Rule of 72: count in doublings

A friend asked me over chai how long his money takes to double.

Not what return he should chase. Not which fund. Just, when does it turn into twice as much. It is the best question I have heard in a while, because doubling is the one thing about compounding your brain can actually picture. A percent slides right past you. "Twice as much" does not.

So I gave him the shortcut I run in my head. 72 divided by your return is roughly the years to double. At 12 percent, money doubles every 6 years. At 8, every 9. At 6, every 12.

Then we did the part that matters. Take Rs 10 lakh and leave it for thirty years. At 12 percent it becomes about Rs 3 crore. At 8, about Rs 1 crore. At 6, roughly Rs 57 lakh. Same money, same three decades.

Look at what that gap actually is. 12 percent buys you five doublings in thirty years. 8 percent buys just over three. The four points between them do not trim a little off the end, they delete close to two doublings, and the last doubling is always the biggest one you ever get.

Here is what made him put the cup down. Shave 12 down to 11, a fee you never notice, one lazy fund, a single panic exit, and about Rs 71 lakh quietly walks off that same ten lakh over thirty years. You did not lose one percent. You lost nearly a quarter of the whole pile.

That is the whole lesson for me. Nobody can hand you a higher market. But the rate you actually keep, after costs, after tax, after your own nerves, decides how many times your money doubles. Count in doublings and investing gets a lot simpler.

The ladder, drawn

At 12 percent, Rs 10 lakh doubles about every six years. Thirty years gives five rungs. Here is what each rung adds.

The exact figure at 30 years is Rs 10,00,000 x 1.12 ^ 30 = about Rs 3.0 crore, a little below the 320 on the ladder because 12 percent doubles in 6.1 years, not exactly 6. The shape is the lesson. Each rung is as big as everything below it. Lose a rung at the end and you lose half the pile.

How many doublings each rate buys

Count doublings over the same thirty years. The exact count is 30 x ln(1 + rate) / ln 2.

Twelve against eleven is 4.90 doublings against 4.52. That 0.38 of a doubling, at the very end of the ladder, is the Rs 71 lakh from the opening: Rs 3.00 crore against Rs 2.29 crore.

How good is the shortcut?

Rule of 72 is an approximation. The exact doubling time is ln 2 divided by ln(1 + rate). Try a few rates and compare.

Between about 6 and 12 percent the shortcut is within a fraction of a year. At 8 percent it is almost exact: 9.00 against 9.01. At 24 percent it says 3.00 years and the truth is 3.22. For high rates, 72 runs a little fast.

The same clock runs on prices

Doubling is not only about growth. It works on anything that compounds, including the cost of living.

At the Aug 2026 inflation rate of 4.82 percent, prices double in about fifteen years. So a rupee that doubles every fifteen years is only standing still. What matters is how many doublings you get above that line, after costs and tax. That is why I keep coming back to the costs side of a portfolio. Nobody controls the market's rate. Expense ratios, turnover and tax are the parts you can actually read before you start.

The free Learn module on mutual fund structure and costs shows how expense ratios and plan choice change what compounds, with quizzes and a certificate. A related trap, quoting returns from a lucky start date, is in the since inception trap.