The order book is a window into the real state of the market. While most traders study charts looking for patterns, professional arbitrageurs look at the order book — and see what no chart ever shows: real liquidity, hidden volume, and the actual price at which a trade will execute right now.
This article covers order books from the basics to practical application in arbitrage.
What Is an Order Book
An order book is a real-time list of all active buy and sell orders for an asset at different price levels.
Order book structure:
SELL (asks) ← red
$2,485.00 | 12.4 ETH
$2,484.50 | 8.1 ETH
$2,484.00 | 22.7 ETH
───────────────────────── ← current price $2,483.80
$2,483.50 | 15.3 ETH
$2,483.00 | 31.2 ETH
$2,482.50 | 9.8 ETH
BUY (bids) ← green
Bid — best buy price (highest among buyers): $2,483.50
Ask — best sell price (lowest among sellers): $2,484.00
Bid-ask spread — gap between best bid and ask: $2,484.00 − $2,483.50 = $0.50
Key Concepts
Bid-Ask Spread: The Hidden Cost
The bid-ask spread isn't just a number on the screen. It's the real cost of entering and immediately exiting a position.
When you buy with a market order — you pay the ask price ($2,484.00).
When you immediately sell with a market order — you receive the bid price ($2,483.50).
Loss from spread alone: $0.50 per ETH = 0.02%.
On top pairs (BTC, ETH), the bid-ask spread is minimal — 0.01–0.05%. On low-liquidity altcoins — 0.5–3% or more. For arbitrageurs this matters: if the combined bid-ask spreads on two exchanges exceed the cross-exchange spread, the trade is unprofitable before it even starts.
Market Depth
Depth is the total volume of buy and sell orders at different price levels. It shows how much volume the market can absorb without significant price movement.
Deep order book:
$2,486.00 | 145 ETH
$2,485.50 | 203 ETH
$2,485.00 | 312 ETH ← large volume close to current price
$2,484.50 | 289 ETH
$2,484.00 | 178 ETH
Thin order book:
$2,490.00 | 2.1 ETH
$2,488.00 | 1.3 ETH
$2,486.00 | 0.8 ETH ← small volume, price will jump sharply
$2,484.00 | 3.2 ETH
$2,482.00 | 1.5 ETH
In a thin book, buying just 5 ETH would push the price by $4–6 — that's slippage.
Slippage
Slippage is the difference between the price you expected and the price your order actually filled at.
Calculating expected average fill price:
Say you want to buy 50 ETH with a market order and the order book looks like this:
| Price (ask) | Volume (ETH) | Cumulative volume |
|---|---|---|
| $2,484.00 | 22.7 | 22.7 |
| $2,484.50 | 8.1 | 30.8 |
| $2,485.00 | 12.4 | 43.2 |
| $2,485.50 | 9.1 | 52.3 |
You buy: 22.7 ETH × $2,484 + 8.1 × $2,484.50 + 12.4 × $2,485 + 6.8 × $2,485.50 = $124,237.05
Average price: $124,237.05 / 50 = $2,484.74 instead of the expected $2,484.00
Slippage: $0.74 = 0.03% — acceptable for a liquid pair.
How to Read the Order Book for Arbitrage
What to check before every arbitrage trade
1. Is there enough volume for your order?
Rule: your order should not exceed 10–15% of visible volume at the needed price level. Otherwise you're moving the market against yourself.
Example: you want to buy $5,000 worth of ETH (~2 ETH). The order book shows 22.7 ETH at the ask ($2,484). Your order = 2 ETH = 8.8% of available volume. Slippage will be minimal — good to proceed.
2. What is the bid-ask spread?
Check the bid-ask spread on both exchanges. The combined bid-ask spread (Exchange A + Exchange B) must be significantly less than the cross-exchange spread.
Example:
- Cross-exchange spread: 0.65%
- Bid-ask on Binance: 0.02%
- Bid-ask on Bybit: 0.02%
- Combined bid-ask: 0.04%
- Remaining after bid-ask: 0.65% − 0.04% = 0.61% — solid
If the cross-exchange spread is 0.25% and combined bid-ask is 0.20%, real profit is almost zero.
3. Is depth symmetrical?
If Exchange A has a deep book (lots of volume near the price) and Exchange B has a thin one, be careful. Slippage on B will consume part of the profit.
4. Are there abnormal walls?
A "wall" in the order book is a massive order at a single price level that artificially restrains price movement. Sometimes it's manipulation: a large player places a big sell order to prevent the price from rising.
$2,490.00 | 15.3 ETH ← normal volume
$2,489.00 | 18.7 ETH
$2,488.00 | 847.0 ETH ← enormous wall!
$2,487.00 | 22.1 ETH
This wall signals that the price is unlikely to pass $2,488 easily. For arbitrage: if your trade hits this wall, the actual fill price will be worse than expected.
Order Types and Their Impact on Execution
Market Order
Executes immediately at the best available price. You agree to pay whatever is needed to buy right now.
Pros: guaranteed immediate execution — critical for arbitrage.
Cons: you pay the taker fee (higher), possible slippage.
When to use: always when speed matters more than price — which in arbitrage is usually the case.
