Key Takeaways
- Slippage is the gap between the expected execution price and the actual average price an order receives.
- Larger orders, relative to liquidity available at and near the best price, commonly experience more slippage.
- Once an order exceeds top-of-book depth, it "walks the book," filling against progressively worse resting prices.
- Slippage as a percentage of order size is not linear; it tends to increase disproportionately past that depth threshold.
- The same order size can slip differently at different times because it depends on liquidity conditions, not size alone.
What Is the Relationship Between Slippage and Order Size?
Slippage vs. order size describes a general, widely observed relationship in market microstructure: orders that are large relative to the liquidity resting at and near the best price tend to experience more slippage than smaller orders. Slippage itself is the difference between the expected execution price, typically the quoted price at the moment an order is placed, and the actual average execution price the order receives once it is filled.
The relationship is about relative size, not absolute size. An order for 500 shares can be "large" in a thinly traded stock and produce meaningful slippage, while the same 500 shares might barely register against the depth available in a heavily traded name. What matters is how the order's size compares with the quantity resting at the best bid or ask, and just behind it, at the moment the order reaches the market.
How Walking the Book Drives Slippage
An order book stacks resting buy and sell orders by price. At any moment, only a limited quantity sits at the very best price. If an incoming order is small enough to be filled entirely out of that top-of-book quantity, it can execute close to the quoted price with little or no slippage.
A large order does not have that luxury. Once it consumes everything available at the best price, it must continue filling against the next price level, then the next, and so on until the full order is complete, a process commonly described as "walking the book." Each successive level is priced less favorably than the one before it (higher for a buy, lower for a sell), so the order's average execution price drifts further from the original quote as it goes deeper.
Because early quantity fills at good prices and later quantity fills at progressively worse ones, slippage does not scale evenly with size. A modest increase in order size that stays within top-of-book depth may add little slippage. The same size increase, once it pushes the order past that depth, can add disproportionately more, which is the core reason slippage as a percentage of order size is commonly described as non-linear rather than a fixed rate.
Worked Example: Walking the Book
Hypothetical example, for education only. Suppose a trader wants to buy shares of a stock quoted at $50.00, and the resting sell-side (ask) liquidity at each price level looks like this immediately before the order arrives:
| Price | Shares Available |
|---|---|
| $50.00 | 200 |
| $50.02 | 300 |
| $50.05 | 500 |
| $50.10 | 1,000 |
A buy order for 200 shares fills entirely at $50.00, the quoted price, with no slippage. A buy order for 1,000 shares, however, must walk the book: 200 shares fill at $50.00, 300 at $50.02, and the remaining 500 at $50.05. The average execution price works out to $50.031, a small amount above the original $50.00 quote.
Now consider a buy order for 2,000 shares. It exhausts every level shown, 200 at $50.00, 300 at $50.02, 500 at $50.05, and 1,000 at $50.10, and the average execution price rises further, to roughly $50.07. Doubling the order size from 1,000 to 2,000 shares more than doubled the gap between the average fill price and the original quote, illustrating how slippage as a share of order size can grow disproportionately once an order pushes past the depth available at the better price levels.
How Traders Think About This Relationship
Traders commonly use the general size-vs-slippage relationship as a rough gut check before sending a larger order, rather than as a precise formula, actual slippage on any given order also depends on how much liquidity happens to be resting in the book at that exact moment, which changes continuously. Some traders compare an intended order size against recent average trading volume or visible book depth as an informal sense check before entering it.
Because a single large order can produce more slippage than several smaller ones spread over time, order-splitting and time-based execution approaches exist specifically to reduce how much of an order competes against thin liquidity at once. This is a general, commonly cited market-structure pattern, not a guaranteed outcome for any individual trade, the amount of available liquidity, and therefore the amount of slippage, varies with the security, the time of day, and overall market conditions.
Limitations and Common Mistakes
- Treating it as a fixed formula. There is no single universal rate at which slippage grows with size; the relationship is directional and non-linear, not a precise equation that applies identically across every security.
- Ignoring that liquidity changes constantly. The depth shown in a book at one instant can look very different a second later, so slippage on a given order size is not fully predictable in advance.
- Comparing absolute size across different securities. An order that is small relative to one stock's liquidity can be large relative to another's; what matters is size relative to depth, not the raw share or contract count.
- Assuming visible top-of-book size is the whole picture. Additional liquidity often exists at levels just behind the best price, and how much of it is displayed varies by venue and order type.
- Overlooking that volatility compounds the effect. Fast-moving conditions can widen the gap between the expected and actual execution price beyond what order size alone would suggest.
Where Order Size Stops Being a Detail
The threshold that matters is not a fixed number of shares. It is the point at which an order stops being small relative to what is resting nearby, and that point moves with the security, the time of day and the conditions. An order that is routine in one name can be the entire visible book in another.
So the check worth doing is relative rather than absolute. How does the intended size compare with the depth currently displayed, and with the volume the security usually trades? Answering that takes a moment and identifies the trades where the rest of this analysis is worth the effort.
The non-linearity is the part that surprises people. Cost does not scale in proportion to size, because each additional tranche fills at a worse level than the one before it, so doubling an order can more than double what crossing costs.
None of this is stable across sessions. The same order can be absorbed quietly one day and move the price noticeably the next, which is why a figure measured once should not be carried forward as a constant.
Frequently Asked Questions
Does slippage increase in a straight line as order size grows?
Not typically. Slippage as a percentage of order size tends to increase disproportionately once an order exceeds the depth readily available at the top of the book, rather than growing at a constant rate with size.
What does it mean for an order to "walk the book"?
Walking the book describes a large order consuming resting liquidity at progressively worse prices once the quantity available at the best price is exhausted, filling against deeper, less favorable price levels until the full order is complete.
How is slippage measured?
Slippage is the difference between the expected execution price, such as the quoted price when the order was placed, and the actual average execution price received across all fills that made up the order.
Why do two orders of the same size sometimes slip by different amounts?
Slippage depends on liquidity available at and near the best price at the moment of execution, not on order size alone, so the same order size can produce different slippage depending on how deep the book is when the order arrives.
Can slippage happen even on a small order?
Yes. A small order can still experience slippage if it arrives when the book is thin, though larger orders relative to available liquidity generally tend to experience more slippage than smaller ones under normal conditions.
How does splitting an order into pieces change the relationship?
Executing in smaller pieces takes less depth at each point and lets the book replenish between them, which usually reduces the concession compared with sending the full quantity at once. The cost is time: the remaining quantity is exposed to price movement while it waits, and in a moving market that can exceed what was saved. The split is a trade between impact and timing risk rather than a way of avoiding cost.
What size is small enough that the relationship stops mattering?
When an order is comfortably within the quantity displayed at the best price, it fills at that price and the depth beyond it is irrelevant. The practical threshold therefore depends on the security rather than on any absolute number, and it changes through the session as displayed size varies. Comparing the intended quantity against the displayed size at the touch is the check that answers this for a specific order.
How does the relationship differ between a buy and a sell?
The mechanics are symmetric, and the book itself frequently is not: displayed depth on the two sides can differ substantially at any moment, so the same quantity can cost more to buy than to sell or the reverse. Assuming symmetry from a single depth reading is a common simplification. Checking both sides before assuming an exit will cost what the entry did is the practical response.
Does the relationship hold in extended-hours trading?
The shape holds and the scale changes considerably. With fewer participants posting, displayed depth is thinner and the quantity required to move the price is much smaller, so an order that would be routine during the regular session can travel several levels. Estimates calibrated on regular-session depth understate the cost outside it, sometimes by a wide margin.