Order Book Depth: Why Top-of-Book Quotes Can Mislead in Betting and Prediction Markets
When you look at a prediction market or sportsbook, the first thing you see is the “top-of-book” price: the best available odds to buy or sell an outcome. But relying solely on these top-level quotes can be risky. In liquid financial markets, the top-of-book is often deep and reliable. In betting and prediction markets—especially on exchanges like Kalshi or Polymarket—top-of-book prices can be shallow, fleeting, or outright misleading. Understanding order book depth is crucial for anyone who wants to trade, arbitrage, or simply get an accurate read of market sentiment.
This article explores how order book depth works, why top-of-book quotes can "lie," and how to navigate these realities using practical, numerate examples. We’ll also show how automated tools like ArbMonster scan full order books (not just top quotes) to surface real arbitrage opportunities, net of all fees and at fillable size.
What Is the Order Book, and What Is Top-of-Book?
An order book is an electronic list of buy and sell (bid and ask) offers for a given contract, sorted by price and time. The top-of-book refers to the best (highest) bid and the best (lowest) ask currently offered. For example:
| Price | Size (YES) | Size (NO) |
|---|---|---|
| $0.80 | 30 | |
| $0.78 | 50 | |
| $0.77 | 70 | |
| $0.76 | 90 | |
| $0.75 | 120 | |
| $0.74 | 50 | |
| $0.73 | 80 | |
| $0.72 | 100 | |
| $0.71 | 150 | |
| $0.70 | 200 |
Here, the top-of-book for YES is $0.74 (best offer to sell YES, size 50), and the top-of-book for NO is $0.80 (best offer to sell NO, size 30). Most sites only show this top quote, but the depth (how much size is offered at each price) is just as important.
Why Top-of-Book Quotes Can Mislead
1. Top-of-Book Size Is Often Tiny
Suppose you see a YES contract on Polymarket quoted at $0.74. You might think you can buy as much as you want at that price. In reality, there may be just $2 or $5 available there. If you want to buy $100 worth, you’ll "walk the book," buying $2 at $0.74, $5 at $0.75, $10 at $0.76, and so on—ending up with a worse average price than the quote you saw.
Example:
- Top-of-book (YES): $0.74, size $2
- Next levels: $0.75 (size $8), $0.76 (size $15), $0.77 (size $35)
- Want to buy $40:
- $2 @ $0.74 = $1.48
- $8 @ $0.75 = $6.00
- $15 @ $0.76 = $11.40
- $15 @ $0.77 = $11.55
- Total spent: $30.43 for 40 shares = average price $0.76075
If you had calculated profitability based on the $0.74 top quote, you’d overestimate the edge.
2. Illusory Arbitrage Opportunities
Suppose ArbMonster scans the market and finds:
- Kalshi: Buy YES @ $0.74 (size $2)
- Sportsbook: Lay NO @ -130 (implied price $0.77, size $500)
At first glance, there’s a $0.03 “arbitrage.” But if you want to trade $100, you’ll get only $2 at $0.74, then have to pay $0.75, $0.76, or higher for the rest. The real arbitrage (net of fees, at fillable size) may vanish entirely.
That’s why ArbMonster scans the full order book and only reports edges that are fillable at real sizes, after all venue fees.
3. Spoofing and Flickering Quotes
Because top-of-book quotes are visible to all, some traders may place (and quickly cancel) small orders to influence the displayed price. Others may set tiny “bait” orders at attractive levels, knowing they’ll be filled only by inattentive traders or bots. In thinly traded markets, this can skew the apparent fair price.
Real-world snapshot:
- Kalshi presidential contract: YES top-of-book $0.51 (size $1)
- Next best sell: $0.54 (size $200)
- True price is closer to $0.54, but the visible quote suggests $0.51
How to Read the True Market Price: Work the Depth
Weighted Average Price (WAP)
When evaluating a trade, always look at the weighted average price you’d pay to fill your intended size. For instance, if you want $100 of exposure and the order book looks like:
| Price | Size |
|---|---|
| $0.74 | $2 |
| $0.75 | $8 |
| $0.76 | $15 |
| $0.77 | $35 |
| $0.78 | $60 |
To buy $40:
- $2 @ $0.74 = $1.48
- $8 @ $0.75 = $6.00
- $15 @ $0.76 = $11.40
- $15 @ $0.77 = $11.55
- Total spent: $30.43
- Weighted average price: $30.43 / 40 = $0.76075
Depth Charts and Slippage
A depth chart is a graphical representation of how much size is available at each price. The steeper the curve, the more slippage you’ll face as you try to fill larger orders. Slippage is the difference between the expected price (top-of-book) and the average price actually paid due to limited depth.
Cross-venue Comparison
True arbitrage is only possible if you can execute both sides at real, fillable sizes. Suppose:
- Kalshi: YES @ $0.74 (size $2), next $0.75 (size $8), $0.76 (size $20)
- Sportsbook: NO @ -130 (implied $0.77, size $500)
For a $20 trade, your Kalshi fill would average $0.755. The arbitrage is $0.77 - $0.755 = $0.015/share, minus all fees. If Kalshi takes 2% per trade and the sportsbook has 5% juice, that edge may disappear or even reverse.
Automated scanners like ArbMonster continuously calculate net arbitrage after all fees, at all available sizes, and across 20+ sportsbooks plus Kalshi and Polymarket. Read more about how this works.
Practical Tips for Navigating Shallow Books
- Always check book depth: Before acting on a quote, look at the full order book to see how much size is available at each price.
- Calculate real average price: Use a spreadsheet or tool to compute your expected average fill for your intended size.
- Watch for spoofing: Treat tiny top-of-book sizes with skepticism, especially if the next price levels are much higher.
- Account for fees: Always include venue fees and transaction costs in your calculations.
- Use automation: If you’re scanning many venues, automated tools like ArbMonster can save time and prevent costly mistakes.
Summary Table: Top-of-Book vs. Realistic Fill
| Venue | Top-of-Book | Top Size | Avg Price for $50 | Avg Price for $200 |
|---|---|---|---|---|
| Kalshi | $0.74 | $2 | $0.760 | $0.783 |
| Polymarket | $0.73 | $5 | $0.753 | $0.790 |
| Sportsbook X | -130 (NO) | $500 | $0.770 | $0.770 |
In this example, the sportsbook offers enough depth to fill $200 at a flat price, while the exchanges' average prices rise substantially as size increases.
FAQ
Q1: Can I always fill my entire order at the top-of-book price?
A1: Usually not. In most prediction and betting markets, top-of-book sizes are small. For larger trades, you’ll pay higher prices as you walk the book.
Q2: How do automated tools like ArbMonster handle order book depth?
A2: ArbMonster scans all available venues, includes all fees, and checks the actual size available at each price. It only reports opportunities that are fillable at real sizes, not just based on the top quote. Learn more here.
Q3: Why do top-of-book quotes sometimes flicker or disappear?
A3: Small or spoofed orders can appear at the top-of-book and get canceled quickly, especially in thin markets. This can make top-of-book prices unreliable for sizing or arbitrage decisions.
ArbMonster is a data service. Nothing here is financial or betting advice; markets carry risk, venues have age and jurisdiction restrictions, and you are responsible for verifying everything before acting.