Direct answer
Slippage is the gap between the price you expect to get and the price you actually receive when an order fills. What “costs” affect slippage are not only the obvious items shown as fees; they also include friction that changes the order’s fill price relative to the quote you saw. Some of these costs are relatively stable (for example, commissions), while others vary with market conditions (for example, spread behavior and execution timing).
Mechanism and definition: where costs enter the slippage gap
Consider a simple framework with clearly stated assumptions: you place an order at time T0 based on a displayed quote or a reference price, and it fills at time T1. The raw slippage is the difference between the expected fill price (from T0) and the actual fill price (at T1). In practice, expected price can come from different references (last price, bid/ask midpoint, or an indicative quote), so slippage measurements depend on your chosen reference.
Costs can affect that gap in two main ways.
- Direct, visible execution costs
- Spread-related cost: If your expected price assumes a mid price but your order can only fill against bid/ask, then the spread effectively sets a baseline difference.
- Commissions and account fees: These often do not change the trade price itself, but they affect the net outcome you attribute to “slippage,” especially if you are comparing expected net price vs actual net price.
- Financing or carry-related charges (when applicable): These can change the net cost of holding positions, which can be confused with execution differences if you do not separate “execution” from “carry.”
- Indirect, condition-driven frictions
- Market impact: A large order or aggressive execution can move available prices at the moment of filling, increasing the difference from the earlier quote.
- Latency and timing: From the moment you submit an order until it reaches the market and executes, quotes can change. Even without any fee changes, time itself can widen the expected-vs-actual gap.
- Liquidity and quote refresh behavior: In thinner markets, the order book can replenish more slowly, so small changes in demand or supply can cause larger jumps in the next available fill price.
Evidence and example: how to distinguish cost types
A practical way to verify what affected slippage is to separate (a) price movement and (b) cost line-items.
Example with explicit assumptions: you expected a fill at a reference price derived from the bid/ask midpoint shown at T0, and the trade fills at T1 at a different price. Assume you have trade confirmation data that includes the executed price and a separate report listing commissions/fees.
Steps you can perform without real-time market data:
- Compute raw price slippage: subtract your expected reference price from the executed price using the same reference definition you used at T0.
- Compute net cost difference: include commissions/fees in the comparison only if your expected net price also included them. Do not mix “execution price slippage” with “fee effects” unless you define both clearly.
- Check timing consistency: compare order submit time vs execution time. If the time gap is large relative to normal quote changes, timing-driven quote changes are a plausible contributor.
- Check for liquidity sensitivity: if slippage is consistently larger during periods associated with thin liquidity (for example, when the spread is wider), that supports a liquidity/quote-refresh mechanism rather than a fixed-fee explanation.
To keep the reasoning grounded, treat any conclusion as conditional on your definitions (expected reference price, which costs you included in “net,” and the timing fields you used).
Limitations, risks, and verification boundaries
- Reference mismatch risk: Different expected-price references (midpoint vs bid/ask vs last trade) can change the calculated slippage dramatically. Without a fixed reference rule, comparisons are unreliable. - Attribution failure mode: Fees like commissions may affect net results but not the execution price. If you attribute all net differences to “slippage,” you may reach incorrect causes. - Variable market conditions: Slippage drivers can change with volatility and liquidity. Historical relationships (for example, “it was small last month”) do not establish future outcomes.