What is an Order Book?

When you buy or sell Bitcoin or Ethereum on a cryptocurrency exchange, have you ever wondered: How does the exchange know who wants to buy, who wants to sell, and how does it determine the trade price?

The answer: The Order Book.

This article will explain how order books work using simple analogies, so the next time you trade, you'll understand what's happening behind the scenes.

Let's Start with Everyday Examples

Scenario 1: Buying at a Market

Imagine you're at a farmer's market buying cabbage:

• Vendor A calls out: "My cabbage is $5 each!"
• Vendor B shouts: "Mine is $4.50 each!"
• You ask: "Can I get it for $4?"

If a vendor is willing to lower their price, the deal is done. If not, you can accept $4.50 or keep waiting for other vendors.

Scenario 2: An Auction

Now imagine an art auction:

• Bidder 1 raises paddle: "I bid $1 million!"
• Bidder 2 raises paddle: "I bid $1.2 million!"
• Seller says: "I won't sell for less than $1.5 million"

When a bidder is willing to pay $1.5 million, the auctioneer strikes the gavel and the deal is done.

The order book is the digital version of these two scenarios. The exchange records all "how much buyers are willing to pay" and "how much sellers want to receive", then automatically matches them for trades.

Order Book Explained in One Sentence

An order book is an electronic ledger used by exchanges to record all buy orders (Bids) and sell orders (Asks). When buy and sell prices can be matched, trades are automatically executed.

It's that simple!

Components of an Order Book

A standard order book has three core elements:

1. Bids — Buyers' Offers

Bids are "I'm willing to buy at this price" orders.

For example:

• Buyer A: "I'll buy 1 BTC at $68,000"
• Buyer B: "I'll buy 0.5 BTC at $67,950"
• Buyer C: "I'll buy 2 BTC at $67,900"

The order book arranges bids from highest to lowest, because the highest bidder is most likely to trade first.

2. Asks — Sellers' Asking Prices

Asks are "I'm willing to sell at this price" orders.

For example:

• Seller X: "I'll sell 0.8 BTC at $68,100"
• Seller Y: "I'll sell 1.5 BTC at $68,150"
• Seller Z: "I'll sell 3 BTC at $68,200"

The order book arranges asks from lowest to highest, because the lowest asking price is most likely to trade first.

3. Spread — The Gap Between Buy and Sell

The spread is the difference between the highest bid and the lowest ask.

Using the above example:

• Highest bid: $68,000
• Lowest ask: $68,100
• Spread = $100

Order Book Example Table

Here's a simplified BTC/USDT order book:

Spread = 68,100 - 68,000 = 100 USDT

How to Read a Depth Chart

The visual representation of an order book is called a Depth Chart.

A depth chart looks like this:

Asks (Red)
        ┌─────────────┐
        │             │
        │             │
────────┘             └────── Current Price
        ┌─────────────┐
        │             │
        │             │
Bids (Green)

How to Read a Depth Chart

• X-axis: Price
• Y-axis: Cumulative quantity (accumulated outward from current price)
• Green area (left): Total cumulative buy orders
• Red area (right): Total cumulative sell orders

Buy Walls vs Sell Walls

• Buy Wall: A large accumulation of buy orders at a certain price, forming a "wall"

  • If it appears below the current price, it indicates strong buying support, making it harder for the price to fall

• Sell Wall: A large accumulation of sell orders at a certain price

  • If it appears above the current price, it indicates heavy selling pressure, making it harder for the price to rise

Order Books and How Trading Works

Now that you understand order books, let's see how trades actually happen.

Two Types of Orders

1. Maker Orders — I'm Not in a Hurry, I'll Wait for a Match

Maker orders mean you set a price and place your order in the order book, waiting for a match.

For example:

• Current BTC price is $68,050
• You place a "buy at $68,000" order
• Your order enters the order book, waiting for someone willing to sell to you at $68,000

Advantage: Lower fees (many exchanges offer Maker discounts or even rebates) Disadvantage: You might wait a long time without a match

2. Taker Orders — I Want to Trade Immediately

Taker orders mean you accept existing prices in the order book and execute immediately.

For example:

• The lowest ask in the order book is $68,100
• You click "market buy"
• Trade executes immediately at $68,100

Advantage: Immediate execution, no waiting Disadvantage: Higher fees

Order Matching Flow

You want to buy 1 BTC, click "market buy"
     ↓
Exchange queries the order book's "asks"
     ↓
Finds lowest ask: $68,100 USDT / Quantity 0.8 BTC
     ↓
Your order takes these 0.8 BTC, trade executes
     ↓
Still need 0.2 BTC, continues to next ask
     ↓
Next ask: $68,150 USDT / Quantity 1.5 BTC
     ↓
Your order takes 0.2 BTC, fully executed
     ↓
Total cost: (0.8 × 68,100) + (0.2 × 68,150) = $68,110 USDT (average execution price)

CEX vs DEX Order Book Differences

Centralized exchanges (CEX) and decentralized exchanges (DEX) handle orders very differently.

Centralized Exchanges (CEX)

Examples: Binance, OKX, Bybit

How They Work:

• Use traditional order books
• All orders recorded on the exchange's servers
• Matching engines can process millions of orders per second
• Fast, deep liquidity, tight spreads

Advantages:

• Extremely fast trading (millisecond-level)
• Deep liquidity, large orders can be filled quickly
• Low fees

Disadvantages:

• Assets stored on the exchange (not in your wallet)
• You must trust the exchange won't go bankrupt or disappear

Decentralized Exchanges (DEX)

Examples: Uniswap (AMM), dYdX (on-chain order book)

Method 1: AMM Automated Market Maker (most DEXs)

• Don't use order books, use "liquidity pools" instead
• Prices determined by mathematical formulas (x × y = k)
• No "bids" or "asks", just "how many tokens are in the pool"

Method 2: On-chain Order Books (few DEXs)

• Orders recorded on the blockchain
• Matching speed limited by blockchain speed (seconds to minutes)
• High gas fees

Advantages:

• Assets in your own wallet, full control
• No need to trust third parties

Disadvantages:

• Slower trading speeds
• Lower liquidity, large orders prone to slippage
• High gas fees (Ethereum mainnet)

Comparison Table

Practical Tips for Beginners

1. Check Order Book Depth Before Trading

Before placing an order, observe the order book:

• Good depth (many buy/sell orders) → Safe to use market orders
• Poor depth (sparse orders) → Use limit orders to avoid slippage

2. Pay Attention to the Spread

• Spread < 0.1%: Excellent liquidity, suitable for fast trading
• Spread > 1%: Poor liquidity, beware of slippage

3. Use Limit Orders to Control Costs

If you're not in a rush, using Limit Orders allows you to:

• Control execution price
• Enjoy lower fees (Maker rates)

4. Beware of "Fake Walls"

Large buy walls or sell walls aren't always real:

• Could be large traders creating false panic or excitement
• May be suddenly canceled when price approaches

5. Avoid Trading During Low-Liquidity Periods

• Asian trading hours: Usually higher volume, better depth
• Weekends, holidays: Typically lower liquidity

Summary

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Ready to start trading on an exchange? We recommend these platforms:

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Further Reading:

• How to Buy Bitcoin — Your first step into crypto
• What is Slippage — Understanding market order risks
• What is DeFi — Learn about decentralized exchanges
• Exchange Comparison — Choose the right exchange for you