The Rule of 72

The Rule of 72 is one of the few pieces of financial mental arithmetic worth memorising. It turns a compounding question into a division you can do in your head, and it is close enough to be useful. It is a maths shortcut, not a claim about what any investment will earn — keep that distinction and it is genuinely handy.

The shortcut

To estimate how many years a sum takes to double at a steady annual rate, divide 72 by the rate written as a whole number. At a rate of r per cent a year, doubling takes roughly 72 ÷ r years — halve the rate and the time roughly doubles, double the rate and it roughly halves. The worked example further down uses an inflation rate to show the arithmetic, deliberately, so that no figure here reads as a return to expect from any investment.

It works both ways

Turn it around and it answers the other question. To double in a set number of years, you need a rate of about 72 ÷ the number of years. That makes it a quick sanity-check on a goal: if a plan needs money to double in a very short time, the rule tells you what annual rate that quietly assumes — and an implausibly high rate is a useful warning that the plan, or the promise, does not add up.

The same rule shows inflation eating value

Compounding is not only something that works for you; inflation compounds against you, and the same rule measures it. At an inflation rate of around 6% a year, prices double in roughly 72 ÷ 6 = 12 years — which is the same as saying your money’s purchasing power halves over that time. This is the honest use of the rule for a saver: here the doubling is a cost you face, not a gain you earn, and it is why money left idle quietly loses ground.

Why it is only a shortcut

The rule assumes one steady rate compounding smoothly. Real investment returns are neither steady nor knowable in advance, so 72 divided by a rate is always arithmetic on an assumption, never a forecast. It is for quick intuition; for a real plan, put your own figures into the SIP calculator or the crorepati math, which show the full illustration rather than a back-of-envelope estimate.

Using it well

Kept in its place, the Rule of 72 is a gut-check. It helps you feel what a rate means in years, spot when a “double your money fast” pitch implies a rate no one can reliably deliver, and see why inflation is not to be ignored. It does not tell you what to buy or what you will earn — it just does the doubling arithmetic quickly so you can think more clearly about the rest.