What is the Rule of 72?

Divide 72 by your annual percentage return to estimate how many years it takes to double your money — 72 divided by 9% gives roughly 8 years.

How do fees affect the years to double my money?

A 1.5% annual fee on a 9% gross return cuts the real rate sharply, extending the doubling time by several more years than the rule suggests.

Investing

Years to Double: CAGR, Rule of 72 and Hidden Fees

5 min readInvesting

There is a tidy party trick that says it takes about eight years to double your money. It leans on the Rule of 72, it ignores your actual fees, and it will comfortably oversell the truth. The honest version goes through CAGR and the costs the rule never charges you.

The shortcut: 72 divided by your rate

The Rule of 72 estimates doubling time: divide 72 by your annual rate. At 9% a year, 72 ÷ 9 ≈ 8 years. At 6%, closer to 12. It is a quick mental frame — fine for a coffee table, dangerous for a retirement plan.

Where the rule starts lying

The rule assumes a constant, fee-free rate and a lump sum held untouched. Real investments carry fees, the rate wobbles, and many people add or pull cash. Each of those assumptions, once violated, shifts the true doubling date. The annualized view computes the real CAGR from final value and years, so you are reading an actual number instead of a rule of thumb.

"The Rule of 72 is an estimate wearing a promise. The most expensive twelve words in investing are 'it should roughly double by then'."

Fees are a silent, compound enemy

A 9% gross return might look like your number, but a 1.5% annual fee leaves roughly 7.4% net. Now doubling takes about 9.7 years, not 8 — over a decade later. Carry that over a long career and the gap is measured in years of your life, not points of return. This is exactly the drag the inflation-adjusted real rate is built to expose.

Inflation makes the honest number sting more

Subtract 3% inflation from a 7.4% net rate and the real annual return shrinks toward 4.3%. Doubling in real purchasing power stretches past 16 years. That is the number that matters for a goal to buy a house or retire — the money you can actually spend, not the digits in a brokerage statement.

Run it backwards from your goal

Instead of asking "when does it double," ask "what do I need to double by a date." Plug your target final value, today's balance and the years into the calculator and read the CAGR required. If it exceeds what your asset class realistically pays after fees and inflation, something has to change — the contribution, the goal, or the expectation. The shortcut told you a comforting story; the calculation tells you the plan.