What Is Slippage in Trading? Why Your Fill Price Isn't Your Order Price

Slippage Is the Gap Between Expected and Actual Price

Slippage is the difference between the price you expected to get on a trade and the price you actually receive when the order fills. It happens on both entries and exits, and in both directions: sometimes it works against you, sometimes in your favor, but in practice it skews against traders more often than not because it tends to spike exactly when you most need a clean fill, during fast or thin markets.

If you place a market order expecting to buy at $50.00 and it fills at $50.08, that's 8 cents of negative slippage. Multiply that across position size and trade frequency, and slippage becomes a real, ongoing cost that many traders underestimate when backtesting a strategy.

Why Slippage Happens

Market Orders Chase Available Liquidity

A market order fills at the best currently available price, not a price you specify. If the order size is larger than what's sitting at the best bid or ask, it "walks the book," filling partially at each successive price level until the full order is filled. Larger orders in thinner markets slip more.

Low Liquidity

Stocks with low average daily volume have wider bid-ask spreads and less depth at each price level. A market cap that's small often correlates with thinner trading and higher typical slippage, simply because fewer shares change hands and the order book is shallower.

Volatility Spikes

During high-volatility events (earnings releases, macro news, flash crashes) prices can move meaningfully in the fraction of a second between when an order is submitted and when it executes. This is the same mechanism that causes stop-loss orders to fill well below their trigger price during a sharp selloff.

Time Delay Between Signal and Execution

Any lag between deciding to trade and the order actually reaching the exchange, whether from a slow platform, a manual order entry process, or network latency, gives price room to move before the order fills.

Slippage in Different Market Conditions

Condition	Typical Slippage	Why
Liquid large-cap, normal hours	Low	Deep order book, tight spread
Illiquid small-cap	High	Thin book, wide spread
Earnings release / major news	High	Rapid repricing, order book thins out
Pre-market / after-hours	High	Lower overall volume, wider spreads
Market open (first minutes)	Elevated	Price discovery after overnight gap

How to Reduce Slippage

Use Limit Orders When Price Matters More Than Certainty

A limit order guarantees a maximum (or minimum) price but not that the order fills at all. For less time-sensitive trades, this trade-off is usually worth it. See Limit Order vs Market Order for when to use each.

Trade During Higher-Liquidity Windows

Spreads are typically tightest during the middle of the regular trading session, once the volatility of the open has settled and before the thinning that can happen near the close. Avoiding the first and last few minutes of the session reduces exposure to wider, less stable spreads.

Size Orders Relative to Average Volume

Comparing your intended position size to the stock's typical daily trading volume gives a sense of how much your own order might move price. A position that represents a meaningful share of a stock's daily volume is much more likely to experience slippage on entry and exit — see Volume Analysis in Trading for how to read a stock's typical volume profile.

Account for Slippage in Backtests

A backtest that assumes every trade fills at the exact signal price will overstate real-world returns, sometimes significantly for higher-frequency strategies. Building a realistic slippage assumption into a backtest is one of the most commonly skipped steps that separates a strategy that looks good on paper from one that actually performs live.

Summary

Concept	Takeaway
Slippage	Gap between expected and actual fill price
Main causes	Thin liquidity, volatility spikes, large order size, execution lag
Worst conditions	Illiquid stocks, earnings releases, pre/after-hours
Mitigation	Limit orders, trading liquid hours, sizing relative to volume
Backtesting impact	Ignoring slippage inflates simulated returns

Slippage is an unavoidable part of live trading, but it's not random. It concentrates in predictable conditions: thin liquidity, high volatility, and oversized orders relative to available volume. Understanding where it shows up is the first step to reducing its cost.

---

Related reading:

• What Is a Limit Order vs Market Order? — trading price certainty for fill certainty
• What Is a Stop-Loss Order? — how slippage affects stop order fills
• What Is Market Cap? — why smaller companies tend to see more slippage
• Volume Analysis in Trading — sizing orders relative to liquidity