Execution & trading costs

Market impact

Market impact is the price movement attributable to executing a trade, as the order consumes available liquidity or prompts other market participants to adjust their prices.

Calculate it: Trading Cost Calculator: Spread, Market Impact and Commission →

What it means

An order can change the price at which its remaining shares can be executed. A buy order may consume shares offered at one price and then reach higher-priced offers; a sell order may similarly reach lower bids. Other participants may also revise their quotes in response to trading. The resulting price movement attributable to the order is market impact.

Market impact is distinct from the bid-ask spread. Paying the current ask rather than the midpoint can create a spread cost even if the order does not move the market. Impact concerns the additional price effect associated with the trade. A move observed after an order is not automatically impact: news and unrelated orders can move prices too.

Impact can be temporary, if prices recover after the trade, or persistent, if the price change remains. Which portion belongs to the order is often difficult to establish from a single transaction.

How it is calculated

One simple way to express estimated impact for a buy order is estimated impact (US$) = shares × reference price (US$/share) × estimated impact rate; for a sell order, the same expression can estimate the magnitude of the cost. Here:

  • Shares is the number of shares in the proposed order.
  • Reference price is the price used to value those shares, such as a specified pre-trade midpoint, in US$ per share.
  • Estimated impact rate is the modeled price effect expressed as a decimal: 0.002 means 0.2%, or 20 basis points.

Multiplying shares by US$ per share and a dimensionless rate produces US$. The rate is an input from an impact estimate, not a fixed percentage that applies to every trade. Models may account for order size relative to trading volume, volatility, liquidity and the intended execution schedule. This formula converts an assumed rate into a dollar estimate; it does not derive that rate or prove how much an executed order moved the market.

Worked example

Suppose, hypothetically, an investor considers buying 1,000 shares at a reference price of US$50 per share. Assume a pre-trade model estimates a market-impact rate of 0.2% for the contemplated execution.

  1. Reference order value: 1,000 shares × US$50/share = US$50,000.
  2. Convert the assumed rate: 0.2% = 0.002 = 20 basis points.
  3. Estimated impact cost: US$50,000 × 0.002 = US$100.
  4. Estimated impact per share: US$100 ÷ 1,000 = US$0.10.

This US$100 is only the illustrative impact component. It excludes any separately estimated bid-ask spread, commissions and other costs. It does not mean every share will fill at US$50.10, or that the market price will rise by exactly US$0.10.

Why it matters before you trade

Market impact can make a larger order more expensive to execute than its quoted price initially suggests. The risk depends not just on its US$ value but on the stock’s available liquidity and the pace of execution. An order representing a larger share of typical daily volume may warrant closer examination than one representing a small share.

A pre-trade analysis can put estimated impact alongside spread and other execution costs before an order is placed. Estimates are uncertain: displayed liquidity can change, and actual fills depend on market conditions, order instructions and competing trading activity.

How PreTrAIde uses it

PreTrAIde includes a market-impact estimate in its deterministic quantitative layer. Its Almgren-Chriss-style model sits alongside estimates of bid-ask spread, participation in daily volume, volatility and liquidity. For a user-built stock basket, these inputs inform pre-trade analysis that returns a GO, NO-GO or MODIFY verdict for each line and the basket, with reasoning and a suggested execution strategy. The verdict is decision support, not an order or an investment recommendation; users trade through their own broker.

PreTrAIde also offers a free trading cost calculator that combines spread and market impact, and trading-cost-by-stock pages showing impact estimates for US$1,000, US$10,000 and US$100,000 orders. These are estimates, not actual fill costs.

Common mistakes

  • Treating impact as the entire trading cost. Spread, commissions and other costs are separate components.
  • Assuming a fixed impact rate. Order size, liquidity, volatility and execution conditions can change the estimate.
  • Calling every subsequent price move impact. Broader market movements and new information may be responsible.
  • Treating an estimate as a guaranteed fill. An impact model cannot know the exact prices an order will receive.

Frequently asked questions

Is market impact the same as the bid-ask spread?

No. The spread is the difference between quoted bid and ask prices; market impact is the price effect attributable to executing the order. Both can contribute to execution cost.

How is market impact estimated before a trade?

A pre-trade model can use order size, trading volume, volatility, liquidity and execution assumptions to estimate an impact rate. Multiplying that rate by the order’s reference value gives an estimated dollar impact, not a guaranteed cost.

Does a bigger order always have more market impact?

Other conditions being equal, a larger order can consume more available liquidity and increase impact. Actual outcomes also depend on the stock, market conditions and how the order is executed.

Related terms

Bid-ask spread Bid-ask spread is the difference between the lowest displayed price to sell a stock (ask) and the highest displayed price to buy it (bid), measured per share. Transaction cost analysis (TCA) Transaction cost analysis (TCA) is the measurement or estimation of a trade’s execution costs against a defined price benchmark, including effects such as spread, market impact and fees. Participation rate Participation rate is the percentage of a stock’s trading volume represented by an order or executed trades over a specified period, indicating how large the trade is relative to market activity. Average daily volume (ADV) Average daily volume (ADV) is the average number of shares traded per trading day over a specified period, used to gauge a stock’s typical trading activity. Almgren-Chriss model Almgren-Chriss model is a framework for scheduling an order over time by balancing expected market-impact costs against the risk of price moves while execution remains unfinished. Execution strategy Execution strategy is a plan for placing and managing a trade, including order type, timing, size, and price conditions, to balance execution cost, speed, and the risk of not getting filled.

Execution & trading costs

Pre-trade analysis Post-trade analysis Effective spread Realized spread Midpoint price Slippage Implementation shortfall Transaction costs All glossary terms →

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Educational content, not investment advice. PreTrAIde is analysis software, not a broker: it never executes orders, and its verdicts are decision support, not recommendations to buy or sell.