Rule of 72: How to Compare Two Investments in Seconds

Why Doubling Time Beats Percentage Returns for Quick Decisions

A 7% return and a 9% return sound close together, but they produce very different outcomes over a decade. The Rule of 72 makes that gap visceral: divide 72 by the rate and you get the years it takes to double your money. At 7%, your money doubles in about 10.3 years. At 9%, it doubles in 8 years.

That 2.3-year gap might not sound dramatic until you realize it means an extra full doubling cycle over a 30-year horizon. Instead of three doublings at 7% (roughly 8x your money), the 9% earner fits in nearly four (roughly 16x). A number like '9% vs 7%' is abstract; 'doubles in 8 years vs 10 years' is something you can actually picture.

A Worked Side-by-Side Comparison You Can Do Right Now

Say you are choosing between a balanced mutual fund averaging 6.5% annually and an index fund averaging 8.2% annually. Divide 72 by 6.5 and you get a doubling time of about 11.1 years. Divide 72 by 8.2 and you get roughly 8.8 years. That 2.3-year difference means if you invest at 30, the index fund doubles your money before your 39th birthday; the balanced fund doesn't get there until you're nearly 41. Try the doubling time calculator to see your own numbers.

Run this for every option on your shortlist and rank them by doubling time. The fund with the shortest doubling time wins on raw compounding power, assuming equal risk. This is a pre-screening step, not a final verdict, but it eliminates the weakest candidates fast. A rule-of-72 doubling time calculator does the arithmetic instantly, so you can run a dozen scenarios in the time it used to take to open a spreadsheet.

Where the Shortcut Quietly Breaks Down

The Rule of 72 assumes a fixed, annual compounding rate. Real fund returns are not fixed. A fund that averaged 8% over ten years might have posted minus 20% in year three and plus 25% in year four. Sequence matters because losses compound too. The rule gives you the average-case trajectory, not the actual path.

The rule also ignores fees. A fund advertising 8% gross with a 1.2% expense ratio is really delivering 6.8% to your account. Run the rule on the net figure, not the headline number. At 8%, doubling takes 9 years. At 6.8%, it takes 10.6 years. That 1.6-year difference is entirely the cost of the fee, and it compounds just as ruthlessly as the return does.

For a clean, repeatable way to stress-test any rate scenario before committing capital, the doubling time calculator at this site handles the math and lets you swap numbers freely. Treat it as a filter, not a forecast.

Using Doubling Time to Set a Minimum Acceptable Rate

Here is a practical way to flip the rule around. Decide first how many years you have until you need the money. Say it's 15 years. Divide 72 by 15 and you get 4.8. That means any investment returning less than about 4.8% per year will not double your money in your window. Use that number as your floor when evaluating options.

This approach is especially useful when comparing a guaranteed instrument like a Treasury bond to an equity fund. If a 5-year Treasury yields 4.7% and you have a 15-year horizon, the rule tells you it doubles once in 15.3 years. An equity fund at 9% doubles twice in roughly 16 years. Suddenly the choice is not '4.7% versus 9%'; it is 'one doubling versus two doublings.' That reframe changes the conversation entirely.