Order book depth shows the resting buy and sell orders displayed at different prices, while slippage is the difference between an execution price and a chosen reference price. Together, they help explain why the price paid or received for a trade can differ from the quote visible before submission.
What an order book actually shows
An order book organizes limit orders into bids, which offer to buy, and asks, which offer to sell. The highest bid and lowest ask form the best available displayed prices. Their difference is the bid-ask spread. Each price level also has an associated quantity, although interfaces may report that quantity in coins, contracts, or quote-currency value.
Depth describes how much displayed quantity is available across a specified range of prices. It is more informative to ask about depth within a defined distance from a reference price than to describe a market as simply deep or shallow. A book can have substantial orders far away from the best quotes while offering little immediately executable liquidity near them.
Why larger orders can receive different prices
A marketable buy order consumes available asks, beginning with the lowest ask. If the quantity at that level is insufficient, execution continues at higher prices, subject to the order’s instructions and venue protections. A marketable sell order similarly consumes bids and may move down through several levels. This process is often called walking the book.
The average execution price depends on the quantities filled at each level. Its formula is the sum of each fill price multiplied by its fill quantity, divided by total filled quantity. Comparing only the first fill with the initial quote hides the cost of later fills. The final fill price and the average fill price answer different questions.
A clearly hypothetical execution example
Suppose, purely as a hypothetical example, a book displays two units offered at 100 quote units and three units offered at 101. A marketable buy for five units would cost 503 quote units if those orders remained available and filled exactly as displayed. The average execution price would be 100.6. Against a reference of 100, adverse price slippage would be 0.6%, before fees.
Those numbers illustrate arithmetic rather than a real market observation. They also assume no competing orders, cancellations, hidden quantities, partial-fill restrictions, or price-protection limits. An actual execution may differ because the book changes between observation and matching. A screenshot is therefore a conditional description of available liquidity rather than a guaranteed execution schedule.
Choose the reference before measuring slippage
Slippage is ambiguous unless the reference price is specified. Possible references include the midpoint when a decision was made, the best ask when a buy order was submitted, or a benchmark covering a longer execution period. A buy can appear expensive against the midpoint even when it fills entirely at the previously displayed best ask, because crossing the spread has a cost.
For a buy, one common adverse-slippage measure is (average fill price minus reference price) divided by reference price. For a sell, reverse the subtraction so a worse execution also produces a positive cost. Reports can instead use signed price changes, so their conventions need checking. Favorable slippage is possible when the received execution improves on the selected reference.
Spread, market impact, fees, and delay
Execution cost has several components. The spread is the gap between buying and selling quotes. Consuming liquidity across levels creates a mechanical price effect relative to the starting book. Fees add another explicit cost. Market movement during transmission or execution can further change the result. These components may overlap in a benchmark-based measurement, so adding them carelessly can double-count costs.
Market impact more broadly concerns how trading changes prices, including other participants’ responses and liquidity replenishment. It cannot always be inferred from a single before-and-after screenshot. A price change occurring after an order may also reflect unrelated market information. Distinguishing causal impact from ordinary movement requires a defined measurement method and suitable comparison data.
Why displayed depth can be misleading
Resting orders can be canceled before they trade. A large displayed order does not prove that its owner will remain willing to transact when approached. Hidden or reserve orders can make executable liquidity greater than the visible book suggests. Neither visible size nor its absence establishes another participant’s intentions, and a large order alone is insufficient evidence of manipulation.
Depth is also venue-specific and instrument-specific. Liquidity shown for a spot pair does not automatically exist in its futures contract, and liquidity elsewhere cannot necessarily be reached by the same order. Contract multipliers, collateral currencies, and lot sizes matter when comparing quantities. Combining books without converting their units can create a misleading picture of available capacity.
How order instructions change the tradeoff
A limit buy specifies the highest acceptable execution price; a limit sell specifies the lowest. A marketable limit order can execute immediately across eligible levels, but its limit prevents fills beyond that boundary. It may fill only partly or remain unfilled. A market order prioritizes execution within the venue’s applicable rules and protections, without guaranteeing the displayed quote.
Other instructions can alter outcomes. Immediate-or-cancel generally removes any unfilled remainder, while fill-or-kill generally requires the full quantity to execute immediately or cancel. Post-only instructions seek to avoid immediately taking liquidity, with handling dependent on venue rules. These controls constrain execution behavior; they cannot guarantee that a desired trade will occur or that the market will remain stable.
A practical way to read depth
Start by identifying the instrument, quantity unit, timestamp, and reference price. Compare the intended order quantity with cumulative executable quantity on the relevant side of the book. Calculate a hypothetical weighted average using those levels, then state that the estimate assumes the displayed orders remain available. Keep estimated fees separate unless the cost measure explicitly includes them.
Finally, distinguish an execution estimate from a trading signal. A deeper displayed book may reduce the immediate cost of a given order under unchanged conditions, but it does not establish direction, fair value, or future liquidity. Reviewing actual fills against a documented benchmark provides a clearer explanation of execution quality than judging a trade solely by its eventual profit or loss.
Key takeaways
- Depth describes displayed quantity across a defined price range.
- Slippage needs a stated reference, direction convention, and cost scope.
- Weighted average fills matter more than the first quoted price.
- Limit instructions constrain prices but introduce incomplete-execution risk.
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