Slippage in trading: measuring the gap between expected and actual fills

Start in plain English

Slippage is the difference between the price you expected and the price you actually got. The glossary holds the short definition; this lesson is about measuring slippage and keeping it small enough that a scalp still makes sense. For a small-cap scalper aiming at 10 or 15 cents, two or three cents lost on the way in and the way out can be a third of the target before the trade has done anything (Illustrative).

The one idea: slippage only means something against a reference price you chose in advance. "I got slipped" is a feeling. "My stop was $4.25 and my average fill was $4.07, so stop slippage was 18 cents a share, or 1.64R on this trade" is a measurement you can count, compare and reduce.

The arithmetic is short. For a long position:

• Entry slippage = average entry fill − reference price.
• Exit slippage = reference price − average exit fill.

For a short position the signs flip. A positive number is a cost; a negative number is price improvement, and it belongs in the count too. Multiply by shares for dollars, and divide by your planned risk for slippage in R.

Two paths from the same start

All prices, sizes, books and costs in this example are Illustrative. Harrow Lumen is fictional.

The position at 10:14 ET. Long 800 shares of Harrow Lumen, filled at $4.36 against an arrival quote of $4.35 × $4.36. Stop order at $4.25, so the planned risk is $0.11 per share, or $88 for the position (1R). Target $4.58 (2R). Time stop 10:24. Commissions and fees $0.005 per share round trip ($4.00 at this size; your plan's rate is covered in ECN fees). A stop order becomes a market order once it triggers, so its fill price is not set in advance; US Day Trading Fundamentals covers the mechanics in Stop and stop-limit orders.

Here is the order book the trader could see when the position was opened.

The review note for all three: "Plan followed; stop at invalidation; size 800; outcome A, B or C." The plan and the 10:14 book were identical. The difference between 1 cent and 18 cents of slippage was decided by what the book did in the next five minutes. The rest of this lesson covers how to choose the reference, why stops slip more than entries, how size and halts change the numbers, and what to log.

Choose the reference before you measure

The same fill can show different slippage numbers, all correct, because each reference answers a different question. Suppose a buy fills at $4.37: the first three rows give three numbers for that fill, and the fourth row applies to stop exits.

In execution research the first reference is called the arrival price. Pick one reference per measurement, write it in your log, and keep it fixed. If you switch references after seeing the fill, the number stops meaning anything.

Count price improvement as well. A limit exit at the target, or an entry filled below the ask, produces zero or negative slippage. Leaving those out makes your slippage look worse than it is; leaving out the bad ones makes it look better.

Entries vs. stops: where slippage is bounded

Order type decides whether slippage has a ceiling.

• Limit and marketable limit orders cap entry slippage at the limit price. The cost of the cap is that the order may fill only partly or not at all. Marketable limit orders covers how to choose that ceiling.
• Market orders have no cap. They take whatever the book offers at the moment they arrive.
• Stop orders become market orders when triggered, so stop slippage has no cap either. FINRA's rule on stop orders makes the same point: the stop price is a trigger, not a guaranteed fill.
• Stop-limit orders cap the price but may not fill. In a fast drop the whole position can stay open below your limit, which turns a slippage problem into an open-loss problem.

There is also an asymmetry in when the orders arrive. You choose the moment of entry. A stop triggers when price is moving against you, often at a level many traders are watching, so your sell order meets other sellers. Whether stop slippage is larger than entry slippage in your own trades is a question your log can answer; as a starting hypothesis, record the two separately.

Size vs. displayed liquidity

A market order walks the book: it takes the best price until that size is gone, then the next level, and so on. Take the Harrow Lumen book from 10:14 and assume it is still there when the stop triggers.

• 800 shares sell 300 at $4.24, 200 at $4.21 and 300 at $4.16 (assuming the $4.25 bid was taken by the print that triggered the stop). Average: ($1,272 + $842 + $1,248) ÷ 800 = $4.2025. Slippage against the $4.25 stop: 4.75 cents a share, $38 in total.
• 300 shares in the same book sell entirely at $4.24: 1 cent a share, $3 in total.

Same stock, same minute, same stop: size changed the slippage per share almost fivefold (Illustrative). Two caveats apply. Displayed orders can be cancelled before you reach them (Path B), and some orders are not displayed at all, so the real book can be thinner or deeper than the screen. Position sizing turns this into a liquidity cap on share count, and Is it liquid enough to scalp? covers depth and spread in more detail.

Below $1, quotes move in hundredths of a cent, so measure with four decimals: a stop at $0.8150 filled at an average of $0.8080 slipped 0.70 cents a share.

Halts and gaps through stops

A stop cannot execute while trading is halted. If a stock hits its lower LULD band and pauses, or is halted for news, the stop waits. When trading resumes, the first prices can be far from your stop, and your order fills there.

