Compound interest: the engine behind FIRE

Compound interest is the engine underneath every FIRE plan. It's the reason a modest monthly amount can turn into a life-changing sum — and the reason waiting a few years costs so much more than it feels like it should. Let's make it click, with a worked example you can actually feel.

What compound interest is

Compound interest is what happens when your returns start earning returns of their own.

Here's the difference. With simple interest, you only ever earn on the money you put in. With compound interest, this year's growth gets added to your pile — and next year, that growth earns too. Then its growth earns. It builds on itself, faster and faster.

A snowball is the classic image, and it's a good one. Roll a small snowball down a long hill and it doesn't grow steadily — it grows faster the bigger it gets, because a bigger ball picks up more snow with each turn. Your money does the same. The longer the hill, the more dramatic the finish.

Why time matters more than the amount

This is the single most important idea in investing, so I'll say it plainly: when you start matters more than how much you start with.

Compounding is exponential, not linear. The growth is slow and unglamorous at first — for years, most of your balance is just money you put in. Then a curve kicks in. The later years are where the real magic happens, because that's when growth is compounding on top of decades of previous growth.

Compound growthWhat your money grows on its own

Illustrative: a fixed amount invested every month for 30 years at 7% a year. The gold area is compound growth — returns on your returns.

Look at the shape of that curve. It's nearly flat early on, then it bends sharply upward. That's why an extra decade at the start is worth so much more than an extra decade of bigger contributions at the end. You can't buy back lost time later by saving harder — the missing years are exactly the ones that would have compounded the longest.

A worked example

Let's put numbers on it. All figures here are round and illustrative, and I'll assume a 7% average annual return — roughly the long-run real return of broad stock markets, though real years are far bumpier than any average.

Say you invest $300 a month — $3,600 a year.

After 10 years, you've put in $36,000. It's worth about $52,000.
After 20 years, you've put in $72,000. It's worth about $157,000.
After 30 years, you've put in $108,000. It's worth about $367,000.

Notice what happens. Between year 10 and year 20 you added $36,000 of your own money — but the balance grew by roughly $105,000. Between year 20 and 30 you again added $36,000, and the balance grew by around $210,000. Same contributions, wildly different growth. That extra $210,000 in the final decade is compounding doing the heavy lifting — most of it is growth on growth, not your deposits.

This is why the number can feel out of reach when you start and then, one day, suddenly doesn't.

The cost of waiting

Now the flip side, and it's the part worth tattooing on the inside of your eyelids.

Take two people, both investing $300 a month at 7%.

Anna starts at 25 and stops at 65. Forty years of compounding.
Ben starts at 35 and stops at 65. Thirty years.

Ben only started ten years later. He invests $36,000 less over his life — hardly a fortune. But at 65, Anna's pot is worth around $790,000 and Ben's around $367,000. Anna ends up with more than double, from a ten-year head start and $36,000 more invested.

That gap — over $400,000 — is the price of a ten-year delay. Not because Anna saved much more, but because her early dollars had the longest hill to roll down. The single most valuable thing you can do for future-you is start now, even if the amount feels embarrassingly small.

Which is really the whole message of FIRE: your money's best friend is time, and the way you put time to work is by investing early and consistently — most simply through broad index funds.

Frequently asked questions

How is compound interest different from simple interest? Simple interest pays you only on your original amount. Compound interest pays you on your original amount plus all the growth it has already earned — so it accelerates over time. For long-term investing, that acceleration is everything.

Does compounding work with stock market returns? Yes, in effect. Reinvested gains and dividends earn future gains, which is the same snowball at work. The difference from a fixed savings rate is that stock returns are uneven year to year — the average compounds, but the path is bumpy.

What return should I assume when planning? A common long-run assumption is around 7% a year after inflation for a stock-heavy portfolio. Treat it as a rough average, not a promise — some years are far higher, some are negative. Being a little conservative in your planning is wise.

Is it too late to start if I'm older? No. The best time was years ago; the second-best is today. You have fewer years to compound, so your own contributions carry more of the load — but starting now still beats waiting another year.