“Commission-free trading” sounds like a gift. And in a lot of ways, it is — you can buy and sell stocks today without handing over $10 every time you click a button, something that would have seemed unthinkable twenty years ago. But here’s the question that doesn’t get asked enough: if online brokers aren’t charging you a commission, how do online brokers make money? Spoiler: they’re not running a charity. Understanding how online brokers make money is one of the most useful things you can learn as an investor, because it explains why the platform you’re using behaves the way it does.
The Short Answer: Online Brokers Make Money in More Ways Than You Think
Most people assume online brokers make money purely from trading fees. That used to be true, but the business model has evolved dramatically. Today, online brokers make money through a mix of revenue streams — some visible, some not so visible — including payment for order flow, interest on cash balances, margin lending, subscription fees, and charges on niche products like options and futures. No single online broker relies on just one of these. They stack several together, and the exact mix is what separates one broker’s business model from another’s.
Payment for Order Flow: The Engine Behind “Free” Trading
If there’s one concept that explains how modern online brokers make money, it’s payment for order flow, commonly shortened to PFOF. Here’s how it works: when you place a trade, your online broker usually doesn’t send it straight to the New York Stock Exchange or Nasdaq. Instead, it routes your order to a market maker — a firm like Citadel Securities or Virtu Financial that specializes in filling retail orders. That market maker pays the broker a small fee for the privilege of executing your trade, then profits from the tiny gap between the buying price and the selling price, known as the bid-ask spread.
This is genuinely one of the biggest reasons commission-free trading exists at all. Online brokers make money on the back end from market makers instead of charging you directly, which is why your $0-commission trade still generates real revenue somewhere in the chain. The payment itself is usually just fractions of a cent per share, but multiply that across millions of trades a day, and it becomes a substantial income stream. Industry-wide PFOF payments have run into the billions annually in the US alone, which tells you just how central this revenue model has become to how online brokers make money at scale.
It’s worth noting this practice isn’t universal or uncontroversial. The UK banned it back in 2012, and the European Union implemented an EU-wide ban that took effect in mid-2026. Interestingly, several EU brokers responded by becoming their own market makers instead — internalizing trades and earning the bid-offer spread directly rather than collecting a PFOF kickback from a third party. The revenue model shifted shape, but the underlying idea — profiting from the mechanics of executing your order — didn’t disappear.
Net Interest Income: Making Money While Your Cash Sits Still
Here’s a revenue stream that often flies under the radar: net interest income. When you deposit cash into your brokerage account and don’t immediately invest it, that money doesn’t just sit there doing nothing for the broker. Online brokers typically sweep uninvested cash into interest-bearing accounts or short-term securities, earning interest on your balance while often paying you a much lower rate — or none at all — in return.
This is one of the quieter but more reliable ways online brokers make money, especially in periods of higher interest rates. For brokers with large numbers of users holding uninvested cash, this line item alone can generate billions of dollars annually. It’s also one reason brokers encourage you to keep a cash cushion in your account rather than transferring every spare dollar out immediately.
Margin Lending: Charging Interest on Borrowed Money
If you’ve ever used margin to trade — essentially borrowing money from your broker to buy more securities than your cash balance would normally allow — you’ve contributed directly to another core way online brokers make money. Margin loans carry interest rates that are often well above what the broker itself pays to access that capital, and the spread between those two numbers is pure profit. For active or leveraged traders, margin lending can be one of the most lucrative parts of a broker’s entire business model.
Fees on Specialized Products
While stock and ETF trades are frequently commission-free, that’s rarely the whole story. Online brokers still make money by charging for options contracts, futures trades, mutual fund transactions, and bond purchases — categories where per-contract or per-trade fees remain common even at brokers that heavily advertise “free trading.” Premium research tools, advanced trading platforms, and managed portfolio services often come with subscription or advisory fees as well, adding yet another layer to how online brokers make money beyond the basic trade execution business.
Why This Matters for You as a Trader
Understanding how online brokers make money isn’t just trivia — it directly affects the quality of your trade execution. Because payment for order flow creates a financial incentive to route your order to whichever market maker pays the most, rather than whichever one offers the best price, some critics argue this creates a built-in conflict of interest. In practice, regulations require execution at or better than the National Best Bid and Offer, so most retail traders see only a marginal difference. But for high-volume or options traders, those marginal differences can add up meaningfully over a year of active trading.
The bigger picture is this: no online broker gives away its services for free. Whether the revenue comes from payment for order flow, interest income, margin lending, or product fees, every broker’s business model is built to be profitable. That’s not a red flag — it’s just business. What matters is transparency. The best online brokers make money in ways that are clearly disclosed, don’t quietly work against your interests, and still leave you with a genuinely lower cost of trading than the commission-heavy world of twenty years ago.
The Bottom Line
Online brokers make money in more layered ways than most traders realize, and “commission-free” almost never means “revenue-free” for the platform. Payment for order flow, interest on cash, margin lending, and product-specific fees all combine to keep these businesses profitable. Knowing exactly how your online broker makes money — and reading its Rule 606 order-routing disclosures if you’re in the US — is one of the simplest ways to make sure the platform you’re trading on is actually working in your favor.
