Orders & execution · Intermediate

Order Book Basics

An order book lists every resting order at every price, with the size behind each one. It is a precise picture of current supply and a poor forecast of anything.

Last reviewed 15 September 2026 by the MarketPro research desk · Editorial policy

In short

An order book is the live list of outstanding buy and sell orders for an instrument, stacked by price. Bids run downward from the highest price a buyer will pay, asks run upward from the lowest a seller will accept, and the gap between the best of each is the spread. The size at each level shows how far price must travel to absorb a large order. It records what is resting now, not what will happen next.

What to remember

The book prices your order before you send it. A 5,000-share buy against 800 at the best ask fills at an average 1.4 cents worse, roughly $70.
Depth decides impact, not order size. The same order is free in a deep book and expensive in a thin one.
Visible size is not all the size. Icebergs, dark venues and spoofed orders all mean the number on screen understates or overstates what is really there.
Spot forex has no central order book, so platform depth is one broker aggregating its own providers, and chart volume is tick count rather than traded volume.

Lesson 9 of 16 in Orders and execution

PreviousChart Timeframes Explained NextTrading Costs: Spreads, Commissions and Fees

Five levels deep on a $42 stock

An order book is the live list of outstanding buy and sell orders for one instrument, stacked by price level. It tells you what is resting in the market right now, on both sides, and how much of it there is. Reading one takes about thirty seconds once you have seen the layout.

Take a stock quoted 42.05 bid, 42.07 ask. Its book might look like this.

Bid size	Bid	Ask	Ask size
1,200	42.05	42.07	800
3,400	42.04	42.08	2,600
900	42.03	42.09	400
6,000	42.02	42.10	11,000
2,500	42.01	42.11	1,500

A market buy for 800 shares fills entirely at 42.07, because 800 shares are resting there. A market buy for 5,000 shares cannot. It takes the 800 at 42.07, then 2,600 at 42.08, then 400 at 42.09, and the last 1,200 at 42.10. The average fill works out at 42.084, which is 1.4 cents worse than the quote on screen, or about $70 on the whole order. Nothing went wrong. The book had already told you what the order would cost, and the only way to find that out without the book is to send the order and read the confirmation.

Bids, asks and the queue behind each price

Bids are outstanding buy orders, listed from the highest price a buyer will pay downward.
Asks, also called offers, are outstanding sell orders, listed from the lowest price a seller will accept upward.
Size is the quantity available at each listed price.

The gap between the best bid and the best ask is the spread, 2 cents in the example above. When an incoming order crosses that gap and matches something resting on the other side, a trade prints and the resting order is reduced or removed. That is the entire matching mechanism.

Within a single price level, most venues fill in the order the orders arrived. So a limit order joining the 6,000 shares bid at 42.02 is behind 6,000 shares. If only 2,000 trade there before price leaves, you were never close to a fill. This is why a limit order at a busy round number is often a worse bet than one a cent inside it.

What a thin book does to a 5,000-share order

Depth is the total size waiting across the levels, and it decides how far price has to travel to absorb you. Put the same 5,000-share order against a book with 40,000 shares resting at 42.07 and it fills at 42.07 in full, at zero cost beyond the spread. The order did not change. The book did.

12 more sections in MarketPro

Depth is also not a fixed property of an instrument. It thins out predictably: in the minutes around a scheduled release, in the first and last few minutes of a session, and in any instrument that is simply not widely traded. A small-cap stock or an exotic currency pair carries a permanently thin book, which is why the same position size behaves completely differently there. Our note on how prices move covers the flow side of the same story.

The practical use of depth is sizing, not prediction. If your intended order is a meaningful fraction of what is showing, you already know to work it in pieces or use a limit.

Icebergs, spoofs and the orders that are not there

The book shows visible orders only, and that is a bigger caveat than it sounds.

