Liquidity and Slippage
The gap between the price on the screen and the price actually filled. In thin securities it is frequently larger than the edge the strategy was pursuing.
MadStockAlerts Research · Updated August 28, 2026
What to take away
- Slippage is the difference between expected and achieved execution price.
- It scales with order size relative to available depth.
- It is worst exactly when it is least welcome: in fast markets and at the open.
- Exit liquidity is the half that gets forgotten until it is needed.
- Backtests that assume stops fill at the trigger overstate results.
MAD Academy Training Video · 0:46
The Price You See Is Not the Price You Get
Slippage is the gap between the quote and your fill, and it scales with your size relative to what actually trades.
This lesson is part of a Stock Alerts + Tools plan.
Where it comes from
The quoted price is available for a stated size. An order larger than that size consumes the top of the order book and reaches further in, filling progressively worse. The average fill is therefore worse than the quote, and the gap grows with the order.
Time is the other source. Between deciding and the order arriving, the market moves, and in a fast market that alone can be substantial. Neither source is anybody's fault and neither can be removed, only measured and planned for.
When it is worst
- The first minutes after the open, when the book is still forming.
- During and immediately after a scheduled release.
- In extended hours, where depth is a fraction of the regular session's.
- In thin and small-cap securities at any time of day.
- When a stop triggers, because a stop fires precisely when price is moving quickly.
The last one deserves emphasis. Stops are designed to trigger during adverse moves, which is when slippage is largest. Backtests assuming stops fill at the trigger price systematically overstate results, and the overstatement is worst in exactly the securities where it matters most.
Scroll the chart sideways to see all of it.
Judging capacity before entering
- 1Compare size to average volumeAn order that is a meaningful fraction of a day's volume will move the price.
- 2Read the spread as a percentageA wide percentage bid-ask spread is a direct, recurring cost on every round trip.
- 3Look at depth, not just the quoteA tight quote for a hundred shares is not a tight market for ten thousand.
- 4Ask about the exitEntering gradually is usually possible. Exiting quickly in a decline frequently is not.
A rough convention is to keep an order below a small single-digit percentage of average daily volume. Beyond that the order is not participating in the market so much as becoming it.
Exit liquidity
Liquidity is not constant, and it is thinnest when everyone wants the same side. A position that could be built comfortably over several sessions can be very difficult to exit in one.
That asymmetry does not appear anywhere on a chart, which is why it is routinely discovered rather than anticipated. The only place it is visible in advance is in the volume and spread data, and only if somebody looks before entering rather than after.
Depth, not spread, is what a size order meets
The spread describes the cost of the smallest possible transaction. A larger order does not pay the spread; it walks up or down the book, taking each resting level in turn until it is filled, and the average price it achieves is worse than the quote it saw.
This is why two securities with identical spreads can have completely different costs at size. A quoted spread of one cent means nothing if there are two hundred shares behind it and the next level is thirty cents away. The relevant measure is how many shares sit within a tolerable distance of the quote, and it is visible in the depth of book rather than in the quote itself.
- Average daily volume is a rough proxy and a poor one, because it says nothing about how that volume was distributed through the day.
- A common convention is to treat some small fraction of average daily volume as the largest position that can be exited in a session without materially moving the price.
- Volume concentrated in the opening and closing auctions means the middle of the day is thinner than the daily figure implies.
- A security whose volume is mostly one large print is less liquid than one with the same volume spread across thousands of trades.
Liquidity is not a constant property of a security. It is highest when it is least needed and evaporates precisely when many participants want the same side, which is the condition every exit plan is implicitly assuming will not occur.
Getting out is a different problem from getting in
Entry is usually voluntary and can be spread over time, split into pieces, or abandoned entirely if the fills are poor. An exit forced by a stop, a margin call or a thesis failure has none of those options, and it happens at the moment other holders are trying to do the same thing.
That asymmetry is the reason exit liquidity is the constraint that should set position size in a thin security. The relevant question is not whether the position can be built but how large a position can be closed in a single session without the closing itself becoming the reason the price falls further.
- Liquidity falls hardest in exactly the conditions that trigger exits, because the buyers who were resting withdraw.
- A position that is a meaningful share of a day's volume cannot be exited in a day without being most of that day's selling.
- Halts remove the option entirely for as long as they last, and they are most common around the news that prompts the exit.
- In extended hours the book is a fraction of its regular-session depth, which is where after-hours reactions are transacted.
The failure mode here is a plan that is sound at every step except the last one, where it assumes the exit is available. It usually is. The occasions when it is not are the occasions the plan existed for.