The Rule of 72: How Long Until Your Money Doubles

🔢 What the Rule of 72 is🔗

Divide 72 by your annual percentage return, and the answer is roughly how many years your money takes to double. That is the whole rule. At 6% a year, 72 divided by 6 is 12, so money doubles in about 12 years. At 9%, it is 8 years.

It works because compound growth is exponential rather than linear, and 72 happens to be a number that approximates that curve well while dividing neatly by 2, 3, 4, 6, 8, 9 and 12. You can do it standing in a queue, which is the point.

🇦🇺 Worked Australian examples🔗

The rule earns its keep when you apply it to real balances. These use illustrative return assumptions, not predictions, and they ignore extra contributions.

Put a number on it. A 30-year-old with $80,000 in a growth super option, adding nothing further, would on an 8% assumption see roughly $160,000 by 39, $320,000 by 48 and $640,000 by 57. The doublings are what does the work, and the last one is worth more than every earlier one combined.

🎯 The essential: This is the real argument for starting early. It is not that early money grows faster. It is that early money gets more doublings, and the final doubling is always the largest.

↩️ Running it backwards🔗

The rule works in reverse too, which is often more useful. If you know how long you have, divide 72 by the number of years to get the return you would need.

• Want to double in 10 years? 72 ÷ 10 = 7.2% a year.
• Want to double in 7 years? 72 ÷ 7 ≈ 10.3% a year.
• Want to double in 5 years? 72 ÷ 5 = 14.4% a year.

That last one is the useful reality check. A required return of 14.4% a year, every year, is far above what broad share markets have delivered over long periods. If a plan depends on it, the plan is fragile, and if a product promises it, that is worth a very careful read of the fund's actual details before anything else.

💸 The fee version, which stings🔗

Fees do not just shave a little off the top. They push out every doubling, and the effect compounds over a working life.

A single extra percentage point costs about a year and a half per doubling. Over a 40-year super balance that is roughly one doubling lost, and one doubling is half the final balance. Our fee impact calculator puts your own numbers through it, and how to choose an ETF covers what to look at beyond the headline fee.

📈 The inflation version🔗

Point the rule at inflation instead of returns and it tells you how quickly prices double, which is the same as how quickly cash loses half its buying power.

At 3% inflation, prices double in about 24 years. At 5%, about 14 years. That is the arithmetic behind the uncomfortable fact that money sitting in a transaction account earning nothing is not holding still, it is shrinking. Our guide to what inflation actually is covers how it is measured in Australia.

🎯 The essential: The number that matters for building wealth is your return after fees, tax and inflation. A 4% savings account against 3% inflation is a real return near 1%, which by the rule takes about 72 years to double in buying power.

⚠️ Where the rule breaks down🔗

• It assumes one steady rate. Real markets do not deliver 8% every year, they deliver a messy sequence averaging something. The rule gives you the shape, not the path.
• It ignores contributions. Most Australians are adding to super every payday, so real balances grow faster than the rule alone suggests.
• It drifts at the extremes. Reliable between roughly 5% and 12%. Outside that, use a calculator.
• It says nothing about risk. A higher assumed return means a wider range of possible outcomes, including bad ones. The rule cannot see that, which is why how the portfolio is built matters more than the arithmetic.

❓ Frequently asked questions🔗