Reading the Order Book

What Is the Order Book?

The order book is a real-time list of all open buy and sell orders for a trading pair. It is the core mechanism through which prices are discovered on exchanges. Every perpetual futures market has its own order book, and learning to read it gives you a direct view into supply and demand.

Bids, Asks, and the Spread

The order book has two sides:

• Bids (buy orders) — listed on the left or bottom, sorted highest price first. These are prices buyers are willing to pay.
• Asks (sell orders) — listed on the right or top, sorted lowest price first. These are prices sellers are willing to accept.

The spread is the gap between the best bid and the best ask. For example, if the best bid for ETH-PERP is $1,902.50 and the best ask is $1,902.80, the spread is $0.30. Tighter spreads mean more liquid markets with lower trading costs.

Understanding Depth

Depth measures the total volume of orders stacked at each price level. Imagine the ETH-PERP order book:

| Bid Price | Bid Size | Ask Price | Ask Size |

|-----------|----------|-----------|----------|

| $1,902.50 | 120 ETH | $1,902.80 | 85 ETH |

| $1,902.00 | 200 ETH | $1,903.00 | 150 ETH |

| $1,901.50 | 350 ETH | $1,903.50 | 90 ETH |

The cumulative bid depth for the top three levels is 670 ETH, while the ask depth is 325 ETH. This imbalance tells you there is stronger buying interest near the current price than selling interest.

Liquidity and Slippage

Liquidity refers to how easily you can enter or exit a position without moving the price. If you want to market-buy 100 ETH in the example above, you would fill entirely at $1,902.80 (the top ask has 85 ETH, and the next level covers the remaining 15 ETH at $1,903.00). Your average fill price would be about $1,902.83 — that $0.03 difference from the best ask is slippage.

On thin order books, slippage can be severe. Buying 300 ETH in the same book would push through all three ask levels.

Wall Orders and What They Mean

A wall is an unusually large order at a single price level — for example, a 500 BTC buy order at $29,800. Walls can act as psychological support or resistance because traders see them and adjust behavior.

However, walls are not guarantees. Large traders sometimes place walls to influence sentiment and then cancel them before they get filled. This tactic is called spoofing. Always treat walls as information, not as certainty.

Reading Order Flow

Watching how the order book changes over time is called reading order flow. Key signals include:

• Bid stacking — large bids accumulating below the current price, suggesting buying interest.
• Ask thinning — sell orders being pulled, potentially signaling an upcoming move up.
• Rapid refreshing — orders appearing and disappearing quickly, indicating algorithmic activity.

Combining order book reading with price charts gives you a more complete picture of market dynamics than either tool alone.