The Rule of 72 is the fastest way to sense how compounding will treat your money without opening a calculator. Divide 72 by the annual return, in per cent, and you get the rough number of years for an amount to double. At an assumed 8% that is about 9 years, at 12% about 6 years. It is an approximation, most accurate for returns between roughly 6% and 10%.
It is not a precise formula, and it does not need to be. Its job is to turn an abstract percentage into a feel for how long doubling takes.
The Rule of 72 at a glance
What is the Rule of 72?
Compounding growth is hard to picture from a percentage alone. Is 8% a year fast or slow? The Rule of 72 answers by converting the rate into a doubling time. Because doubling is something we can imagine, the rate suddenly means something.
The mechanics are one division. Take the annual return as a whole number, say 9, and divide 72 by it. The result, 8, is the approximate number of years the money needs to double, assuming the return holds and the gains stay invested. You can run it the other way too: if you want money to double in 6 years, 72 divided by 6 says you would need about a 12% annual return.
A worked table, illustrative only
The table below applies the rule across a few assumed returns. These rates are for illustration, not forecasts; market-linked returns vary and are never guaranteed.
| Assumed annual return | 72 ÷ return | Approx years to double |
|---|---|---|
| 6% | 72 ÷ 6 | 12 years |
| 8% | 72 ÷ 8 | 9 years |
| 9% | 72 ÷ 9 | 8 years |
| 12% | 72 ÷ 12 | 6 years |
Notice how sensitive the doubling time is to the rate. Moving from an assumed 6% to 12% does not shave a little off the wait, it halves it, from 12 years to 6. That steep relationship is compounding in miniature, and it is why even small differences in long-run return matter so much.
How accurate is the Rule of 72?
It is an approximation of a precise logarithmic formula, and it is deliberately traded for simplicity. In the 6% to 10% range it lands within a few months of the exact answer, which is close enough for any mental estimate. Outside that band it drifts a little, overstating slightly at very low rates and understating at very high ones. For the returns most long-term plans assume, the small error does not change any decision.
The inflation version: how fast prices double
The same division works on the other side of your money. Divide 72 by the inflation rate and you get the years for prices to double, which is the years for your purchasing power to halve. At 6% inflation, 72 divided by 6 is about 12 years. That is a stark way to see why money left idle in a low-return account loses value: prices are compounding even when your savings are not.
That arithmetic is the case for owning growth assets over long horizons. If inflation is doubling prices every dozen years or so, a return that merely matches it leaves you standing still in real terms.
What the rule does not tell you
The Rule of 72 is a feel, not a plan. It assumes a single, steady return, which real markets never deliver, so a portfolio that averages a rate will not double on a tidy schedule. It also ignores taxes, fees and charges, all of which lower what you actually keep and stretch out real doubling. And it says nothing about the risk taken to earn a rate. For the full picture of how the growth itself builds, see the power of compounding.
Related NYVO guides
- The Power of Compounding, Explained: the full mechanics behind the doubling shortcut.
- Asset Allocation: The Decision That Matters Most – the mix that shapes the return you might assume.
- Saving vs Investing: Which Comes First? explains why beating inflation needs growth, not just saving.
- What is a SIP? How Systematic Investment Plans Work – compounding applied one instalment at a time.
- Goal-Based Planning 101 puts a doubling estimate to work per goal.
- What Inflation Really Does to Your Money applies the same doubling arithmetic to prices instead of returns.
Kept in its lane, the Rule of 72 is a genuinely useful reflex: it turns a bare percentage into a length of time you can reason about. The question it cannot answer, which mix of assets might earn that percentage, is where asset allocation begins.
