# Best execution

Best execution is a broker’s duty to seek the most favorable reasonably available outcome for a client’s order, considering factors such as price, costs, speed and likelihood of execution.

## What it means

When a broker handles a stock order, the lowest displayed price is not necessarily the best available outcome. The broker may need to weigh the price against fees, the likelihood of a fill, speed, order size and the client’s instructions. The exact legal standard varies by jurisdiction; best execution does not mean a guaranteed best price on every order.

A limit price, for example, can restrict what price is acceptable while leaving the order unfilled. A market order may fill promptly, but its final price can differ from the quote seen when the order was entered. Best execution concerns the broker’s handling of these trade-offs, not whether the stock later rises or falls.

## How it is calculated

Best execution has no single universal formula or pass mark. A useful *cost measure* for comparing two completed buy orders, when other terms are comparable, is:

`All-in buy cost (US$) = (average fill price − reference price) × shares filled + broker charges`

- **Average fill price** is the share-weighted average price actually paid, in US$ per share.
- **Reference price** is a specified benchmark, such as the quote midpoint when the order arrived, in US$ per share. Its timestamp matters.
- **Shares filled** is the number of shares executed, not merely requested.
- **Broker charges** are applicable commissions and other explicit broker fees, in US$.

The result is US$; lower cost can be preferable *if* the alternatives offer comparable timing, fill probability and other relevant conditions. For a sell, reverse the price difference to measure cost: `(reference price − average fill price) × shares filled + broker charges`. Neither formula alone establishes whether a broker met its legal obligation.

## Worked example

Suppose, hypothetically, an investor requests a buy of 100 shares. At order arrival, the reference midpoint is US$50.00 per share. Consider two possible completed executions under otherwise comparable conditions:

1. **Execution A:** 100 shares fill at an average of US$50.04; broker charges are US$1.00. Price difference: `(US$50.04 − US$50.00) × 100 = US$4.00`. All-in buy cost: **US$5.00**.
2. **Execution B:** 100 shares fill at an average of US$50.02; broker charges are US$4.00. Price difference: `(US$50.02 − US$50.00) × 100 = US$2.00`. All-in buy cost: **US$6.00**.

B has the lower fill price but the higher measured all-in cost. If B instead filled only 20 shares, these totals would not be directly comparable: the remaining 80 shares and the possibility of no further fill would matter.

## Why it matters before you trade

Before placing an order, an investor can distinguish a quoted price from an achievable execution. The bid-ask spread, available liquidity, order size relative to trading volume and potential market impact can affect the fill. A limit order sets a price boundary but introduces non-execution risk; a market order does not set that boundary. Explicit fees also matter when comparing outcomes.

A pre-trade estimate is not an audit of a broker’s conduct. After a fill, execution-quality measures can compare actual prices and costs with a clearly timed benchmark, while accounting for partial fills and the order’s instructions.

## How PreTrAIde uses it

PreTrAIde does not receive, transmit or execute orders, so it does not perform a broker’s best-execution duty or certify that a broker met it. Its pre-trade analysis evaluates a user-built stock basket before orders are placed. The quantitative layer estimates bid-ask spread, participation in daily volume and market impact; its analysis provides execution scores and a suggested execution strategy alongside a GO, NO-GO or MODIFY verdict and reasoning. These are decision-support outputs, not execution results or recommendations. Users place trades themselves with their own broker.

## Common mistakes

- **Equating best execution with the lowest displayed price.** Fees, available size and whether an order fills can change the outcome.
- **Treating a benchmark as an available trade.** A midpoint is a reference, not a promise of a fill at that price.
- **Ignoring order instructions.** A restrictive limit may prevent an unfavorable price but also prevent execution.
- **Judging execution by later stock performance.** A subsequent price move does not, by itself, show whether the broker handled the order well.
- **Calling a pre-trade estimate proof.** Estimated costs cannot establish the actual fill price or a broker’s compliance.

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Source: https://pretraide.com/glossary/best-execution