Suppose Path B had included a pause: Harrow Lumen pauses near $4.15 and reopens at $3.80, and the stop fills at $3.78. Result: ($3.78 − $4.36 − $0.005) ÷ $0.11 = −5.32R (−$468.00), against a planned −$88 (Illustrative). No stop placement prevents this; only size limits it. FennEdge's articles on trading halts and limit up / limit down (LULD) cover the rules, and What happens to your orders during a halt, in US Day Trading Fundamentals, covers order handling. A FennEdge data study is planned to measure halt reopening prices against halt prices.

Observe vs. infer

At 10:14 ET in the Harrow Lumen example (fictional; all figures Illustrative), separate what the screen showed from what a trader might assume about the stop.

Only the left column was usable at 10:14.

Odds box

Look at the last bucket. Three exits in a hundred produce about 36% of the total slippage cost (1.35 of 3.72 cents, computed from the assumed split). An average hides that shape, which is why the log should keep every individual fill, not only a monthly average. When the FennEdge data study of halt reopening prices is published, the halt row can use measured reopening gaps instead of an assumption.

What shifts the odds

No FennEdge sample yet links these conditions to slippage, so treat them as hypotheses to record, not findings. Log them with every fill:

• Your size vs. displayed size within a few cents of the reference price.
• Spread at arrival, in cents and as a share of your target.
• Speed of the tape in the minute before the fill: number of prints and how fast the quote changed.
• Order type: market, marketable limit, stop, stop-limit.
• Time of day: the first minutes after the open and the premarket often have thinner books (a FennEdge data study of volume and spread by time of day is planned).
• Price tier: sub-$1 stocks quote in hundredths of a cent; a "cent" of slippage is a larger share of the price.
• Crowded levels: a stop just under a round number or an obvious low exits at the same moment as other stops.
• Recent halts: a stock that paused recently can pause again.

Cost of being wrong

Using the Harrow Lumen plan at 800 shares (Illustrative):

Small slippage repeats as well: one cent on each side of 10 round trips a day at 800 shares is $160 a day (Illustrative). A plan that breaks even before slippage loses money after it.

How many trades before this means anything

Slippage is skewed: most fills are close, and a few are far. That shape makes small samples misleading. If 3% of stop exits gap through on a halt, the chance of seeing none in 20 stop exits is 0.97²⁰, about 54% (computed from the assumed split, Illustrative). Half of the traders who log 20 clean stops would conclude their slippage is small, and their logs would contain no information about the tail. Track the median and the worst 10% of fills separately, and read How many trades before your numbers mean anything before treating either as settled.

What this lesson does not claim

• It does not claim any typical slippage figure for small caps. Every number above is Illustrative; your broker, routing, size and stocks set your own.
• It does not claim that limit orders are better than market orders. A limit trades price risk for fill risk, and a missed exit can cost more than slippage.
• Harrow Lumen, its book and every price in this lesson are fictional.

Go deeper

• FennEdge first.
  • The Trading Journal syncs real fills and fees from supported brokers and, on Plus and above, shows net P&L after fees, with fee detail on Pro and Max (as of 2026-10-05). It does not record the quote at your decision time, so note your arrival quote yourself if you want to measure entry slippage against it. It holds broker-synced trades only; simulated fills belong in your practice log.
  • The Scanner (paid: Plus, Pro, Max, as of 2026-10-05) alerts on LULD halts and resumes of Nasdaq-listed stocks, useful context before you size into a stock that has paused. It does not cover news halts and has no Level 2 depth.
  • Articles: trading halts and LULD.
• Implementation shortfall. Perold (1988) measured execution against the decision price, including the shares you meant to trade and did not. Unfilled shares are a cost too when the price moves without you.
• Partial fills. A partly filled exit leaves a smaller position open at a worse moment. Log filled and unfilled shares.
• Simulated fills. Simulators tend to fill stops at or near the stop price. Paper trading shows how to add a slippage penalty.
• Related lessons: Stop-loss placement, VWAP trading strategy.

Exercises

Six exercises follow this lesson: three concept checks, two calculations (entry slippage across two partial fills, and stop slippage when a sub-dollar order walks the book), and one replay of the Harrow Lumen book that asks you to separate what was knowable at 10:14 from hindsight and inference. Exercises, answers and progress are available after signing in.

Primary sources

• Perold, A. F. (1988). The implementation shortfall: Paper versus reality. The Journal of Portfolio Management 14(3).
• FINRA. Rule 5350: Stop orders.
• LULD Plan. Limit Up-Limit Down Plan.
• FINRA. Frequent Intraday Trading: Understanding the Basics.

Sources checked 2026-10-05. Harrow Lumen and all prices, books, fills, slippage splits and costs in this lesson are fictional and Illustrative. This lesson is education, not a recommendation to buy or sell any security. Past patterns do not indicate future results.