The Rule of 72: Why the Same Latte Costs More at 25 Than at 45

The Rule of 72 is a quick way to estimate how long it takes money to double at a given rate of return. Divide 72 by the annual return, and the answer is roughly the number of years to double. At a 7% average return, that's 72 ÷ 7, or about 10 years. Money doubles roughly every 10 years, then doubles again from that new, larger amount, and again after that.

The part people miss is what that means for a dollar spent today versus a dollar spent later. The same purchase, at the same price, does not cost the same thing in real terms depending on how many of those 10-year doubling windows are still ahead of it. A dollar spent at 25 has given up far more doublings than the identical dollar spent at 45.

I've watched this play out in real time. The last several years have been a strong market, and plenty of people have seen their money roughly double since the COVID-era lows. There's no guarantee that continues, but it's a useful live example of the Rule of 72 doing exactly what the math says: given enough years and a high enough return, doubling isn't rare. It's just what compounding looks like on a normal timeline, roughly a decade at a time.

The same habit, 3 different ages

Take a $5 coffee, 5 days a week. That's about $1,300 a year. Now imagine, just once, taking that $1,300 and investing it instead of spending it, then leaving it alone until age 65. The number it grows to depends entirely on how many doublings are left between the age you invest it and retirement.

The same $1,300 habit is worth roughly 4 times more, in the money it gives up, at 25 than it is at 45. At 35, it's still worth about twice what it is at 45. Nothing about the coffee changed. What changed is how many doubling periods that money had left to work with before retirement.

This is the same logic behind why starting to invest early matters more than almost any other single decision, just applied to the spending side instead of the saving side. A dollar not spent at 25 isn't worth a dollar at 65. It's worth whatever that dollar could have doubled into, roughly 4 times over, given a 40-year runway.

Why this matters more than the math on any one purchase

No single latte, ever, changes anyone's financial trajectory. The Rule of 72 isn't really about coffee. It's a mental shortcut for seeing that time is the scarcest input in compounding, scarcer than the amount of money itself in many cases. Someone who understands that 20s dollars have more doublings ahead of them than 40s dollars is better equipped to decide, deliberately, which small recurring costs are worth keeping and which aren't, at whatever age they're making that call.

Frequently Asked Questions

What is the Rule of 72 and how do you use it?

The Rule of 72 is a quick way to estimate how long it takes money to double at a given rate of return. Divide 72 by the annual return, and the answer is roughly the number of years to double. At a 7% average return, that's 72 divided by 7, or about 10 years. Money doubles roughly every 10 years, then doubles again from that new, larger amount.

Why does the same purchase cost more when you're younger?

A dollar spent today gives up every doubling period between now and when you'd have needed that money. A $5 coffee, 5 days a week, comes to about $1,300 a year. That same $1,300 habit is worth roughly 4 times more, in money given up, at 25 than it is at 45, because more 10-year doubling windows sit between 25 and retirement than between 45 and retirement.

Is it realistic for money to actually double every 10 years?

At a 7% average annual return, the Rule of 72 puts the doubling time at about 10 years, and real market periods have shown this happening. Money has roughly doubled for plenty of investors since the COVID-era lows during the recent strong market. There's no guarantee that continues, but it shows doubling isn't rare given enough years and a high enough return.