An order book is a live, constantly updating list of every buy order (bid) and sell order (ask) waiting to be filled for a given asset on an exchange. It’s the closest thing crypto trading has to X-ray vision into supply and demand — and learning to read one is the first real step from ‘buying crypto’ to ‘trading crypto.’
Key Takeaways
- Bids are buy orders, listed with the price a buyer is willing to pay; asks are sell orders, listed with the price a seller wants.
- The ‘spread’ is the gap between the highest bid and the lowest ask — a tight spread usually signals a liquid, actively traded market.
- ‘Market depth’ refers to how many orders sit at each price level; deep order books can absorb large trades with less price impact.
- A market order fills immediately against the best available price in the book; a limit order sits in the book until someone matches it (or you cancel it).
- Thin order books (common on low-volume tokens) can cause large price swings (‘slippage’) from relatively small trades.
Reading an Order Book
| Term | What It Means |
|---|---|
| Bid | A standing offer to buy at a specific price |
| Ask | A standing offer to sell at a specific price |
| Spread | Difference between the best bid and best ask |
| Depth | Total volume of orders at each price level |
| Slippage | The gap between expected and actual fill price on a large or fast-moving order |
Our Take
New traders often treat the order book as a curiosity and jump straight to clicking Buy, but it’s genuinely useful information, especially before placing a sizable order. A wide spread with thin depth on both sides is a warning sign: your market order could fill at a noticeably worse price than the last traded price you saw on the chart. Watching how quickly the book refills after a large order gets absorbed also tells you something charts alone don’t — whether the current move has real conviction behind it or is running on thin liquidity that could reverse just as fast.
The practical habit worth building is simple: before any trade larger than a token amount, glance at the depth on both sides of the book, not just the last price. On a liquid pair like BTC or ETH against a major stablecoin, that’s rarely necessary. On a smaller-cap token, it can be the difference between a clean fill and a trade that moves the market against you.
FAQs
What’s the difference between a bid and an ask?
A bid is an order to buy at a set price; an ask is an order to sell at a set price. The current market price sits between the highest bid and the lowest ask.
Why does the order book matter if I only place market orders?
Market orders fill against whatever is sitting in the book, so a thin book can mean your trade fills at a worse average price than expected — this is called slippage.
📎 Source: Coinbase Learn — What is an order book?