Limit Order
Executes only at your specified price or better. If the price doesn't reach your level — the order won't fill.
Pros: you pay the maker fee (lower), no slippage.
Cons: no guaranteed execution — in arbitrage this is a risk.
When to use: in balance arbitrage where there's no race against time, and when the spread is large enough to wait for a better price.
Post-Only Order
A special limit order type that guarantees maker execution (never taker). If it would execute as a taker — it automatically cancels.
Useful for reducing fees in high-frequency trading.
How to Analyze Order Books Across Exchanges
Tools for viewing order books
On the exchanges themselves: every major exchange displays the order book in the trading interface — typically in the center of the screen.
Display settings:
- Price grouping ($0.01, $0.10, $1, $10) — affects visible depth
- Number of levels (10, 20, 50, 100) — use 20–50 for arbitrage
- Cumulative volume — shows total volume up to each price level
In SpreadScan: the Market section lets you compare order books across multiple exchanges simultaneously.
Comparing two exchange order books
For arbitrage, compare the order books on both sides of the trade:
| Parameter | Exchange A (Binance) | Exchange B (KuCoin) | Conclusion |
|---|---|---|---|
| Best ask | $2,484.00 | $2,497.50 | Spread: 0.54% |
| Ask volume | 22.7 ETH | 8.3 ETH | B less liquid |
| Bid-ask spread | $0.50 (0.02%) | $1.00 (0.04%) | B costs more in spread |
| Depth ±1% | 450 ETH | 180 ETH | B is 2.5× thinner |
Conclusion: there's a spread, but KuCoin has lower liquidity — for orders above 5–7 ETH, slippage on KuCoin becomes significant.
Hidden Orders and Iceberg Orders
Major exchanges support hidden orders and iceberg orders — they don't appear fully in the order book.
Iceberg order: only part of the volume is visible (the "tip"). When that portion fills — the next portion automatically appears at the same price.
Example: the order book shows 5 ETH for sale. After those 5 ETH fill — another 5 ETH appears at the same price. And another. This is an iceberg.
How to identify: if volume at a price level doesn't decrease after fills — you may be looking at an iceberg.
Significance for arbitrage: an iceberg in the path of your trade means more real volume at that level than visible. Sometimes helpful (extra liquidity), sometimes a wall of resistance.
Practical Checklist: Evaluating the Order Book Before an Arbitrage Trade
Quick order book check on both exchanges before every trade:
✅ Volume
- Visible volume at the target level ≥ 7× your order size
- No abnormal "walls" right at or near the current price
✅ Spread
- Bid-ask spread on both exchanges < 0.10%
- Combined bid-ask spread doesn't consume the cross-exchange spread
✅ Depth
- Sufficient depth within ±0.5% of current price on both exchanges
- No sudden "cliff" in liquidity just past the first level
✅ Slippage
- Calculated expected slippage for your order size
- It doesn't exceed 0.10% for liquid pairs
All checks pass — execute. Any doubts — skip or reduce size.
How Order Books Change During Volatility
In calm markets, the order book looks balanced — volume distributed evenly across many levels. During volatility, the picture changes dramatically.
During a sharp upward move:
- Sellers (asks) pull back higher or disappear entirely
- Buyers (bids) execute rapidly
- Bid-ask spread widens sharply
- The book "thins out" — depth falls
This is exactly when large cross-exchange spreads appear. But simultaneously, slippage increases and execution risk rises.
For arbitrageurs: high volatility creates larger opportunities and larger risks simultaneously. Trade smaller sizes or apply more conservative thresholds during these periods.
Conclusion
Reading order books is one of the core skills of a professional arbitrageur. Without understanding market depth and liquidity, it's easy to enter an attractive-looking trade that turns unprofitable due to slippage.
Three key rules:
- Always check volume at your target level — your order shouldn't consume more than 10–15% of available volume
- Include bid-ask spread in your profit calculation
- In a thin order book — reduce your size or skip the trade
Open SpreadScan with liquidity data →
This article is for educational purposes only and does not constitute financial advice.
Frequently Asked Questions
What are bid and ask in simple terms? Bid is the best price someone is currently willing to pay to buy. Ask is the best price someone is currently willing to accept to sell. The gap between them is the bid-ask spread.
Why do order books differ across exchanges? Each exchange is an independent market with its own buyers and sellers. Orders on one exchange are invisible to another. This is exactly what creates cross-exchange spreads for arbitrage.
What does "thin order book" mean? A thin order book means few orders and small volumes at each price level. In such a market, any trade significantly moves the price — slippage will be large.
How do I calculate expected slippage? Walk through the order book level by level and calculate the volume-weighted average price for your order size. The difference between this average price and the current best price is your expected slippage.
What is a "wall" in the order book? An abnormally large order at a single price level. Often used by large players to manage price direction. For arbitrageurs — a signal to verify real liquidity at that level before assuming the price will move through it cleanly.
Does the order book show all orders? No — iceberg orders and hidden orders aren't fully visible. The displayed book shows only the visible portion of icebergs and none of the hidden orders. Real available volume may be higher than what you see.