What is the Rule of 72?

The Rule of 72 is a mental-maths shortcut for working out how long an investment takes to double. You divide 72 by the annual return percentage: at 7% a year, 72 ÷ 7 ≈ 10.3 years to double; at 9% it is about 8 years. You can flip it around too — divide 72 by the years you have to estimate the return you would need. It works because of compounding (earning returns on your returns), and it is accurate enough to do in your head without a spreadsheet.

How accurate is the Rule of 72?

The Rule of 72 is an approximation, and it is most accurate for returns between roughly 6% and 10% — the range most diversified investments fall into. At 8% it gives 9.0 years versus an exact 9.0 years (a near-perfect match). For very low returns (say 2%) it slightly overstates the time, and for very high returns (say 20%) it slightly understates it. For precise figures the calculator above also shows the exact doubling time using the formula ln(2) ÷ ln(1 + r).

How do I double my money faster?

Because the doubling time is driven by your return, even small increases in return cut the time sharply — going from 6% to 9% takes the doubling time from 12 years to about 8 years. In practice that means keeping costs low (fees directly reduce your net return), staying invested so compounding can work, and not letting cash sit in low-interest accounts when your goal is long-term growth. There is no shortcut without taking on more risk, so match your investments to your timeframe and risk tolerance. General information only — not financial advice.

Rule of 72 Calculator

Enter your expected annual return to see how long it takes to double your money. Add a starting balance to see the dollar value at each doubling, and compare the Rule of 72 estimate against the exact compounding figure.

Last verified: 5 May 2026

How long does it take to double my money?

The Rule of 72 says your money doubles in roughly 72 ÷ your annual return years. At 7% that's about 10.3 years; at 9% about 8 years; at 6% about 12 years. It works because of compounding, and it's accurate enough to do in your head. The rule is most reliable for returns between about 6% and 10% — outside that band, use the exact figure (ln 2 ÷ ln(1 + r)) the calculator also shows. Source: ASIC MoneySmart.

Worked example. You invest $10,000 at a 7% annual return. 72 ÷ 7 ≈ 10.3 years to reach $20,000. Leave it another 10.3 years and it doubles again to $40,000, then $80,000, then $160,000 — four doublings in roughly 41 years. That compounding is why starting early matters far more than picking the perfect investment.

General information and estimates only — not financial, tax, or legal advice. Always verify with a licensed adviser or the ATO.

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