SERIES 02: BEGIN WITH THE END IN MIND

The Rule of 72 Is the Only Math You Actually Need

I’m going to teach you one piece of math. One. And it’s going to change how you evaluate every financial decision you make for the rest of your life. No exponent keys, no spreadsheets, no calculator apps. Just division.

Take 72. Divide it by your annual return. The answer is roughly how many years it takes for your money to double.

That’s it. That’s the whole thing.

How it works in practice

Earning 6% on your investments? 72 ÷ 6 = 12 years to double. Got a high-yield savings account at 4%? 72 ÷ 4 = 18 years. Found a fund returning 10% over a long stretch? 72 ÷ 10 = 7.2 years.

Now chain the doublings. You invest $10,000 at 7% annual return. After about 10 years, you’ve got $20K. After 20, $40K. After 30, $80K. After 40 years — the span of a working career — that $10,000 has become $160,000 without you adding another cent. Sixteen times your original investment from one deposit and four decades of patience.

The rule isn’t exact. It’s an approximation that gets a little fuzzy above 15% or below 3%. But for the range most people care about — savings accounts on the low end, market returns on the high end — it’s accurate enough to make decisions with.

The flip side: how fast debt doubles

The rule works on anything that compounds. Including the things compounding against you.

Credit card at 18% APR? 72 ÷ 18 = 4 years to double. A $5,000 balance you ignore for 4 years becomes $10,000. Eight years, $20,000. From the same original purchase. That jacket or that weekend trip or whatever it was — it’s now four times what it cost, and you don’t even own it anymore.

Try it with a car loan. A personal loan. A “buy now pay later” balance. The rule makes the cost of carrying debt viscerally obvious in a way that a monthly statement never does.

Why this matters more than budgeting tips

Most financial advice focuses on the monthly view. Cut this expense, track that category, move $50 from dining to savings. And that stuff matters. But the rule of 72 reframes the whole game. It zooms out.

When you’re standing in a store debating a $300 purchase, the rule lets you instantly calculate what that $300 could become. At 7%, it doubles in 10 years. So that $300 is really $600 in a decade. $1,200 in twenty years. $2,400 in thirty. You’re not spending $300 — you’re spending the future value of $300. [See: Compound Interest Is Just a House Party That Got Out of Hand]

You don’t have to do this with every purchase. That would make you insufferable at brunch. But for the big recurring ones — the car payment, the apartment upgrade, the subscription stack — running the rule of 72 takes three seconds and might save you six figures over a career.

A dinner party trick that’s actually useful

Next time someone asks what you’d do with a million dollars, you can answer with uncomfortable precision. At 7%, a million doubles to two million in about 10 years. Retire at 55 with a million, and by 65 it’s two million — assuming you live off other income or draw conservatively. The money doesn’t stop growing just because you stopped working.

Flip it around: how much do you need to invest today to have a million at 65? If you’re 25, you’ve got 40 years. Four doublings at 7%. Work backwards: a million divided by 16 is about $62,500. Invest $62,500 at 25 and leave it alone, and the rule says you’ll cross a million around 65. That’s a single lump sum. Combine it with monthly contributions and the picture gets a lot more interesting. [See: What If You Started Investing at 22 vs 32?]

Use it on your own numbers

The rule of 72 is the napkin math. iF is the full picture. Plug in your current savings, your monthly contribution, your expected return, and watch the doubling happen in real time on a growth curve. The rule tells you the rough shape. iF shows you the exact trajectory with your life’s variables baked in.

Bottom line: 72 ÷ rate = years to double. Works for investments. Works for debt. Works on a napkin at dinner. It’s the one formula worth memorizing because it turns abstract percentages into something you can actually feel.