Fill Probability and Queue Position

When you place a limit order at a price where other orders are already waiting, you join a queue. On most exchanges, orders at the same price fill in the order they arrived. Your place in that queue, together with how much trading happens at your price, decides whether your order fills. Understanding queues explains why a limit order can sit untouched while the price trades at your level, and why getting filled is not always good news.

How the queue works#

Under price time priority, all orders at the best bid form a line. Incoming sell orders fill the line from the front. See Matching Engines.

What affects your fill probability#

A simple estimate: if typical volume trading at a price before it moves is smaller than the queue ahead of you, your fill chance is low.

Adverse selection: when fills are bad news#

The trades most likely to fill your passive order are the ones where the market moves through your price. If you are bidding at $10.00 and a large seller appears, they fill the entire queue, yours included, and then the price drops to $9.98. You got filled exactly because the price was about to fall.

Meanwhile, when the price is about to rise, your bid often does not fill: buyers lift the offer at $10.01 and the price moves away. This asymmetry, called adverse selection, is a core reason passive trading is harder than it looks, and why Market Makers and Liquidity Providers invest heavily in predicting short term moves.

Queue position in practice#

• Round numbers attract long queues. Bids at $50.00 can be much longer than at $50.03. Placing slightly inside a round number can improve your odds.
• Improving the price puts you first. If the spread is wider than one tick, placing a bid one tick above the current best bid makes you first in a new queue.
• Modifying an order usually loses its place. Changing the price, and on many venues increasing the size, sends your order to the back.
• Hidden and reserve orders queue behind displayed ones at the same price on most exchanges. See Hidden and Iceberg Orders.
• Pro rata markets allocate fills in proportion to size rather than time, so queues work differently there.

Seeing the queue#

Level 2 data shows the total size at each price, but usually not individual orders or your exact place. Some professional platforms estimate queue position from the order book and trades since your order was entered. Most retail traders do not need precise queue tracking, but it helps to remember that touching your price is not the same as trading through it. See The Order Book and Market Depth.

Practical tips#

1. For entries where missing the trade is costly, consider a marketable limit instead of joining a long queue. See Market vs Limit Orders.
2. For patient entries, place limits at levels from your plan rather than at the obvious round number.
3. Count missed fills in your Trading Journal to see what passivity is really costing you.
4. Expect passive fills to be followed by some adverse movement, and set stops accordingly.

Frequently asked questions#

Why did the price touch my limit but my order did not fill?#

Because orders ahead of yours in the queue absorbed all the trading at that price before it moved away.

Does a bigger order fill faster?#

Under price time priority, no; time of arrival matters, not size. In pro rata markets, larger orders receive more of each fill.

Does changing my limit order lose my place?#

Changing the price almost always does. Reducing size often keeps your place, while increasing size usually does not.