What is compounding, and why does it matter?

Compounding is what happens when the returns you earn start earning returns of their own. That single feature — growth feeding on itself — sits underneath every other idea in investing, and it's why time, not the size of your first deposit, is the most powerful lever you have. Left at an 8% annual return, $10,000 becomes about $100,000 over 30 years — a tenfold gain, most of it arriving in the final decade. Everything else here is about how to get onto that curve and stay on it.

What is compounding, in plain terms?

It's earning a return on your returns, not just on the money you first put in. Picture $100 growing at 10% a year: after year one you have $110, and that $10 is your return. In year two you earn 10% on the whole $110 — not the original $100 — so you gain $11 and reach $121; year three takes you to $133. The gain grows every year because the base it works on grows, and stretched over decades that difference between "a return on the original" and "a return on everything so far" is what turns a straight line into a sharply rising curve.

How fast does money double? The Rule of 72

Divide 72 by your annual return — that's roughly how many years it takes to double. The Rule of 72 is the one piece of mental arithmetic worth memorising: at 7% a year money doubles in about ten years, at 10% in about seven, at 15% in about five. It works backwards too, on anything that eats a percentage every year — 3% inflation halves your cash's purchasing power in about 24 years (72 ÷ 3), and a fund fee grinds down your wealth by the same math. One number, and you can size up any rate in your head.

Why does starting early beat starting big?

Because compounding rewards time far more than size, and the final years do the heavy lifting. Follow that $10,000 at 8%: about $21,600 after ten years, $46,600 after twenty, and $100,600 after thirty. The last decade alone adds around $54,000 — more than the first two decades combined.

That is the uncomfortable truth about patience: the years that feel slowest are the early ones, and the largest gains show up only for the people who don't quit. A 25-year-old investing a modest sum is working the 30-year column; a 45-year-old investing five times as much is working the 10- or 20-year column — a head start no plausible difference in deposits fully closes. (The full multiplier table is on the compounding chart.)

Do costs really compound too?

Yes — every recurring fee runs through the same machine, in reverse. A 1.5% annual fee doesn't cost you 1.5%; it permanently downgrades your growth rate. An investor earning the market's 8.5% but paying 1.5% is really compounding at 7% — and over 30 years that is the difference between 11.6 and 7.6 times your money, roughly a third of the end wealth handed to costs. This is why keeping fees low (see how to start investing) is not penny-pinching — it is protecting the rate the whole machine multiplies.

What compounding looks like in a real portfolio

It stops being theoretical the moment it runs on real money over real years. The portfolio has compounded in public since 2020, every full year in profit, with every position visible and the full six-year curve published against the S&P 500. Rather than quote a figure that moves with the market, the compound-interest calculator is pre-loaded with the portfolio's real CAGR against the S&P 500, so you can see what that live edge turns into over ten, twenty or thirty years — the same curve as above, running on actual results instead of round numbers. Compounding is not a promise about the future; it is the mechanism, and the live record is what it looks like when you let it run.

The takeaway is short enough to act on: start as early as you can, keep costs off your rate, and then do the hardest thing in investing — leave it alone. If you're right at the beginning, how to start investing is the practical next step; the principles the whole portfolio runs on are set out on the philosophy page.