Partial Fills Explained: Why You Didn't Get the Price You Expected

Introduction

Partial fills happen when a broker executes only part of your buy or sell order at the price you requested. If you placed an order for 1,000 shares and you only receive 300, that is a partial fill. Understanding why this happens helps you avoid surprises and execute trades more reliably.

Why did you only get part of your order filled, and what can you do about it? This article explains order types, order queues, market liquidity, and practical steps you can take when markets are thin. You will learn how to size orders, use limit and special instructions, and read the basic signs of low liquidity.

• Partial fills occur when not enough shares are available at your requested price, or when your order is behind other orders in the queue.
• Market orders seek speed, so they can fill at worse prices in thin markets; limit orders control price but can be partially filled.
• Use order instructions like IOC and FOK, break large orders into smaller chunks, and check the order book or average volume to reduce surprises.
• Thin markets and wide bid-ask spreads increase partial fills and price slippage; large-cap stocks like $AAPL and $AMZN usually have deeper liquidity than small-cap names.
• Be patient, use limit prices, and consider order timing to improve execution quality in low-liquidity situations.

How Orders Work and Why Partial Fills Happen

When you place an order, the exchange routes it into a queue where it waits to be matched with opposite orders. If there aren’t enough shares available at the price you set, only part of your order will match and the rest may remain unfilled. That basic imbalance between supply and demand creates partial fills.

Different order types behave differently. A market order asks for immediate execution at the best available price, so you'll usually get a full fill in highly liquid stocks. A limit order sets a maximum buy price or a minimum sell price, so it will only trade at your price or better, which can result in partial fills if only some shares meet that condition.

Example: Simple math of a partial fill

Imagine you place a limit buy for 1,000 shares of a thinly traded stock at $10. The order book shows only 300 shares available at $10 and 700 at $10.10. Your broker will fill the 300 at your $10 limit and leave the remaining 700 unfilled, unless you change the order to accept higher prices. You received a partial fill of 30 percent.

Order Queues and Priority

Order matching on exchanges follows rules about price and time. Orders at the same price are ranked by who sent them first. That means if you enter an order late, you are behind existing orders at that price. This time priority is a core reason for partial fills.

Some venues and brokers may route parts of your order to different places. Dark pools and multiple exchanges add complexity. You might get a partial fill because one venue has buyers ready while another does not. Your broker's smart router aims to find the best execution, but it can still return partial fills when supply is fragmented.

Special order instructions

Two common instructions help control partial fills. IOC, immediate or cancel, executes whatever can be filled immediately and cancels the rest. FOK, fill or kill, requires the entire order to fill immediately or it cancels the whole order. Use IOC when you only want immediate liquidity, and use FOK when you need the full size or nothing.

Liquidity, Market Depth, and Thin Markets

Liquidity means how easily shares can be bought or sold without moving the price much. Market depth shows available shares at each price level. Thin markets have shallow depth, so even modest orders can wipe out the best bid or ask and cause partial fills or large price movement.

Large, heavily traded stocks like $AAPL and $AMZN typically have deep order books and tight bid-ask spreads. Small-cap, newly listed, or low-volume names may have wide spreads and few shares at each price. Those conditions are where partial fills and slippage become common.

How to read the signs

Before you trade, check average daily volume and the current bid-ask spread. If average daily volume is low relative to your intended order size, you are likely to face partial fills. A wide spread means trading costs are higher and executions may be partial or at worse prices. Level 2 data shows how many shares sit at each price level and helps you estimate the likely fill size.

Practical Strategies to Reduce Partial Fills

You can’t eliminate partial fills, but you can reduce them with practical actions. The strategies below are easy to apply even if you’re new to trading.

