Market depth is the quantity available to buy or sell across a range of prices. In a displayed order book, it is the resting size at individual price levels, or the cumulative size through a specified price. For a trading desk, depth turns “the offer is 101” into a more useful question: how much can we buy before paying more?
The distinction matters whenever an order is larger than the quantity at the best price. A narrow bid–ask spread can coexist with very little size. A strategy that assumes its whole position trades at the best quote can therefore overstate its executable profit.
Depth needs a side, a range and a unit
A buyer consumes asks; a seller consumes bids. Ask depth through a price includes displayed shares offered at that price or lower. Bid depth down to a price includes displayed shares bid at that price or higher. State whether the units are shares, contracts or another instrument quantity.
If ask prices are and displayed quantities are , cumulative ask depth through a maximum price is
Depth “at the best ask” and depth “through several ask levels” answer different questions. A count of levels alone also misses how far apart their prices are. A useful comparison fixes the price range, the side, the observation time and the venue or feed coverage. CME's liquidity guide treats book depth, bid–ask spread and size-dependent cost to trade as distinct views of liquidity.
A 70-share order meets 50 shares at the best ask
Consider this synthetic, single-venue snapshot, with prices in USD per share. Assume price priority and a frozen book, with no fees, hidden liquidity or competing orders while matching occurs.
| Ask price, USD/share | Shares at this price | Cumulative ask depth |
|---|---|---|
| 101 | 50 | 50 shares |
| 102 | 100 | 150 shares |
A buy for 70 shares cannot fit entirely at 101. Under these assumptions, it takes 50 at 101 and 20 at 102. The cash purchase amount is
The order's fill-weighted average price is
This is the execution VWAP of this order's fills, rather than a session-wide market benchmark. The last share costs 102; the average is lower because most shares trade at 101. Eighty shares remain offered at 102 in this frozen calculation.
Original frozen, single-venue example. Depth includes quantities at or below the stated ask price. Prices are USD per share and sizes are shares; fees, hidden liquidity and competing updates are excluded.
Paying 101 for all 70 shares would understate the cash outlay by USD 20. That extra amount is the static walking cost against the initial ask, before fees. No bid or midpoint was specified, so it is not a cost against the midpoint or a complete transaction-cost estimate.
Price protection changes the answer. A buy limit of 101 for the same 70 shares permits only the first 50 shares to trade in this snapshot, leaving 20 unfilled. The rest cannot trade at 102 under that cap. This follows the directional protection described by Investor.gov; the remainder's treatment depends on the order instructions and venue rules.
What the depth display leaves unresolved
Availability changes. Another trader may consume the displayed size first; quotes can be cancelled, added or replenished before your order arrives. A static book walk is conditional arithmetic, not a guaranteed fill. It also does not identify the complete causal market impact of submitting the trade: that requires a model of how the market responds, including what would have happened without it.
Coverage is limited. A display may include only selected levels on one venue, and hidden liquidity is absent from displayed size. Nasdaq's TotalView explanation explicitly concerns depth on Nasdaq. An instrument being listed elsewhere does not turn that view into an aggregate of all trading venues.
Resting orders have a different question. A large quantity on your own side does not tell you when your passive order will fill. Priority, incoming orders and cancellations matter. Nor does a large bid automatically predict a rising price: displayed depth is trading interest at an instant, while trading volume records executed trades over a period.
An observation has a boundary. If a requested buy exceeds the displayed quantity you have observed, the snapshot cannot price the whole request. Report the covered quantity and the unpriced remainder. Dividing the partial cash amount by the original request would create an artificially low average.
For a worked treatment of these distinctions, Order Books develops cumulative depth, queue allocation and snapshot-based execution models. The lesson has a public preview; full access requires membership.
Sources
- Rishi K. Narang, Inside the Black Box: A Simple Guide to Quantitative and High-Frequency Trading, 2nd edition (Wiley, 2013), Chapter 7: “Aggressive versus Passive,” pp. 120–121; “Large Order versus Small Order,” p. 127; and “Where to Send an Order,” pp. 127–128. All example prices, quantities and the figure are original.
- CME Group: CME Liquidity Tool User Guide, “Types of Analysis” — depth by level, spread and order-size-dependent cost to trade.
- Nasdaq: Finding Depth to Inform Markets of the Future, introduction — the venue scope of TotalView depth.
- Investor.gov: Types of Orders — buy-limit price protection.