How to Read a Crypto Exchange's Order Book

An order book looks like noise until you know which side to read and which column to trust. This covers what the columns mean, why depth matters more than the top price, and where order books get faked.

The two sides of the book

An order book lists open buy orders on one side and open sell orders on the other, sorted by price. Bids are what buyers will pay, asks are what sellers will accept. The best bid is the highest buy price, the best ask is the lowest sell price. The gap between them is the spread, and it is the venue's raw revenue on every fill.

A market buy walks down the ask column. A market sell walks down the bid column. That is the whole mental model: your order consumes a queue, and the cost is the queue itself.

Depth is the number that actually matters

The top of the book tells you the price available right now. Depth tells you the price you will get for the size you actually want. If you want to buy ten thousand dollars of an asset and only six hundred sits beneath the best ask, the remainder fills at worse levels, and the average is your real cost.

Read the cumulative column. Sum the size at each level and ask where your order ends. That gap between a quoted price and an executed price is why large orders on thin pairs look expensive in hindsight and cheap in the chart.

The patterns that show up

• A wall. A very large single level. Sometimes genuine support, sometimes an order placed to be cancelled before it fills.
• A thin book. Very little size behind the spread. Small orders move price far, and your exit can be much worse than your entry.
• A book that refills instantly. Levels that reappear the moment they are eaten, usually a market maker working both sides.
• Identical size on both sides at one price. Often a single participant posting two orders rather than two independent traders.

Practical checks before you trade

• Depth at your size. Walk the book and find where your order finishes. That average price is closer to your real cost than the best bid or ask.
• Spread against your size. A spread small in absolute terms can be enormous relative to the profit you are trying to make.
• Consistency across venues. If two reputable venues disagree materially on depth for a major pair, one is showing you something inaccurate.
• Behaviour under stress. Watch the book during a volatile period rather than a calm one. Thinning out exactly when you need to exit is the normal failure mode.

The cost of crossing the spread

Every market order pays the spread immediately. That is the price of immediacy. A limit order at the best bid may never fill, and may fill slowly in a fast market. Both choices have a price, and it is worth deciding which one you are paying rather than discovering it in a fill report.

A short habit worth keeping

Before placing an order, glance at the book, note the size at the best bid and ask, and decide whether you want to add liquidity or take it. Placing a limit order near the touch of the book often costs no more than crossing the spread and occasionally earns the spread instead. On a thin pair that choice is worth more than most of the analysis you might do about the asset.

Why order books get faked

Nothing on a screen is guaranteed. Orders can be placed and cancelled in milliseconds, stacked layers can collapse the instant price approaches them, and some venues report different books to different clients. Wash trading creates volume that never existed, which makes a thin book look busy.

The defence is to treat the book as one input. Check spread and depth on more than one venue, look for volume that survives a price move rather than volume that spikes with one, and never treat a visible wall as a promise.

Frequently asked questions

Does a large order at one price stop the market?

Sometimes, and it is worth respecting when it holds. But a large order can be cancelled before you reach it, and some venues route orders across several participants. Treat the wall as information about intent rather than a guarantee.

Why did my fill price differ from the price I saw?

Your order consumed multiple levels of the book. The average fill is weighted across every size you took, and between the moment you looked and the moment your order landed, other trades moved the queue underneath you.

How much depth is enough?

Enough that your order is a small fraction of it. If your trade is a large share of the size sitting at the levels it will consume, you are the price impact. Splitting the order into smaller pieces reduces impact but exposes you to price moving against you in the meantime.