1. Use limit orders instead of market orders, so you control the price you're willing to pay or accept. This prevents unexpectedly bad fills in thin markets.
2. Break large orders into smaller chunks. Smaller orders are more likely to match existing liquidity without walking the book.
3. Use IOC or FOK when you want immediate action or want to avoid partials entirely. Be aware that FOK may cancel your order if full size isn't available.
4. Check the order book and average daily volume. If your order is a meaningful percentage of typical daily volume, reduce the size or trade over time.
5. Place post-only or limit maker orders if your broker supports them to add liquidity rather than take it, which can reduce fees and avoid immediate partial fills.

Real-world example: Splitting an order

You want to buy 5,000 shares of a small-cap ETF where average daily volume is 20,000 shares. Placing a single 5,000-share order could move the market and cause partial fills. Instead you split it into ten 500-share orders placed across the day. Each smaller order is more likely to find matching liquidity, lowering the chance of partial fills and reducing market impact.

Real-World Examples

Example 1: Liquid stock. You submit a limit buy for 1,000 shares of $AAPL at the current ask. $AAPL typically has thousands of shares available at that price, so you usually get a full fill quickly. Partial fills are rare for such large-cap names, unless news causes a sudden imbalance.

Example 2: Thin stock. You place a limit buy for 2,000 shares of a small-cap name at $2. The order book shows 400 shares at $2, 300 at $2.01, and small amounts higher. You get a partial fill of 400 shares at $2. The remaining 1,600 shares remain open and are unlikely to fill unless price moves. You can either raise your limit to eat into higher levels or cancel and rework the order.

Example 3: Market order risk. You enter a market sell for 10,000 shares of a low-volume ETF near market close. There are few bidders, so your order sweeps through lower prices and may fill in pieces at progressively worse prices. The partial fills and price slippage increase your realized cost compared to what you expected.

Common Mistakes to Avoid

1. Using market orders in thin markets, which can lead to large fills at unfavorable prices. How to avoid it: use limit orders and check liquidity first.
2. Placing oversized orders relative to average volume. How to avoid it: size your trade as a small percentage of average daily volume or split the order into smaller parts.
3. Ignoring the order book and relying only on the last trade price. How to avoid it: inspect level 2 depth or your broker's order book to see where liquidity sits.
4. Not using appropriate order instructions like IOC or FOK when you need control. How to avoid it: select IOC if you only want immediate execution, or FOK if you need full size instantly, but know that both can cancel your order.
5. Chasing executions during volatile news events, which increases the chance of partial fills and slippage. How to avoid it: trade with more caution around earnings, macro news, or after-hours sessions.

FAQ

Q: Why did I get a partial fill even though the stock looked liquid?

A: Liquidity can change quickly. Even liquid stocks can have temporary imbalances during news, option expirations, or near market open and close. Your order may also have been behind other orders in the time priority queue.

Q: Will my broker automatically cancel the unfilled portion of a limit order?

A: That depends on the order duration you chose. Day orders expire at the close unless filled. Good-till-canceled orders remain until filled or canceled. Use IOC to cancel unfilled portions immediately.

Q: Should I use a market order to avoid partial fills?

A: Market orders prioritize speed over price and can cause poor fills in thin markets. They may reduce partial fills in deep markets, but they increase the risk of large price differences. Use limit orders to control price when liquidity is uncertain.

Q: How can I tell whether my order will likely be partially filled before I submit it?

A: Check the current order book depth at the price you plan to trade, compare your order size to average daily volume, and look at the bid-ask spread. These indicators help estimate the probability of a partial fill.

Bottom Line

Partial fills are a normal part of market structure. They happen because there aren’t always enough shares at your requested price, or because you’re behind other orders in the queue. You can reduce partial fills by using limit orders, checking market depth, breaking large orders into smaller pieces, and choosing appropriate order instructions like IOC or FOK.

At the end of the day, patience and preparation pay off. Before you trade, look at volume, spread, and the order book. Adjust your size, pick the right order type, and you’ll avoid many common execution surprises. Keep practicing these steps and you'll improve your trade execution over time.