What is an order book, and how does it set the price you pay?
Every trading screen is built on the same simple ledger of buyers and sellers. Learn to read it and you will understand why the price you see is rarely the price you get.
Education only, not investment, tax or legal advice. Crypto-assets are high-risk — risk disclosure.
The short answer
An order book is an exchange's live list of unfilled buy orders (bids) and sell orders (asks) for one asset, sorted by price. A trade happens when a new order crosses the gap between the best bid and best ask; orders are then filled by best price first, earliest first.
Key takeaways
- The order book lists resting limit orders: bids to buy on one side, asks to sell on the other, each with a price and a quantity.
- The spread is the gap between the highest bid and the lowest ask; a wide spread is a cost and a sign of uncertainty.
- Most exchanges match orders by price-time priority: the best price fills first and, at the same price, the earliest order fills first.
- A market order walks through the book until it is filled, so large orders in thin books can fill well away from the last traded price.
What does an order book actually show?
An order book is the exchange's running ledger of everyone who wants to trade an asset but has not traded yet. Each line is a limit order: an instruction to buy or sell a stated quantity at a stated price or better. The U.S. Treasury's Office of Financial Research describes the book as a publicly visible mechanism that shows bids and offers, each with a price and a quantity2.
The book has two sides. Bids are buy orders, listed from the highest price down. Asks (also called offers) are sell orders, listed from the lowest price up. The highest bid and the lowest ask are together called the top of the book, and they are the two numbers most price tickers summarise into a single "price".
An illustrative order book for a token quoted in US dollars (made-up numbers)
| Side | Price (USD) | Quantity | What it means |
|---|---|---|---|
| Ask | 100.60 | 3.5 | Someone will sell 3.5 units, but only at 100.60 or more |
| Ask | 100.30 | 2.0 | Next-cheapest seller |
| Ask | 100.15 | 0.8 | Second-best offer |
| Ask (best) | 100.10 | 1.2 | Lowest price any seller will currently accept |
| Bid (best) | 100.00 | 1.5 | Highest price any buyer will currently pay |
| Bid | 99.95 | 2.2 | Second-best bid |
| Bid | 99.80 | 4.0 | A deeper buyer, further from the market |
Nothing in the table is a trade. It is a queue of intentions. A trade only appears in the separate "trades" or "time and sales" feed once a buyer and a seller agree on a price.
What is the bid-ask spread and why does it matter?
The spread is the gap between the best ask and the best bid. In the example above it is 100.10 − 100.00 = 0.10 dollars, or about 0.10% of the mid-price of 100.05. Anyone who buys at the ask and immediately sells at the bid loses that gap, so the spread is a real, if hidden, trading cost.
The spread also carries information. The OFR paper notes that a wider spread reflects greater uncertainty about the asset's price2. Spreads tend to widen when fewer participants are quoting, for example in small tokens, at quiet hours or during sudden news.
Figure · Tight book versus thin book
How are orders matched?
A trade happens when a new or modified order crosses the spread: a buy order priced at or above the best ask, or a sell order priced at or below the best bid. The incoming order then trades against the best opposing orders using price-time priority2: the best price is filled first and, among orders at the same price, the one that arrived first is filled first.
Figure · From order to trade
Price-time priority rewards traders who quote competitive prices early, which is why the top of a busy book is usually crowded with automated quotes. Not every venue allocates fills the same way, so it is worth reading a platform's rulebook before relying on a queue position.
The book is also why our explainers on market and limit orders and on liquidity and slippage keep coming back to the same picture: every order type is just a different way of interacting with this queue. The rest of our Markets section builds on it.
What happens when a market order hits the book?
A market order says "fill me now at whatever price is available". The SEC's investor education site warns that a market order generally executes at or near the current ask for a buy (or bid for a sell), but that the last traded price is not necessarily the price you will get1. The order book shows exactly why.
Worked example
Buying 3 units with a market order
Using the illustrative book above, a market buy for 3 units fills 1.2 units at 100.10, 0.8 units at 100.15 and the last 1.0 unit at 100.30. Total cost: 120.12 + 80.12 + 100.30 = 300.54 dollars, an average of 100.18 per unit.
That is about 0.08% above the best ask you saw on screen, before any trading fee. The same order in a book ten times deeper would have filled entirely at 100.10.
