Fill
What Is a Fill?
A fill is the action of satisfying an order to buy or sell a financial asset. It represents the actual execution of the trade. The "fill price" is the price at which the execution actually occurred.
In trading, placing an order is just a request. A "fill" is the confirmation that the request has been completed. Until an order is filled, you do not own the stock (if buying) or have the cash (if selling). The fill is the moment of truth where the theoretical plan meets market reality. • Partial Fill: If you order 1,000 shares but there is only a seller for 400 shares at your price, you get a partial fill of 400. The remaining 600 stay as an open order. • Fill Price: This is critical. You might see a stock trading at $100 and hit "Buy Market," but if the price jumps effectively instantly, your fill might be at $100.05.
Key Takeaways
- A fill occurs when a buyer is matched with a seller.
- An order can be "filled," "partially filled," or "unfilled" (working).
- Market orders guarantee a fill but not the price.
- Limit orders guarantee the price but not the fill.
- Slippage is the difference between the expected price and the fill price.
Fill Quality: Market vs. Limit
How order types affect your fill.
| Order Type | Fill Probability | Price Certainty | Risk |
|---|---|---|---|
| Market Order | 100% (Immediate) | Low (Takes best available) | Bad fill price (Slippage) |
| Limit Order | Uncertain (Only if price hits) | High (Exact or better) | No fill (Missing the trade) |
Real-World Example: Slippage on a Fill
A trader tries to sell a thinly traded "penny stock" during a panic.
Algorithmic Trading and Fills
For institutional traders, getting a "good fill" on large orders is an art. They use execution algorithms (like VWAP or TWAP) to break large orders into tiny pieces. This disguises their intent and prevents their own buying pressure from driving the price up before they are fully filled. The goal is to minimize "market impact" and achieve a fill price close to the market benchmark.
FAQs
Often heard in the phrase "Fill or Kill" (FOK). This is an order instruction that tells the broker: "Fill this entire order immediately at my price or better, or cancel (kill) the entire thing." It prevents partial fills.
This is called "price improvement." If you place a Limit Buy at $50.00, but a seller comes in at $49.95 at that exact millisecond, your broker should fill you at $49.95. You saved money.
If it is a Limit Order, the market price simply hasn't reached your limit yet. Or, there are other traders ahead of you in the queue at that same price (Time Priority). If it is a Market Order not filling, trading might be halted, or the market is closed.
A bad fill is when the execution price is significantly worse than the market price observed when the order was entered. This usually happens in fast-moving markets or illiquid assets using market orders.
The Bottom Line
In trading, the fill is the only thing that counts. Strategy and analysis are theoretical until the order is filled. Understanding the mechanics of execution—how liquidity, order types, and latency affect your fill price—is essential for preserving profit margins. Experienced traders worry less about the chart and more about the quality of their fills.
More in Trade Execution
At a Glance
Key Takeaways
- A fill occurs when a buyer is matched with a seller.
- An order can be "filled," "partially filled," or "unfilled" (working).
- Market orders guarantee a fill but not the price.
- Limit orders guarantee the price but not the fill.