On This Page
Key Takeaways
- Market impact is the price change caused by executing an order.
- Compare order size with available depth and trading volume when planning execution.
- The worked buy costs $22.50 above its arrival midpoint.
- That amount contains $10 of initial half-spread and $12.50 of additional book-walk cost.
- Separate execution benchmarks from estimates of the order’s causal price effect.
What is market impact?
Market impact is the price change caused by executing an order. An order that demands liquidity moves the price against itself: a buy pushes prices up and a sell pushes them down. A trader evaluates that effect alongside spread, explicit fees and price movement during execution.
How is execution cost measured against the arrival midpoint?
A named benchmark makes the execution comparison concrete. For a buy, subtract the arrival midpoint from the average fill and multiply by quantity. This measures the cost against that benchmark; estimating the order’s own causal effect is a separate analysis.
The fixed-book example starts at $50.00 bid and $50.02 ask, with a $50.01 midpoint. The 1,000-share buy executes 300 at $50.02, 200 at $50.03, 450 at $50.04 and 50 at $50.05.
Total cost is $50,032.50 and average fill is $50.0325. Relative to the arrival midpoint, the order costs an additional $22.50.
The initial half-spread accounts for $10: the one-cent difference between $50.02 and $50.01 across 1,000 shares. The additional book-walk cost is $12.50 above buying the entire quantity at the initial offer. Together they equal $22.50.
Formula
Buy cost versus arrival midpoint = (average fill − arrival midpoint) × shares; modeled impact return = Y × daily return volatility × √(order size ÷ average daily volume)- The midpoint comparison measures benchmark cost, including spread and other execution effects.
- Y is the fitted coefficient in the selected impact model.
- Order size divided by average daily volume is a fraction of normal daily volume. Execution participation instead compares the order with market volume over its execution interval.
- Multiply an impact return by the reference share price to express the modeled move in dollars per share.
Worked Example
At release, the bid is $50.00 and ask is $50.02, giving a $50.01 midpoint. In a fixed-book example, a 1,000-share buy fills 300 at $50.02, 200 at $50.03, 450 at $50.04 and 50 at $50.05. Explicit fees are zero.
- Total cost: $50,032.50
- Average fill: $50.0325
- Cost above arrival midpoint: ($50.0325 − $50.01) × 1,000 = $22.50
- Initial half-spread: ($50.02 − $50.01) × 1,000 = $10.00
- Additional book-walk cost: ($50.0325 − $50.02) × 1,000 = $12.50
- Reconciliation: $10.00 + $12.50 = $22.50
The four fills reconcile to $22.50 above the arrival midpoint. That total includes the initial half-spread. The additional cost above the initial best offer is $12.50.
How order size, depth and pace affect market impact
Read available quantity at each price and compare the order with normal trading volume. An execution schedule determines how much quantity is released at each step and over what interval.
Passive, marketable, reserve and block order instructions reach liquidity in different ways. A reserve or hidden order’s price still determines whether it executes immediately against available liquidity.
Choose the quantity, price instruction and pace together, and after execution, average every fill against the same benchmark used in the plan.
The square-root market impact model
The square-root impact model relates impact return to daily volatility and the square root of quantity divided by average daily volume. Execution participation uses a different denominator: market volume during the order’s execution interval. Keep those inputs distinct when comparing plans.
Limitations of market impact estimates
- Separate the order’s own effect from the benchmark cost. The $22.50 arrival-midpoint cost also holds spread, market movement during execution and other traders’ orders alongside the order’s own effect.
- Live books cancel and replenish while the order executes, so price the order with a limit and measure the cost from the fills you get.
- Refit the square-root model to your own fills before you size from it. Its coefficient, volatility and average daily volume inputs set the estimate, and each sample period gives its own.
- The split between temporary and permanent impact comes from the measurement model and window, so measure the post-trade price at the same horizon every time.
- Balance pace against exposure. Spreading an order over time takes less from the book at each step but leaves the unfilled quantity exposed to price movement for longer.
Bottom line
Market impact is the cost a trader’s own order adds by consuming liquidity, and it grows with order size relative to depth and volume. Measure execution cost against a named benchmark such as the arrival midpoint, size and pace the order against available depth, and calibrate any impact model on your own fills.
Clear answers
Frequently Asked Questions
Sources and Further Reading
- CFA Institute: Trade Strategy and Execution
- CFA Institute: Trading Costs and Electronic Markets
- Federal Reserve: Order Flow Imbalances and Amplification of Price Movements
How entries are sourced, calculated and updated: editorial methodology.