An order book is a live list of the buy and sell orders waiting to be filled in a market, organised by price. It shows how many shares or contracts people are willing to buy at each price below the current market and how many they are willing to sell at each price above it. Learning to read one gives you a clearer sense of how prices are formed and why they sometimes move in sudden steps.
How the book is organised
An order book has two sides. The bid side lists buy orders, with the highest price at the top. The ask side, sometimes called the offer side, lists sell orders, with the lowest price at the top. The gap between the best bid and the best ask is the bid-ask spread.
Each level shows a price and the total size waiting there. For example, there might be 5,000 shares bid at 250.0p, 3,000 at 249.9p and 8,000 at 249.8p, while sellers offer 4,000 at 250.2p and 6,000 at 250.3p. When a buyer arrives wanting more than 4,000 shares straight away, their order will eat through the first level and start filling at 250.3p.
This deeper view is often called Level 2 data or depth of market. Level 1 shows only the best bid and ask.
Where order books are used
Exchanges such as the London Stock Exchange run central order books for many shares, matching buyers and sellers electronically. Futures exchanges do the same for contracts on indices, bonds and commodities. In these markets, the order book is the market.
Foreign exchange is different. There is no single central exchange, and prices come from banks, market makers and electronic platforms. Some brokers show a depth-of-market view, but it reflects their own liquidity rather than the whole market. If you trade spread bets or CFDs, your broker’s price is usually derived from the underlying market, and the depth you see may not be a full exchange order book.
What the book can tell you
Liquidity. A book with large orders stacked close to the current price is deep, meaning big trades can happen without moving the price much. A thin book with small orders spread out means even modest trades can shift the price. Our guide to liquidity in trading explains why that matters for costs.
Potential support and resistance. Large clusters of orders at a particular price can act as a temporary floor or ceiling, because a lot of buying or selling has to be absorbed before the price can move through.
How your own order will fill. If you place a large market order in a thin book, you may be filled at several prices, giving you a worse average price than the one you saw quoted. That is a form of slippage.
What the book cannot tell you
The order book is a snapshot of intentions, and intentions change. Orders can be cancelled in milliseconds. Some large traders deliberately hide their size using iceberg orders, which show only a small portion of the total. Others may place orders they never intend to fill, hoping to influence how others behave, a practice that is illegal when done to mislead and is known as spoofing.
Much trading also happens away from public books, in dark pools and through off-exchange trades. Our explainer on dark pools and retail price action explores that side of the market.
The point is that a big order sitting on the book is a clue, not a guarantee. Reading the book well means watching how it changes as price approaches, not just what it shows at a single moment.
Should beginners use it?
For most beginners trading on daily or four-hour charts, the order book is more useful as a concept than as a screen to stare at. Understanding it explains why spreads widen at certain times, why stops can be filled at worse prices, and why some levels hold while others break easily.
Short-term traders in futures and shares do use depth data actively, but it takes practice to read and can be overwhelming at first. Watching it during calm and busy periods is a good way to build intuition before trying to act on it.
Bringing it together
An order book lists waiting buy and sell orders by price, revealing how deep or thin a market is at any moment. It helps explain spreads, slippage and why prices move in steps. Treat it as a live, shifting picture of intentions rather than a fixed map of where price will go.
If you would like to understand how ready you are to manage execution and risk with real money, our free trader assessment is a good place to start.
