The quick version. An order book is a live list of unfilled limit orders on one venue, sorted by price, with buyers on one side and sellers on the other. The gap between the best of each is the spread, and the size stacked behind them is depth, which tells you roughly how far an order of a given size would push the price. What the book cannot tell you is intent, because every order in it can be cancelled in an instant.
What you are actually looking at
The book has two sides. Bids are offers to buy, listed from the highest price down. Asks, sometimes called offers, are willingness to sell, listed from the lowest price up. Most interfaces aggregate all orders at the same price into a single row with a total quantity.
Between the two sides sits a gap, because the engine would already have paired anyone selling cheaply with anyone buying dearly. Everything in the book is an order nobody has yet been willing to trade against.
Hold one thing in mind from the start: this is a single venue’s book. Crypto has no consolidated book across exchanges, so the depth you are staring at is only the liquidity available where you happen to be looking. A short definition sits in our order book glossary entry.
The spread, and what it costs you
The best bid and the best ask are the two prices you could actually trade at right now: sell at the bid, buy at the ask. The distance between them is the spread, and crossing it is an immediate cost you pay before the market has done anything at all.
A tight spread generally means competition among market makers and reasonably healthy interest. A wide one means the opposite, and spreads tend to widen precisely when conditions get volatile, because quoting tightly becomes riskier for whoever is providing the price.
The mid price sitting neatly between the two is a convention. Useful for charting and measuring, but not a price at which anyone has agreed to trade with you.
Depth, and the only question that really matters
Depth is the cumulative size waiting at and beyond the top of the book. Many interfaces show it as a running total, or as a shaded chart sloping away from the mid price.
The productive way to read it is not to admire the shape but to ask something specific: for the size I am considering, how far down the book does that order reach? That question turns a wall of numbers into an estimate of price impact, which is why depth matters more than the headline volume figure.
Depth is also frequently lopsided. A book can look well supplied on the buy side and be hollow on the sell side, or the reverse. The side that matters is whichever one you will eventually need, and exit liquidity has an unhelpful habit of thinning when everyone wants it at once. The mechanics are in liquidity and slippage, and the resulting cost is slippage.
Why “walls” are not commitments
Sooner or later you will see an unusually large order sitting a little away from the current price, and someone will describe it as support, or as a wall holding the market up. Treat that reading with caution.
A resting order is not a promise. It is an offer its owner can withdraw at any moment, usually at no cost, and orders of that kind are routinely pulled the instant price approaches them. The wall you were relying on can cease to exist a fraction of a second before your own order arrives.
Placing orders with no intention of filling them, to create a false impression of interest, is a recognised manipulation pattern, prohibited on regulated venues and harder to police elsewhere. You cannot tell from the screen which large orders are genuine.
Books can also hide size. Iceberg and reserve orders display only a slice of their true quantity, refreshing as each slice fills, so a modest-looking level may be far deeper than it appears.
Safety: Sizing a position around a visible wall means betting on data that can vanish before your order is matched. If a large resting order is the main reason for a decision, the decision has no foundation you can verify. Nothing in the book is binding until it trades.
The book versus the tape
The order book shows intentions. The trade history, often labelled time and sales or simply the tape, shows what actually happened: executed trades, with size, price and direction.
That difference is worth internalising. A trade is evidence, because somebody committed capital and cannot take it back. A resting order is a proposal. When the two disagree, the tape is the one telling you the truth.
Market orders never appear in the book at all. They arrive, consume resting liquidity and show up only afterwards on the tape, which is why a book can look calm right up until it is emptied.
A sane way to read a book
You do not need to watch it flicker to get value from it. A handful of questions, asked before you trade rather than after, extract most of what is genuinely there.
- How much size sits within a band you care about? Pick a percentage away from the mid that would bother you, and total the depth inside it.
- How does that compare on both sides? Balanced depth and one-sided depth behave very differently under pressure.
- Does this venue hold a meaningful share of the asset’s turnover? A deep-looking book on a minor venue can still be the thin end of the market.
- Does the depth persist? Watch whether size stays put as price approaches, or evaporates and reappears elsewhere.
Those answers tell you what an order of your size would plausibly cost, which is the book’s practical purpose. They tell you nothing about direction, and no arrangement of resting orders ever has. For the wider context of turnover and size see market cap, volume and liquidity explained, and for how each order type meets the book, order types explained.
Key takeaways
- Bids sit below, asks above, and the spread between them is a cost you pay the moment you cross it.
- Depth is only useful compared to your own intended size. That comparison estimates price impact.
- Resting orders can be cancelled instantly and for free, so large visible walls are not commitments.
- Iceberg orders hide size and market orders never appear, so the book is incomplete even in good faith.
- The tape records what happened; the book records what somebody says they might do.
Frequently asked questions
Does a big buy wall mean the price will hold?
No. A large resting bid tells you only that somebody has an order in the book at this instant. It can be cancelled before price ever reaches it, and orders placed to create an impression rather than to trade are a known manipulation pattern. Even genuine size gets withdrawn when conditions change.
Why do the books look different on two exchanges?
Because each venue keeps its own independent book, with its own users, market makers and fee structure. There is no consolidated view across crypto exchanges. Arbitrage tends to keep quoted prices roughly aligned, but depth can vary enormously, so the liquidity actually available to you depends on where you trade.
What is the difference between depth and volume?
Volume is what has already traded over some period, usually the last day. Depth is what is waiting to trade and has not yet done so. Volume tells you a market has been active; depth tells you what capacity exists right now for your specific order. Depth is the one that determines your price.
Do I need to watch the order book to trade sensibly?
Not continuously. Most of the value comes from a single check before you act: whether enough size sits near the current price for the amount you intend to trade, on both sides. Staring at a flickering book invites you to read meaning into cancellations that are largely automated and rarely mean what they appear to.
Educational content, not financial advice. Crypto is volatile and high-risk; never share your seed phrase or private keys with anyone. Always do your own research.
Last updated Jul 25, 2026
