Market depth: the difference between a price and a price you can get size at
Market depth is the amount of resting size sitting at each price level of the order book, not just the best bid and offer. The quoted price only applies to the size available at the top level, so an order larger than that walks into the next levels and fills at progressively worse prices. Depth is therefore the schedule of what size costs, and the quote is only the first line of it.
Market depth is the quantity of orders waiting at each price level of the book, above and below the current quote. The headline price applies only to the size sitting at the best level, so the moment your order is bigger than that, you are buying from the next price up, and the one after that.
That is the whole gap between a price and a price you can get size at. A quote is a price with a quantity attached. Depth is the list of all the other quantities, each with its own, worse price.
A quote is a price with a quantity stapled to it
An order book is a queue of unexecuted limit orders sorted by price. On the bid side, people have offered to buy at various prices; on the offer side, people have offered to sell. The best bid and best offer are just the two innermost entries. The number next to each price is as important as the price itself: it is the amount someone is willing to transact at that level, and not one unit more.
A market order does not request a price. It requests a quantity and accepts whatever prices are needed to fill it. The matching engine takes the best level, exhausts it, moves to the next, and keeps going until the quantity is filled. Your fill is the volume-weighted average of every level you consumed.
This is why depth exists as a concept at all. Someone has to be willing to stand on the other side of your order, and the people doing that are pricing risk. A market maker will quote a small size close to the mid price because the inventory risk is small. For larger size, the risk of being left holding a position while the market moves is higher, so the price they ask for is worse. The ladder of prices in the book is a ladder of risk premiums, sorted from cheapest to dearest.
Two things follow immediately. First, the shape of the book, not the spread, determines what size costs. Second, depth is not a fixed property of a market. It is the current opinion of a set of participants who can withdraw their quotes at any time, and who withdraw them precisely when the market gets difficult.
Why a tight spread can sit on top of nothing
The spread is the distance between the best bid and the best offer. It describes the cost of a round trip in the smallest tradeable size. It says nothing about the second level, the fifth, or the twentieth.
A venue can show a very narrow top of book with tiny quantities behind it. This is not deception. It is a rational response to competition: whoever is at the front of the queue gets the flow, so there is strong pressure to be a tick better than everyone else, but only for an amount small enough to be safe. The result is a book that looks excellent and behaves badly for size.
The reverse also exists. A book with a wider top level but substantial quantity at every level behind it can fill a large order more cheaply than the tight one, because the order never has to travel far.
A quick comparison of three books, all with the same mid price:
| Book | Top-of-book spread | Size at best level | Size in next five levels | Better for |
|---|---|---|---|---|
| Thin and tight | Very narrow | Small | Small | Small orders only |
| Wide and deep | Noticeably wider | Large | Large | Larger orders |
| Tight and deep | Very narrow | Large | Large | Everything |
The third row is what a healthy, actively traded market looks like. The first row is what many markets look like at odd hours, around a scheduled event, or in a less traded instrument.
What this costs you, with made-up numbers
All figures below are invented for illustration. They are not anyone’s real prices.
Suppose a book for an asset looks like this on the offer side:
- 100 units offered at $50.00
- 150 units offered at $50.05
- 200 units offered at $50.20
- 500 units offered at $50.60
Say the best bid is $49.98, so the mid price is $49.99 and the spread looks like 2 cents. You buy 400 units with a market order.
You take all 100 at $50.00, costing $5,000. You take all 150 at $50.05, costing $7,507.50. You then need 150 more from the third level at $50.20, costing $7,530. Your total is $20,037.50 for 400 units, an average of $50.09.
Measured against the mid price of $49.99, you paid about 10 cents per unit, or roughly $40 on a $20,000 position. The spread suggested a cost of about 1 cent per unit if you measured from the mid. The depth made it ten times that. Nothing malfunctioned. You simply bought from four sellers, three of whom were more expensive than the first.
Now consider the exit. If the bid side is shaped the same way, selling 400 units costs you a similar walk in the other direction. Your round trip is not the spread. It is the spread plus the depth cost on both sides, which in this invented case is around $80 rather than the $8 you might have assumed. For how this flows into position sizing, see /tools/position-size-calculator/, and for leveraged positions where the exit price determines survival, see /tools/liquidation-price-calculator/.
The mistake: reading the ladder as a promise
The specific error is treating the displayed depth as inventory you have a claim on. It is not. Every quantity shown is a resting limit order that its owner can cancel, and cancellation is usually the fastest operation in the system. By the time your order arrives, the level you aimed at may be gone, and the level behind it may have been taken by someone whose order arrived first.
This matters most in the situations where people lean on depth hardest. A stop order placed in a quiet market assumes the book will look roughly the same when it triggers. It triggers because the market moved, and the move is exactly what makes market makers pull their quotes. So the stop converts to a market order and walks a thinner ladder than the one you were looking at when you set it. That is the main engine behind the gap between a stop level and a stop fill, covered in /guides/what-is-slippage/.
The second error is less dramatic and more expensive over time: measuring execution quality by the spread alone when you routinely trade more than the top-of-book size. If your usual order is several times the size at the best level, the spread is close to irrelevant to you. Your real cost is the average depth cost at your size, and it is worth measuring on your own fills rather than inferring from the quote. Comparing venues on quoted spread while trading size is comparing the wrong number: see /compare/ for what else differs between them.
The practical consequence is not caution for its own sake. It is that size and urgency are the two things you actually control. A limit order declines to walk the ladder, at the cost of possibly not filling: the trade-off is laid out in /guides/market-order-vs-limit-order/, and the hybrid case in /guides/what-is-a-stop-limit-order/. Breaking an order into pieces gives the book time to refill between them. Trading when more participants are present means more levels are populated. None of this removes the cost of size. It just stops you paying for depth you did not need to consume. Terms used above are defined in /glossary/.
Education only, not financial advice, and trading carries a risk of loss.
Key facts
- A quote describes one price and one quantity, not an unlimited supply at that price.
- An order larger than the top level consumes each level in turn and fills at the volume-weighted average of the levels it touched.
- Spread and depth are independent: a book can show a narrow top-of-book and almost nothing behind it.
- Depth is made of resting limit orders, which can be cancelled faster than most orders can arrive.
- The effective cost of size can be measured as the gap between the mid price and the average fill price.
- Depth thins when market makers widen their risk, so it is usually smallest at the moment size is most urgent.
Frequently asked
Is a tighter spread always better execution?
Only for size that fits inside the top level. The spread tells you the cost of a minimum-sized round trip. If your order is larger than the quantity shown at the best bid or offer, the levels behind it decide your fill, and a book with a wider top but deep levels behind it can be cheaper for real size.
Why did my order fill worse than the depth display suggested?
Two reasons, usually together. The displayed size is a snapshot of resting limit orders, and those orders can be cancelled in the time between your click and your order reaching the matching engine. Also, other orders may be queued ahead of yours and may have taken the levels you were aiming at.
Does depth exist in forex if there is no central order book?
Yes, but it is fragmented. Liquidity sits with individual providers rather than one public venue, so you see depth for the stream your broker aggregates, not for the whole market. The mechanism is identical: a price is attached to a quantity, and larger size reaches further into the stack.
How do I estimate the cost of size before I trade?
Work out the volume-weighted average price your order would reach across the visible levels, then compare it with the mid price. The difference is your expected cost of taking liquidity, separate from commission. If that number is uncomfortable, the order is too large for the book at that moment.
Risk warning: trading is high risk and most retail accounts lose money. Education only, not financial advice.