This effect is called slippage, and it grows with order size and shrinks with depth. Depth is the ability to buy or sell a given amount without moving the quoted price2, so a deep book absorbs large orders quietly, while a thin book lets even modest orders push the price.
How to read depth before you trade
- 1
Find the top of the book
Note the best bid and best ask and work out the spread as a percentage of the mid-price.
- 2
Add up quantity near the top
Total the quantity on the side you would trade against within, say, 0.5% of the best price.
- 3
Compare with your order size
If your order is a large share of that quantity, expect slippage; consider a limit order or a smaller size.
- 4
Check the fee schedule
Many platforms charge different fees to orders that add liquidity (resting limits) and orders that remove it (market orders).
Are crypto order books different from stock market order books?
The mechanics are the same: crypto trading platforms also run limit order books with bids, asks and a matching engine. The differences are around the book rather than inside it. Crypto markets trade around the clock, the same token can trade on many unconnected platforms at slightly different prices, and many platforms operate the exchange, the broker and the custodian under one roof.
That last point matters for protection. In a 2023 investor alert, the SEC warned that crypto asset platforms may lack important investor protections and that, at the time, none of the major crypto asset entities was registered with it as a broker-dealer, exchange or investment adviser3. A tidy order book on screen tells you nothing about who holds your money or whether the volume is genuine. Our guide on how to check whether a firm is regulated covers that side, and the Policy section explains who regulates what.
Same machine, different surroundings
| Feature | Typical listed stock | Typical crypto platform |
|---|---|---|
| Trading hours | Set exchange sessions | 24 hours, 7 days a week |
| Where it trades | Linked venues under one market-wide rulebook | Many separate platforms, prices can differ |
| Who holds the asset | Broker and separate custodian | Often the platform itself |
| Matching rule | Price-time priority is common | Price-time priority is common |
What mistakes do beginners make with order books?
Common beginner mistakes
Treating the last price as your price
The ticker shows the last trade. Your market order fills against whatever asks or bids are resting now, which may be worse.
Ignoring the spread
On thinly traded tokens the spread alone can cost more than the trading fee. Work it out before you click.
Using market orders in thin books
A large market order can walk several price levels. A limit order caps the price you pay, at the risk of not being filled.
Trusting a busy-looking book
Quantity on screen can be cancelled in milliseconds, and displayed depth may not match what you can actually trade. Depth is a snapshot, not a promise.
Risk warning
Price risk is not the only risk
Crypto-assets are highly volatile and the platforms that list them may not offer the protections you expect from a regulated stock exchange. Never trade money you cannot afford to lose. Read our risk disclosure before acting on anything you learn here.
Frequently asked questions
Is the order book the same as the price chart?
No. A price chart plots trades that have already happened. The order book shows orders that are waiting to trade, so it describes where the price could move next rather than where it has been.
Why do orders disappear from the book?
An order leaves the book when it is filled, when the trader cancels it, or when it expires. Automated traders update quotes constantly, so the book can change many times per second.
Can I see every order in the book?
Usually you see the visible part. Many platforms let traders hide some or all of an order's size, and screens often group orders by price level, so the displayed depth is a simplified view.
Does a limit order guarantee a fill?
No. A buy limit order can only execute at your limit price or lower, and a sell limit only at your limit price or higher1. If the market never reaches your price, nothing happens.
What is a market maker in an order book?
A market maker is a firm or program that keeps quoting both bids and asks, earning the spread in exchange for supplying liquidity. Their quotes are a large share of the orders near the top of busy books.
The bottom line
The order book is the queue behind every price. Read the spread, check how much quantity sits near the top, and remember that a market order fills against the book as it is now, not against the last price on the ticker. On crypto platforms, a healthy-looking book says nothing about who holds your funds, so check regulation separately.
Sources
- Types of Orders — Investor.gov, U.S. Securities and Exchange Commission Primary source
- Effects of Limit Order Book Information Level on Market Stability Metrics (OFR Working Paper 14-09) — Office of Financial Research, U.S. Department of the Treasury, 2014 Primary source
- Exercise Caution with Crypto Asset Securities: Investor Alert — Investor.gov, U.S. Securities and Exchange Commission, 2023 Primary source
How we checked this page: every figure above links to the numbered source it came from. Spotted an error? Tell the desk — see our editorial policy.