# Fill Probability and Queue Position

> Your place in the order queue decides whether a limit order fills. Learn how queues work, how to estimate fill probability and why fills can be a warning sign.

Source: https://learn.tradelabsai.com/orders/queue-position/  
Track: Orders and Execution · Level: Intermediate · Updated: 2026-10-03  
Publisher: TradeLabs AI (https://tradelabsai.com). Education, not financial advice.  
Cite as: TradeLabs Learn, "Fill Probability and Queue Position", https://learn.tradelabsai.com/orders/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](https://learn.tradelabsai.com/orders/matching-engines/).

**Example: Waiting in line**
The best bid is $10.00 with 20,000 shares waiting. You add a buy limit for 500 shares at $10.00, so you are behind 20,000 shares. Over the next minute, 12,000 shares trade at $10.00, and some orders ahead of you are cancelled, removing 3,000 more. You are now 5,000 shares from the front. If the price then ticks up to $10.01, the queue never reaches you and you miss the trade.

## What affects your fill probability

| Factor | Effect on your chance of filling |
|---|---|
| Shares ahead of you | More shares ahead, lower chance |
| Volume trading at your price | More volume, higher chance |
| Cancellations ahead of you | Cancellations move you forward |
| Price moving away | Ends your chance until it returns |
| Price moving through your level | Guarantees a fill, often at a bad moment |

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](https://learn.tradelabsai.com/market-structure/market-makers/) 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](https://learn.tradelabsai.com/orders/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](https://learn.tradelabsai.com/market-structure/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](https://learn.tradelabsai.com/orders/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](https://learn.tradelabsai.com/start-here/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.

## Continue learning

- Next lesson: [Transaction Costs](https://learn.tradelabsai.com/orders/transaction-costs/)
- Previous lesson: [Best Execution and Execution Quality](https://learn.tradelabsai.com/orders/best-execution/)
- Related: [Best Execution and Execution Quality](https://learn.tradelabsai.com/orders/best-execution/): Best execution is a broker's duty to get you the most favourable terms on orders. Learn what it covers, price improvement, routing reports and how to judge fills.
- Related: [Limit Orders](https://learn.tradelabsai.com/orders/limit-orders/): A limit order trades only at your price or better. Learn how buy and sell limits work, why they may not fill, queue priority and how to set a smart limit price.
- Related: [Matching Engines](https://learn.tradelabsai.com/orders/matching-engines/): A matching engine is the system that pairs buy and sell orders on an exchange. Learn price time priority, pro rata matching, auctions and why it matters to you.
- Related: [The Order Book and Market Depth](https://learn.tradelabsai.com/market-structure/the-order-book-and-market-depth/): The order book lists every waiting buy and sell order by price. Learn to read market depth, what imbalances show, spoofing risks and how depth affects fills.
- Related: [Market Makers and Liquidity Providers](https://learn.tradelabsai.com/market-structure/market-makers/): Market makers quote prices to buy and sell all day, earning the spread. Learn how they make money, manage risk, why they matter and the myths about them.
- Related: [Hidden and Iceberg Orders](https://learn.tradelabsai.com/orders/hidden-and-iceberg-orders/): Iceberg orders show only part of a large order; hidden orders show none of it. Learn how they work, their priority trade off and how to spot icebergs in the book.