Rule of 72: A Three-Second Check

Michael Whitman · Trading since 2017, full-time since 2020 · Reviewed 2026-09-06

How it works

Divide 72 by the annual percentage rate. At six per cent, twelve years. At eight per cent, nine years. At twelve per cent, six years. That is the whole rule.

The exact answer at eight per cent is 9.01 years. The shortcut gives nine — an error of four days across a decade, from arithmetic you can do in your head while somebody is still talking.

Accuracy is best in the range that matters. Between about six and ten per cent the error is negligible; at two per cent the rule says thirty-six years against an exact thirty-five, which is close enough for the purpose.

Why the later doublings dominate

Doublings multiply rather than add. Two doublings is four times the original, three is eight and four is sixteen — which is the part intuition consistently gets wrong.

Each doubling adds more than every previous one combined. Ten thousand becoming twenty adds ten; eighty becoming a hundred and sixty adds eighty. The final years of a long investment produce most of the result, which is the real argument for starting early and for not interrupting.

What the fee does to it

A charge reduces the rate before the division happens. Eight per cent doubles in nine years; eight minus one and a half doubles in eleven.

Across four doublings that is eight extra years. Thirty-six becomes forty-four to reach the same multiple — which is the fee expressed as time rather than as a percentage, and it is a far more legible version of the same fact.

Where else it applies

Run it on inflation and it gives a halving time. At three per cent, money loses half its purchasing power in twenty-four years — which is why a fixed income is a shrinking income.

On debt the doubling is what you owe. At eighteen per cent an unpaid balance doubles in four years, which is the same arithmetic pointed at you.

Subtract inflation first for an answer in today’s money. Eight per cent nominal against three per cent inflation is five per cent real, which doubles purchasing power in about fourteen years rather than nine.

None of it predicts anything. The rule converts an assumed rate into a time; whether the rate arrives is a separate question it cannot answer.

In practice: testing a claim

Run it backwards on any performance claim. Somebody promising to double money in two years is claiming thirty-six per cent a year, sustained. Divide 72 by the time and you have the implied rate immediately.

That single reversal is what the rule is actually for. It converts a claim expressed in a flattering form — a promise about doubling — into an annual rate that can be compared with what markets, businesses and professional managers actually deliver. Most impressive-sounding claims fail this check in about three seconds, which is considerably faster than any other form of due diligence available.

One extension is worth knowing because it answers the question people actually have. The rule of 114 gives the time to triple and the rule of 144 gives the time to quadruple, using exactly the same division. At eight per cent that is fourteen years to triple and eighteen to quadruple.

Putting the three together makes the shape of compounding visible. Nine years to double, fourteen to triple, eighteen to quadruple — the gaps between them shrink, which is the acceleration everybody describes and almost nobody quantifies. Nine years for the first doubling and nine more for the second is a very different statement from doubling every nine years, and the second version is the one that is true.

What the rule of 72 is not

It is not exact. It is an approximation, best between 6% and 10%.

It is not a forecast. It assumes a rate somebody supplied.

It is not only for gains. Inflation and debt work the same way.

And it is not a substitute for the real calculation on anything large.

When it fails

It drifts at extreme rates. Above about twenty per cent the approximation loses accuracy noticeably, and at very low rates a variant using 69 or 70 is closer.

The second failure is applying it to a volatile return. An investment averaging eight per cent with wide swings does not double in nine years reliably, because the sequence matters and the average conceals it.

A third is forgetting to subtract costs. The rate to divide into is the rate you actually receive, after fees and after tax.

A fourth is using a nominal rate for a real question. Doubling money is not doubling what it buys.

And a fifth is treating the output as a plan. It is an estimate produced from an assumption, and the assumption is doing all the work.

The original data

Of the 31,760 trading and investing videos in this site’s corpus, 2 have “rule of 72” in the title at a median of 663,162 views across 2 channels. “Compound” returns 7 at a median of 81,015, “passive income” returns 90 at a median of 14,872 across 83 channels, and “index fund” returns 30 at a median of 74,230. The counts are in research/corpus-coverage.json, produced by site/measure_corpus.py.

The figures in the diagrams on this page are illustrative and the arithmetic in them is exact — 72 divided by 8 is 9, and the precise answer is 9.01 years.

A median of 663,162 views across two videos is the highest per-video figure of any term measured in this corpus. One piece of mental arithmetic, almost nobody covering it, and enormous demand for it. The version worth memorising is the reverse one: divide 72 by the promised doubling time to get the annual rate being claimed, and judge that number instead of the promise.

Related

Compound interest is the mechanism this approximates. Index funds is where the fee arithmetic bites hardest. And passive income is the goal the doubling times are usually measured against.

This page is educational, not financial advice. Test every idea on your own charts before risking money.