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- A crypto futures order book<\/strong> is a live list of buy orders (bids) and sell orders (asks), ranked by price, for a futures contract.<\/li>\n\n\n\n
- The gap between the best bid<\/strong> and best ask<\/strong> is the bid-ask spread, the baseline cost of trading right now.<\/li>\n\n\n\n
- Market makers<\/strong> post limit orders that rest in the book. Market takers<\/strong> cross the spread with market orders that consume that resting size.<\/li>\n\n\n\n
- The matching engine fills orders using Price-Time Priority<\/strong>: best price wins first, and ties go to whichever order arrived first.<\/li>\n\n\n\n
- Market depth<\/strong>, the size resting near the current price, decides how much slippage a large order causes. Deep books absorb size, thin books do not.<\/li>\n\n\n\n
- Stop-market orders<\/strong> always trigger but can fill at a worse price. Stop-limit orders protect your fill price but can fail to fill at all.<\/li>\n\n\n\n
- Reading the book alongside metrics like open interest, funding rate, and the taker buy\/sell ratio gives a fuller signal than price or depth alone.<\/li>\n<\/ul>\n\n\n\n
What Is a Crypto Futures Order Book?<\/h2>\n\n\n\n
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Definition: Crypto Futures Order Book<\/strong> A real-time, exchange-run list of every open buy and sell order for a futures contract, ranked by price and matched by an automated engine.<\/p>\n<\/blockquote>\n\n\n\n
A crypto futures order book works the same way as a spot order book, with one key difference. In spot, buyers and sellers trade the actual coin. In futures, they trade a contract that tracks the coin’s price, so the order book records bids and asks on that contract, not on BTC or ETH itself.<\/p>\n\n\n\n
Every order you place, whether it fills instantly or sits waiting, passes through this book first. It is the single mechanism behind every price you see quoted on a futures exchange. Since almost every resting order in it is a limit order waiting for a match, you’ll also see it called a limit order book<\/strong>, the two terms describe the same thing.<\/p>\n\n\n\n
How Is a Crypto Futures Order Book Structured?<\/h2>\n\n\n\n
An order book has two sides. The bid side<\/strong> lists buy orders, sorted from the highest price down. The ask side<\/strong> lists sell orders, sorted from the lowest price up. The order at the very top of each side is called the best bid and the best ask.<\/p>\n\n\n\n
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- Best Bid<\/strong>: the highest price any buyer is currently willing to pay.<\/li>\n\n\n\n
- Best Ask<\/strong>: the lowest price any seller is currently willing to accept.<\/li>\n\n\n\n
- Bid-Ask Spread<\/strong>: the gap between the two, and the cost of an instant trade.<\/li>\n\n\n\n
- Size<\/strong>: how many contracts sit at each price, shown next to every level.<\/li>\n<\/ul>\n\n\n\n
The diagram below shows a simplified BTC-USDT futures book. The best bid and best ask sit closest to the middle, and the spread between them is $3.50 in this example.<\/p>\n\n\n\n
![\"\"](\"https:\/\/mudrex.com\/learn\/wp-content\/uploads\/2026\/07\/Screenshot-2026-07-14-at-3.50.48\u202fPM-1024x607.png\" "\\"\\"")
<\/figcaption><\/figure>\n\n\n\n \n
Definition: Bid-Ask Spread<\/strong> The difference between the best bid and the best ask, representing the minimum cost of buying and immediately selling a contract.<\/p>\n<\/blockquote>\n\n\n\n
In liquid futures markets, like BTC or ETH perpetuals, this spread is often just a few cents to a few dollars. In a thin altcoin futures market, the same spread can be far wider, which quietly adds cost to every trade you place. Mudrex breaks down what widens or narrows this spread in its guide to how bid-ask spread works<\/a>.<\/p>\n\n\n\n
Who Are the Market Makers and Market Takers in an Order Book?<\/h2>\n\n\n\n
Every price you see in the book was placed by one of two types of traders, and the difference matters for how your own orders behave.<\/p>\n\n\n\n
Market makers<\/strong> place limit orders, an instruction to buy or sell only at a set price or better. These orders do not fill right away. They sit in the book, adding size and building market depth, which is why makers are also called liquidity providers.<\/p>\n\n\n\n
Market takers<\/strong> place market orders<\/a>, an instruction to fill immediately at whatever price is currently resting in the book. Takers cross the spread and consume the maker’s resting size the instant their order lands.<\/p>\n\n\n\n
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- Limit orders<\/strong> give you price control, but there’s no guarantee of a fill.<\/li>\n\n\n\n
- Market orders<\/strong> guarantee a fill, but there’s no guarantee of your price.<\/li>\n\n\n\n
- Makers<\/strong> add liquidity to the book and often pay lower fees for doing so.<\/li>\n\n\n\n
- Takers<\/strong> remove liquidity from the book and usually pay a slightly higher fee.<\/li>\n<\/ul>\n\n\n\n
How Does the Matching Engine Decide Which Orders Fill First?<\/h2>\n\n\n\n
A crypto futures exchange can process thousands of orders every second using an algorithm called Price-Time Priority<\/strong>. Two simple rules govern every single match.<\/p>\n\n\n\n
Rule 1, Price Priority<\/strong>: the best price always fills first. A buyer offering a higher price gets matched before a buyer offering a lower one, and a seller asking a lower price gets matched before one asking higher.<\/p>\n\n\n\n
Rule 2, Time Priority<\/strong>: if two orders sit at the exact same price, the one placed first gets filled first. A trader who places a limit buy at $65,100 one second before another trader at the same price will always get filled first, in full, before the second order gets any size at all.<\/p>\n\n\n\n
This is why two traders can enter the “same” price and still get very different outcomes. The engine does not know or care about your intent, only your price and your timestamp.<\/p>\n\n\n\n
What Is Order Book Depth and Why Does It Matter for Slippage?<\/h2>\n\n\n\n
Market depth<\/strong> is the total size resting at price levels near the current market price, and it is the single biggest factor in how much a large order actually costs you.<\/p>\n\n\n\n
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Definition: Market Depth<\/strong> The cumulative size of buy and sell orders sitting at each price level away from the current market price, usually shown as a depth chart.<\/p>\n<\/blockquote>\n\n\n\n
A deep order book has thick size packed close to the current price. It can absorb a large market order with barely any price movement. A shallow order book has thin, scattered size, so a market order bigger than what is resting at the best price has to “walk the book,” filling in slices at progressively worse prices until it is done.<\/p>\n\n\n\n
This gap between the price you expected and the price you actually got is called slippage<\/strong>. The diagram below shows a 2 BTC market buy order walking through three ask levels because the first two levels alone did not have enough size to fill it.<\/p>\n\n\n\n
![\"\"](\"https:\/\/mudrex.com\/learn\/wp-content\/uploads\/2026\/07\/Screenshot-2026-07-14-at-3.59.02\u202fPM-1024x554.png\" "\\"\\"")
<\/figcaption><\/figure>\n\n\n\n The table below breaks down exactly how that fill happens, level by level.<\/p>\n\n\n\n
- Market orders<\/strong> guarantee a fill, but there’s no guarantee of your price.<\/li>\n\n\n\n
- Best Ask<\/strong>: the lowest price any seller is currently willing to accept.<\/li>\n\n\n\n
- The gap between the best bid<\/strong> and best ask<\/strong> is the bid-ask spread, the baseline cost of trading right now.<\/li>\n\n\n\n