Stock Trading
Trading Costs: What Zero Commission Does Not Cover
Commission was a single line on a statement, and usually the smallest one. Everything that replaced it sits inside the execution price, where your frequency decides how much it matters.
Commission-free trading was a genuine improvement. It also produced a widespread and expensive belief that trading has become free, which is a rather different claim and one the arithmetic declines to support.
Commission was a single line item. It was often the smallest of the costs involved, and it had the distinction of being the only one that ever appeared on a statement, which is most of why its removal felt to a great many people like the end of the matter. Everything that replaced it is embedded in prices. That makes it easier to ignore. It does not make it smaller.
The costs that survived
1. The bid-ask spread
The largest cost most active traders carry. Also the one they consider least.
Every stock carries two prices at the same moment. The bid is the highest a buyer will pay. The ask is the lowest a seller will accept. Your purchase fills at the ask and your sale fills at the bid, which means you cross that spread twice on every completed round trip, once on the way in and once on the way out.
| Instrument | Typical spread | Cost per round trip |
|---|---|---|
| Mega-cap (e.g. a $200 index heavyweight) | $0.01 | ~0.01% |
| Liquid mid-cap at $50 | $0.02–0.05 | ~0.08% |
| Small-cap at $12 | $0.05–0.15 | ~0.8% |
| Thin OTC stock at $0.30 | $0.03 | ~10% |
The bottom row is accurate. A 10% round-trip cost means the stock has to rise 10% before you reach break-even, which is one reason penny stocks are a harder game than their low share prices suggest.
2. Slippage
The distance between the price you saw and the price you received. It appears when markets are moving quickly, when liquidity is thin, and when your order is large relative to the volume that normally trades.
A market order guarantees a fill at whatever price the book happens to offer. In a fast-moving stock, the distance between that price and the one on your screen can run to several percent. This is the main practical argument for making limit orders your default, since a missed entry costs nothing at all while a poor fill costs money immediately.
3. Payment for order flow
Market makers pay zero-commission brokers to route orders to them. Whether this meaningfully worsens execution remains a live debate, and I would not claim to settle it here. The structure itself is not in dispute. The service is free because your order flow carries value, and whatever you pay for it sits inside the fill price, where no statement will ever display it.
4. Tax
Frequently the largest cost of all for a profitable active trader, and the one most often left out of strategy calculations. In the US, gains on positions held under a year are taxed as ordinary income, while longer holds qualify for lower long-term rates, and the gap between the two can run to twenty percentage points or more.
A strategy returning 15% a year before tax may return considerably less once tax has been paid, and the after-tax figure is the only one that buys anything.
5. Currency conversion
Trade in a currency other than your own and your broker will typically charge a spread on the conversion, often 0.5% or more in each direction. On a US stock bought and sold from abroad, that single item can exceed every other cost put together.
What it does to an edge
Suppose you hold a genuine edge worth 0.3% per trade before costs, which is respectable.
| Trades a year | Gross | Cost at 0.1% | Net |
|---|---|---|---|
| 20 | 6.0% | 2.0% | 4.0% |
| 100 | 30% | 10% | 20% |
| 500 | 150% | 50% | 100% |
| 2,000 | 600% | 200% | 400% |
On paper, more trading looks better. Now raise the cost to 0.35% per round trip, which is entirely ordinary for small caps or for anything carrying a wide spread, and the 2,000-trade row falls to zero. The edge is the same. The skill is the same. Frequency alone removed the result.
Reducing costs, in order of impact
- Trade less. By some distance the largest lever. Most traders would improve their net results by halving their trade count and altering nothing else whatsoever about the way they select, enter or manage a position.
- Trade liquid instruments. Tighter spreads, less slippage, better fills.
- Use limit orders. You set the price you pay. The order waits until the market comes to meet it.
- Hold longer where the strategy allows. Lower frequency and, often, better tax treatment.
- Avoid trading the open. Spreads sit at their widest during the first few minutes of the session, while the market is still working out what the price ought to be.
- Check currency and inactivity fees. Small, recurring, and straightforward to eliminate.
Calculate what a trade netted after costs with the stock profit calculator, and read risk management for why the figure that decides whether a strategy works is net expectancy, which is the figure a backtest quietly leaves out.
Frequently asked questions
If my broker charges zero commission, is trading free?
No, and commission was often the smallest of the costs in the first place. You still pay the bid-ask spread on every round trip, you still suffer slippage when markets move quickly, and you still owe tax on your gains. For an active trader those three together usually come to rather more than commissions ever did.
What is the bid-ask spread and how much does it cost me?
It is the gap between the highest price a buyer will pay and the lowest price a seller will accept. You buy at the ask and sell at the bid, so you cross that gap twice on every complete trade. On a liquid large cap it might be 0.01%. On a thin small cap it can exceed 3% per round trip, which is larger than most traders' expected profit per trade, and that comparison is the one worth dwelling on.
What is slippage?
The difference between the price you expected and the price you actually received. It widens when markets move fast, when liquidity is thin, and when your order is large relative to the volume that normally trades. Market orders are far more exposed to it than limit orders, which is the main argument for making limit orders your default.
How does payment for order flow affect me?
Your broker routes your order to a market maker, who pays for the privilege, and that is how zero-commission brokers make their money. Whether you end up with a meaningfully worse price than you would have got elsewhere is genuinely disputed. The undisputed part is that the arrangement is the reason the service appears free, and that whatever you pay for it sits inside the execution price where no statement will show it.
Do trading costs really matter that much?
They scale with how often you trade, which is exactly why they fall hardest on active traders. A cost of 0.1% per round trip is irrelevant at ten trades a year and decisive at a thousand. The variable that determines whether costs consume your edge is frequency, and cost per trade is only half the calculation.