What it means
A stock’s order book shows bids to buy and offers to sell at specified prices. Its depth describes how many shares are displayed at each price level, not just at the best bid and best ask. A deeper book has more displayed shares available across the levels being examined.
Depth is side-specific: bids matter when selling, while offers matter when buying. It also depends on the price range you include. Saying a stock has “10,000 shares of depth” is incomplete without identifying the side, price range and time of observation.
Displayed depth is a snapshot, not a promise that an order will fill. Orders can be canceled, added or executed before an incoming order reaches them. Some available liquidity may not appear in the displayed book.
How it is calculated
One practical measure is cumulative displayed depth: add the displayed share quantities on one side of the book through a stated price level.
Cumulative depth (shares) = Σ displayed shares at eligible price levels on one side
- Cumulative depth is the total displayed quantity, measured in shares.
- Σ means add the quantities together.
- Eligible price levels are the bids or offers within the stated range. For a buy order, these could be offers at or below a specified maximum price; for a sell order, bids at or above a specified minimum price.
- One side means bids and offers are calculated separately, rather than combined into a single fillable quantity.
If the question is how much of a particular order the displayed book could accommodate at or better than a price, compare its share quantity with cumulative depth on the opposing side. This is a snapshot calculation, not a fill forecast.
Worked example
Suppose a hypothetical stock has these displayed offers at one instant:
| Offer price | Displayed shares |
|---|---|
| US$50.00 | 200 |
| US$50.01 | 300 |
| US$50.03 | 500 |
A hypothetical investor is checking a 600-share buy order with a maximum price of US$50.03.
- Add the offers at or below the maximum: 200 + 300 + 500 = 1,000 shares of cumulative displayed ask depth.
- The first two levels contain 500 shares. On this static snapshot, another 100 shares would have to come from the US$50.03 level to account for all 600 shares.
- If those quantities remained available and the order interacted with them in that sequence, the illustrative total would be
(200 × US$50.00) + (300 × US$50.01) + (100 × US$50.03) = US$30,006. Dividing by 600 gives an average of US$50.01 per share.
The calculation excludes commissions and other costs. Actual fills can differ because the book and the order’s place in the execution queue can change.
Why it matters before you trade
An order larger than the displayed quantity at the best opposing price may reach additional price levels. That can change the average execution price even when the quoted bid-ask spread looks narrow. Comparing order size with depth across relevant prices helps distinguish a small top-of-book quote from the liquidity available over a wider range.
Depth is not the same as average daily volume: one measures displayed orders at a moment; the other summarizes shares traded over a period. Neither alone establishes what a future order will cost. In a pre-trade check, consider depth alongside order size, spread, volatility and market-impact estimates.
How PreTrAIde uses it
PreTrAIde’s published product facts do not say it measures live order-book depth. Its pre-trade analysis instead estimates bid-ask spread, participation in daily volume and market impact, and calculates liquidity scores and tiers. It returns basket-level and per-stock verdicts with reasoning and a suggested execution strategy. Those estimates relate to execution conditions, but they should not be read as a live depth display or a guarantee of a fill.
Common mistakes
- Treating best-quote size as total depth. Additional price levels may hold more displayed shares, at different prices.
- Adding bids and offers together. A buy order interacts with offers; a sell order interacts with bids.
- Assuming displayed size is guaranteed. Quotes can change, and hidden liquidity may also affect execution.
- Ignoring the price boundary. Depth of 1,000 shares across several prices does not mean all 1,000 are available at the best price.