Why Your Trade Filled at a Different Price—and What It Costs You

The price you saw is not the price you got

You click buy. The screen says one price. The confirmation says another, slightly worse. That gap has a name—slippage—and it is the difference between the price you expected for a trade and the price at which it actually executed. It can go either way: sometimes you pay a little more, sometimes a little less. Traders tend to remember the painful direction.

Here is the useful reframe. Slippage is not a platform malfunction or a random insult. It is a variable cost of doing business, sitting alongside spreads and commissions, and it should be budgeted for rather than treated as bad luck. The moment you accept that, the interesting question changes from "why me?" to "which of the three causes just bit me?"

Because there are three, and they behave very differently: latency, liquidity and volatility. Untangling them is called slippage attribution, and it is the difference between adjusting your execution and adjusting your expectations.

Cause one: latency, the time your order spends travelling

Latency is the delay between sending an order and having it reach execution. It is not slippage itself—latency is the delay, slippage is the price difference—but delay creates the opportunity for prices to move while your instruction is in transit.

The journey is longer than it feels. Your order leaves your platform, crosses your internet connection, reaches the broker's server, gets processed, is matched against available liquidity, and the result travels back. A slow connection, unstable Wi-Fi, an overloaded platform or simple physical distance can each add time. One educational guide estimates home internet can add roughly 50–200 milliseconds, while a properly positioned virtual private server (VPS)—a rented machine in a data centre near the broker—can bring that down to single-digit milliseconds.

For a long-term investor, milliseconds are a rounding error. For someone scalping tiny moves, they are the whole game.

Cause two: liquidity, or whether anyone is on the other side

Liquidity means how much trading interest sits at each price. A displayed quote is not a promise to trade unlimited size at that level; it is closer to a shelf that may already be half empty by the time you reach it. When your order is bigger than the volume available at the best price, the matching engine walks the book—filling the remainder at the next levels, then the next, producing an average price worse than the headline you saw.

This is where size matters, and not politely. Market impact grows disproportionately with order size, which is why large orders get sliced into smaller pieces over time. Two common schedules are TWAP (time-weighted average price), which spreads an order evenly across a period, and VWAP (volume-weighted average price), which leans into moments when the market is naturally busier.

Liquidity also has a calendar. Quiet hours, market opens and closes, holidays and the daily rollover between sessions all thin the book. A narrow spread on screen can still hide shallow depth—the quote looks inviting until you try to use it.

Cause three: volatility, when price simply outruns everyone

Volatility is rapid price movement, and it can cause slippage even when your connection is fast and your order is small. Around major economic releases, liquidity providers widen spreads or pull quotes to protect themselves precisely when everyone wants to trade. Prices can skip levels entirely. Weekend gaps do the same thing on Monday mornings, absorbing news released while markets were shut.

Scheduled data releases are the classic example, and the conditions are well documented: during high-impact events, spreads can widen sharply from normal levels, and stop-loss orders—which become market orders once triggered—may fill well beyond the intended level. A stop at one price filling several pips worse turns a planned loss into an unplanned one. That is not a broker conspiracy; it is a market with nobody obliged to trade at your level.

Separating the three: the attribution question

So how do you tell which cause produced a particular bad fill? You measure, using benchmarks. Arrival price—the price when your order entered the system—is the natural starting point: it captures everything that happened after your decision. VWAP over the execution window is another. Transaction cost analysis (TCA) is the formal name for comparing actual fills against these yardsticks.

Slippage itself can be split into components: price slippage (actual versus expected price), timing slippage (actual versus intended execution time) and market impact (price movement your own order caused). If your fill was worse mainly because time passed, blame latency. If it was worse because your order consumed several price levels, blame size against available liquidity. If the whole market jumped, blame volatility.

Each diagnosis points somewhere different. Latency suggests infrastructure. Liquidity suggests order type, sizing or timing. Volatility suggests you were trading in a window where friction is simply the price of admission.

What actually helps—and what it costs

Market orders prioritise getting filled over getting a good price, which makes them the most slippage-prone choice in fast conditions. Limit orders only execute at your price or better, capping negative slippage but accepting the real risk of no fill at all. Stop-limit orders add a price boundary after a stop triggers, though they can leave you stranded if price leaps past. There is no free lunch here, only a trade-off between certainty of execution and certainty of price. Some platforms also offer a maximum deviation setting, which rejects a fill beyond a tolerance you set.

Beyond order types: trade when participation is naturally deeper, keep position sizes honest relative to available liquidity, and track expected versus actual fills by session, instrument and order type. A record of your own fills is more honest than any assumption—and notably more honest than a backtest that assumes perfect execution, which is a bit like practising swimming on a dry kitchen floor.

The honest limits

Slippage cannot be eliminated. It can be measured, budgeted, and reduced at the edges, but fast markets will always find a way. Be sceptical of anyone promising perfect fills. Be equally careful with rules and platform features, which vary by jurisdiction, broker and venue—what applies to one market structure may not apply to yours, and nothing here is a recommendation about what you personally should trade or how.

The goal is modest and worthwhile: know which of the three effects is costing you, and stop paying for the wrong one.