Iceberg orders display a small slice of a much larger order and refresh as each slice fills. A level showing 500 shares can absorb 50,000.
Dark venues match orders that never appear on the lit book at all. A significant share of equity volume never shows.
Spoofing is the placing of large orders with no intent to trade them, pulled the instant price approaches. It is illegal and prosecuted, and it still happens, which is why a 20,000-share wall is not a ceiling.

The book also changes many times a second. What your screen shows is a frame from a film, refreshed at whatever rate your platform manages. Treating a snapshot of resting intentions as a forecast of where price will go is the single most common misreading, and it is why order-flow screens sell so well to people who have not yet noticed that the wall keeps disappearing.

Spot forex has no central book at all

Everything above describes an exchange. Spot forex is not one. There is no central venue, so there is no single order book, and what your platform labels depth of market is your broker aggregating quotes from its own liquidity providers. Two brokers will show you different books for EUR/USD at the same instant. Volume on a forex chart is tick count from that one feed, not traded volume, which is why volume analysis transfers badly from equities.

The reasoning still transfers even where the data does not. Thin conditions mean your order moves price further, spreads widen when providers pull back, and the cost lands on market orders rather than resting ones. Knowing which hours are thin does the same job as reading a book you cannot see, and the session map is where that starts.

Three things worth doing with it

Before any order that is large for the instrument, glance at the size at the top two or three levels and decide whether you are taking liquidity or providing it.
When you use a limit, place it where there is genuine size nearby rather than at the obvious round number where the queue is longest.
Record the quote you saw and the price you got. Over a few dozen trades that difference is a measured number, and it is the only honest read on what your execution actually costs. The glossary has the terminology if any of it is new.

An order book is a good instrument and a poor oracle. It describes current supply precisely and says nothing at all about the next order to arrive.

Finish this lesson in MarketPro

The rest of Order Book Basics, a quiz that checks it landed, and your place kept in the curriculum.

12 more sections of this lesson
All 320 lessons, with no cap and no paywall
Free on any account, including a free one

In your browserContinue on webor continue on web

Same account as the app · No install · Free to start

Or scan with your phone camera to get the MarketPro app.

Download on theApp Store Get it onGoogle Play

Free account, no card needed.

Not investment advice. Past performance is not indicative of future results.

Run the numbers while this is fresh

Work it through on your own position size. Nothing leaves your browser and there is no sign-up: Position size calculator, Margin calculator, Leverage calculator.

Written and reviewed by the MarketPro research desk

MarketPro is a trading-signal and trading-education app operated by Harajuku Holdings LTD (Cyprus). Our signal desk publishes and tracks every trade idea in-app, and every page here is checked against the app's live behaviour before publishing. Last reviewed 15 September 2026. How we produce signals and content · About MarketPro

FAQ

Order Book Basics: questions

What does market depth actually tell you?

It tells you how much size is resting at each price on both sides of the market right now. That lets you estimate what a given order would cost to fill, because anything larger than the size at the best price has to reach into worse prices. It is a measure of current liquidity, not a prediction, and it can change completely within a second.

Can you predict price direction from the order book?

Not reliably. The book shows resting intentions that can be cancelled at any moment, and large visible orders are frequently pulled as price approaches. Hidden and iceberg orders mean the displayed size is often wrong in the other direction too. Most professional use of book data is about execution quality, deciding how to fill an order, rather than about direction.

Why does my forex platform show a different depth of market to someone else?

Because spot forex has no central exchange. Each broker aggregates quotes from its own set of liquidity providers, so two platforms genuinely show different books for the same pair at the same moment. Neither is wrong. It also means forex chart volume is a count of price updates on that one feed rather than traded contracts.

Trade · Learn · Earn

The whole academy is free in MarketPro

All 320 lessons, 12 guided paths, quizzes and a daily streak. Free on every account, in the app or your browser, with vetted free signals alongside it.

In your browserContinue on webor continue on web Download on theApp Store Get it onGoogle Play

Same account as the app · No install · Free to start

Not investment advice. Past performance is not indicative of future results.