Rule of 72: how long it takes to double your money

The rule in one line

Years to double ≈ 72 ÷ annual return (in %).

At 6% a year, money doubles in about 12 years. At 12%, about 6. No calculator needed, which is the point: it lets you compare an FD, a PPF account and an equity fund in your head.

How accurate is it?

Very, in the range most savers deal with. The exact doubling time is ln 2 ÷ ln(1 + r), and here is how the rule compares:

Between about 6% and 10% the rule is almost exact. At higher rates it runs slightly fast, so at 18% and above, 76 or 78 gives a closer answer. For everyday planning, 72 is close enough.

Tripling and quadrupling: 114 and 144

The same idea extends:

• Rule of 114: years to triple ≈ 114 ÷ rate. At 12%, about 9.5 years (exact: 9.7).
• Rule of 144: years to quadruple ≈ 144 ÷ rate. At 12%, 12 years, which is also just two doublings of 6.

Five ways to use it

1. Comparing what you own. A savings account at 3% doubles your money in 24 years; a 7% FD in about 10; an equity fund compounding at 12% in about 6. Over a 30-year career that is the difference between one doubling and five. A gap of a few percentage points sounds small; in doublings it is enormous.

2. Seeing inflation. The rule works on prices too. At 6% inflation, the cost of living doubles in about 12 years. That means ₹1 lakh of monthly expenses today becomes about ₹2 lakh in 12 years and ₹4 lakh in 24. The inflation calculator gives exact figures, and how inflation eats your savings works through what it does to cash.

3. Seeing the cost of debt. Credit card interest in India often runs around 3.5% a month. At that rate an unpaid balance doubles in about 20 months. That is why clearing card debt comes before any investment; see snowball vs avalanche.

4. Measuring a real track record. Over the ten years to September 2026, the Nifty 50 returned about 11.5% a year with dividends, by our estimate. The rule says that should double money in about 6.3 years, so ten years is about 1.6 doublings. And indeed ₹10,000 became about ₹29,800, nearly three times. Our post on Nifty 50 returns has every period.

5. Seeing the cost of fees. A 1% annual cost on a 12% return lowers the rate to 11%, stretching the exact doubling time from 6.1 to 6.6 years. Across 30 years that leaves the final corpus about a quarter smaller. The impact of 1% calculator shows it in rupees.

Where the rule misleads

It assumes a steady rate. Equity returns arrive in lumps, with bad years in between. The Nifty fell 51.79% in 2008 and rose 75.76% in 2009; the rule of 72 knows nothing about that path. Use it on long-run averages, not on next year.

It ignores tax. Use the after-tax rate. An FD at 7% for someone in the 30% bracket is really about 4.9%, which takes about 14.5 years to double, not 10.

It ignores inflation. Doubling your rupees is not doubling what they buy. Subtract inflation first: a 12% return with 6% inflation is roughly a 6% real return, so your purchasing power doubles in about 12 years, not 6.

It does not apply to SIPs. The rule describes one lump sum left alone. In a SIP each instalment has a different time to grow, so use the SIP calculator instead. For goal planning the years-to-goal calculator works backwards from a target.

Running it backwards

The rule also answers the reverse question: what return do I need to double my money in N years? Divide 72 by the years. To double in 8 years you need about 9% a year; in 5 years, about 14.4%, which is beyond what any low-risk product offers. Anyone promising to double your money in two or three years is promising 24–36% a year. Our compound interest calculator and the guide to the magic of compounding take it further.

This article is for education, not investment advice. Returns shown are illustrations; actual returns vary and are not guaranteed.