In Detail

A fill occurs when a trading order is matched with a counterparty and executed at a specific price. Understanding fills is essential because the price you see on your screen is not always the price you get — the quality of your fill directly impacts your profitability. There are two types of fills: complete fills (your entire order executes at once) and partial fills (only a portion of your order executes, which happens when there is not enough liquidity at the desired price to absorb the full order). Market orders fill immediately at the best available price, but in fast-moving markets that price can differ substantially from what you saw when you clicked the button — this difference is called slippage. Limit orders guarantee your price or better but may never fill if the market does not reach your price. The quality of a fill depends on several factors: the liquidity of the asset (heavily traded stocks fill more easily than thinly traded ones), the order size relative to available volume, market conditions at the time (volatile moments produce worse fills), and the execution quality of your broker or exchange. Professional traders track their fill quality meticulously because even small differences in execution price compound significantly over hundreds or thousands of trades.

How to Use in Trading

Always review your actual fill prices after a trade executes and compare them to the price you intended. If you consistently experience poor fills (slippage of more than a few cents on liquid stocks), investigate whether your broker is providing best execution or routing your orders to market makers who fill at inferior prices. For better fill quality, use limit orders instead of market orders whenever possible — you control the exact price and avoid unexpected slippage. When entering a position in a fast market, consider using a limit order slightly above the current ask (for buys) or below the current bid (for sells) to get filled quickly while still capping your worst-case price. For large orders that might move the market, break them into smaller pieces executed over time (this is called working an order), use iceberg orders that only display a portion of the total size, or use a VWAP or TWAP algorithm to spread execution across the session.

Example

You want to buy 500 shares of a mid-cap stock trading at $42.50 (bid) / $42.55 (ask). With a market order, you might get 200 shares at $42.55, 200 at $42.58, and 100 at $42.62 — your average fill is $42.575, which is $0.025 worse than the initial ask price. That is $12.50 in slippage on a single trade. Instead, you place a limit order for 500 shares at $42.56 — one cent above the current ask. Over the next 30 seconds, all 500 shares fill at $42.55 or $42.56 as sellers come to your price, giving you an average fill of $42.555. By using a limit order, you saved $10 in execution cost on this single trade. Over 200 trades per month, that discipline saves $2,000 per year in fill quality alone.

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Related Terms

Market Order Limit Order Slippage

More Trading Terms

Ask Price

The lowest price a seller is willing to accept for an asset. Also called the offer price.

ATH (All-Time High)

The highest price an asset has ever reached in its trading history.

ATL (All-Time Low)

The lowest price an asset has ever reached in its trading history.

Bid Price

The highest price a buyer is willing to pay for an asset.

Breakout

When price moves above resistance or below support with increased volume, potentially signaling a new trend.

Definition