Partial Fills
Definition
A partial fill occurs when only part of an order is executed at the requested price, with the remainder filled later at different prices or not filled at all. Partial fills are most common in low-liquidity or fast-moving markets and can materially affect trade costs and outcomes, making broker execution quality and liquidity access important comparison factors.
In Plain English
In plain terms, a partial fill means your order is only filled in pieces. Instead of your entire trade executing at once, some of it is completed while the rest waits for more buyers or sellers. This can result in multiple execution prices or an incomplete trade.
How It Works
- You place an order for a specific size.
- The broker attempts to execute the order at available prices.
- Only part of the order can be matched immediately due to limited liquidity.
- That portion is executed and confirmed.
- The remaining portion stays pending or is cancelled, depending on order rules.
- Additional fills may occur at the same or different prices.
- The final average execution price reflects all filled portions.
- Unfilled portions may expire or be cancelled based on time-in-force settings.
Why This Matters for Traders
Partial fills introduce uncertainty around execution price and trade size. For larger orders or trades placed in illiquid conditions, partial fills can increase average entry costs, distort risk–reward calculations, and complicate position management. Traders relying on precise position sizing or tight risk limits are particularly affected.
Common Misunderstandings
- Partial fills are broker errors: they are usually caused by limited liquidity.
- Partial fills only affect large traders: small trades can also be affected in thin markets.
- The quoted price guarantees full execution: quoted prices reflect limited available size.
- Partial fills always complete eventually: unfilled portions may never execute.
- All brokers handle partial fills the same way: policies and reporting differ.
How This Affects Broker Choice
Broker handling of partial fills varies and affects real trading costs. When comparing brokers, users should consider:
• Liquidity depth for frequently traded instruments.
• Transparency in reporting partial executions and average prices.
• How unfilled portions are handled (cancelled vs left pending).
• Availability of execution controls such as Fill-Or-Kill or Immediate-Or-Cancel.
• Consistency of behaviour across platforms.
From a monetisation and comparison perspective, partial-fill behaviour highlights execution quality differences that are not visible in headline spreads, supporting broker reviews and execution-focused comparisons.
Risks & Common Mistakes
• Ignoring how partial fills affect effective position size.
• Assuming stop losses or take-profit orders apply uniformly across all fills.
• Trading illiquid instruments without adjusting order size.
• Overlooking average execution price after multiple fills.
• Choosing brokers with limited transparency around order execution.
Risk note: partial fills can increase trading costs and execution risk, particularly in volatile or low-liquidity markets and when leverage is used.
Real-World Example
You place a limit order to buy 10 contracts at £100.
• Only 4 contracts are available at £100 and execute immediately.
• The remaining 6 contracts fill later at £101.
Your average entry price becomes £100.60, altering both risk and potential reward compared to the original plan.
What to Check Before Trading
- How deep is liquidity for your typical trade size?
- Does the broker clearly report partial fills and average prices?
- Can you control order behaviour with time-in-force options?
- How are stop losses and take profits handled on partially filled orders?
- Are execution rules consistent across platforms?
- Does the broker disclose execution statistics?
- Are illiquid instruments clearly flagged?
Related Concepts
Limited liquidity is the primary cause of partial fills.
Execution rules determine how partial fills are handled and reported.
Limit orders are most commonly affected by partial fills.
Time-in-force settings control whether unfilled portions remain active.
Multiple fills at different prices can increase effective slippage.
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