What it means
The price shown when you consider a stock trade may differ from the price at which it fills. Execution cost measures that difference against a stated reference price. For a buy, paying more than the reference is a cost; for a sell, receiving less is a cost. The reference might be the quote midpoint or the price when the trading decision was made. The choice matters because different references produce different answers for the same fill.
Execution cost can include the bid-ask spread and market impact. Fees may be added, but they should be identified separately so a price-only estimate is not confused with an all-in figure. A pre-trade estimate is not an actual fill or a guaranteed cost.
How it is calculated
For one fully filled stock order, a straightforward measure is:
Execution cost (US$) = side × (average fill price − reference price) × shares filled + included fees
- Side is +1 for a buy and −1 for a sell. This makes an unfavorable price difference a positive cost for either side.
- Average fill price is the share-weighted average of the prices actually paid or received, in US$ per share.
- Reference price is the specified benchmark, also in US$ per share, observed at a specified time.
- Shares filled is the number of shares executed. For a partial fill, calculate the filled portion separately; the unfilled portion has no fill price.
- Included fees are any commissions or other explicit charges counted in the calculation, in US$. Set this term to zero for a price-only measure.
A negative result is possible: it means the fill was favorable relative to that reference, after any included fees. To compare differently sized trades, divide the dollar cost by reference price × shares filled and multiply by 10,000 to express it in basis points. One basis point is 0.01%.
Worked example
Consider a hypothetical buy of 100 shares. Suppose the reference price is US$50.00 per share, the average fill price is US$50.10, and the included commission is US$1.00.
- Price difference: US$50.10 − US$50.00 = US$0.10 per share.
- Price-related cost: US$0.10 × 100 shares = US$10.00.
- Add the commission: US$10.00 + US$1.00 = US$11.00 total execution cost against the US$50.00 reference.
- Reference value: US$50.00 × 100 = US$5,000. Cost rate: US$11.00 ÷ US$5,000 × 10,000 = 22 basis points, or 0.22%.
The US$11.00 is not a claim that a particular component, such as spread or market impact, caused the entire price difference. It is the observed difference from the chosen reference plus the stated fee.
Why it matters before you trade
A quote does not establish the final price of an order. The bid-ask spread can create an immediate cost, while a larger order relative to available liquidity may affect its own fill prices. Timing, order size and execution conditions can therefore change the estimated cost. Comparing estimates requires the same reference price, fee convention and units.
Execution cost is distinct from whether a stock subsequently rises or falls. An order can have a low execution cost and still lose value afterward; a costly fill does not determine the stock’s later return. Checking potential costs before placing an order helps make the trade’s price assumptions explicit, without predicting its investment outcome.
How PreTrAIde uses it
PreTrAIde provides pre-trade analysis for stock baskets before users place orders with their own brokers. Its quantitative layer estimates bid-ask spread and market impact and measures participation in daily volume. It also produces execution scores and a suggested execution strategy as part of its decision-support verdicts for the basket and each line.
PreTrAIde’s free trading cost calculator covers spread plus market impact. Its trading-cost-by-stock pages show estimated spread and market impact for US$1,000, US$10,000 and US$100,000 orders. These are estimates, not actual fills. PreTrAIde does not receive, transmit or execute orders.
Common mistakes
- Omitting the benchmark: A cost figure is incomplete without its reference price and time.
- Counting components twice: If a fill-price difference already captures spread or impact, adding the same effect again overstates cost.
- Mixing dollars and basis points: US$10 is a dollar amount; its basis-point equivalent depends on the trade’s reference value.
- Treating an estimate as a bill: Actual cost depends on what fills, at what prices, and which fees are charged.
- Ignoring partial fills: Costs measured on executed shares do not describe the outcome of shares that never filled.