the years you have gives the return you would need to double in that time.
- how many years does it take to double money at 12 percent
- About six years at 12% a year, from the Rule of 72: 72 ÷ 12 = 6. The rule is a shortcut for compound doubling time and is close enough for mental maths in the 6–10% range. The precise figure is a little over six years because the rule slightly understates longer doublings, but for a quick estimate 72 divided by your return rate is all you need.
- is the rule of 72 accurate
- It is an approximation, and most accurate for returns between about 6% and 10%, where its error is only a few months. Above that range it slightly overstates the doubling time and below it slightly understates, so at extreme rates a more precise version uses 69.3 or 70. For everyday Indian equity and fixed-deposit assumptions, 72 is close enough that the small inaccuracy never changes a decision.
- rule of 72 for inflation meaning
- Applied to inflation, the Rule of 72 tells you how fast prices double and your money’s purchasing power halves: divide 72 by the inflation rate. At 6% inflation, prices double in about twelve years, so ₹100 of spending today needs ₹200 in twelve years to buy the same thing. It is the clearest way to see why idle cash loses value, and why a return merely equal to inflation preserves nothing in real terms.
- the rule that estimates how long an investment takes to double is called
- The Rule of 72. You divide 72 by the annual rate of return to get the approximate number of years for money to double, or divide 72 by a target number of years to get the return you would need. It is a mental shortcut for compound growth, accurate enough between about 6% and 10%, and it applies equally to inflation eroding purchasing power.