Rule of 72

The Rule of 72 is one of the most useful quick-reference tools in personal finance, allowing investors to rapidly estimate the doubling time of any investment without a calculator. The rule works because 72 is evenly divisible by many common interest rates, and the approximation is remarkably accurate for rates between about 2% and 20%.

If your portfolio earns 6% per year, 72 divided by 6 equals 12 — your investment doubles roughly every 12 years. At 8% annual returns, money doubles every 9 years (72 / 8 = 9). At 4%, doubling takes 18 years. At 12%, just 6 years. These simple calculations make the long-term impact of return differences and the power of time instantly intuitive.

The rule works in reverse as well, and this version is often more sobering. Inflation erodes purchasing power through the same compounding dynamic. If inflation runs at 3% per year, 72 / 3 = 24 years for your purchasing power to be cut in half. At 6% inflation, purchasing power halves in just 12 years. This illustrates why keeping too much money in a low-yield savings account earning 1% — when inflation runs higher than that — represents a slow, invisible loss of real wealth.

The Rule of 72 also applies to debt. Credit card debt at 18% interest doubles in just 4 years (72 / 18 = 4). A $5,000 credit card balance carried without payment would grow to $10,000 in four years, $20,000 in eight years, and $40,000 in twelve years — a financially devastating outcome that the quick mental math makes immediately clear.

For investors comparing options, the rule sharpens decision-making. The difference between a 6% and 8% annual return might seem modest, but the 12-year doubling cycle versus the 9-year cycle means an extra doubling roughly every 36 years — turning $100,000 into $200,000 versus $400,000 over that same span. Small return differences, compounded over long periods, produce massive wealth divergence.