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Key Takeaways
- Best execution requires a broker-dealer to use reasonable diligence to seek the most favorable customer price under prevailing conditions.
- Evaluate price together with size, speed, fill likelihood, costs and order instructions.
- Covered firms using regular and rigorous review must perform it at least quarterly, by security and order type.
- Compare routing destinations with competing markets and address material differences in execution quality.
- In the worked example, both routes average $30.04; full completion takes 40 milliseconds on A and 600 milliseconds on B.
What is best execution?
Best execution is a broker-dealer's duty to use reasonable diligence to find the best market for a security and seek a customer price as favorable as possible under prevailing conditions. FINRA Rule 5310 applies this obligation to its member firms. Price is the starting point; size, speed, fill likelihood, costs and the order’s instructions also count.
The order’s instructions establish the objective. A market order seeks prompt execution. A limit order sets the acceptable price boundary. Quantity, urgency and venue instructions shape the available choices.
Formula
Analytical execution shortfall = side-adjusted (fill price − decision benchmark) × executed quantity + explicit fees- For a buy, fill price above the benchmark is adverse; for a sell, fill price below the benchmark is adverse.
- The benchmark can be order-arrival price, midpoint, or another documented reference suited to the analysis.
- Execution shortfall puts a dollar cost on each order, and the broker’s compliance review weighs it alongside speed, size and fill likelihood.
Best execution example: same price, different speed
A customer sends a market order to buy 2,000 shares when the NBBO is $30.00 by $30.04. Route A fills all 2,000 shares at $30.04 in 40 milliseconds. Route B fills 1,000 at $30.03 immediately, then 1,000 at $30.05 after 600 milliseconds, when the quoted ask has risen to $30.05. Explicit fees are zero on both routes.
- Route A: 2,000 × $30.04 = $60,080; full completion in 40 milliseconds
- Route B: 1,000 × $30.03 + 1,000 × $30.05 = $60,080; full completion in 600 milliseconds
- Both routes average $30.04 per share
- Arrival midpoint = ($30.00 + $30.04) / 2 = $30.02
- Cost above arrival midpoint = 2,000 × ($30.04 − $30.02) = $40 on each route
Both routes cost $60,080. Route A completes the order 560 milliseconds sooner. Route B’s first half improves by one cent, while its second half fills one cent above the original ask. Compare the complete order alongside its price, speed and available size.
Both routes cost $60,080 and average $30.04. Against the $30.02 arrival midpoint, each costs $40 above the benchmark with zero explicit fees. Route A completes 560 milliseconds sooner. Route B’s initial improvement is offset by the later fill.
Judge the routes on the whole order. At the same $60,080, Route A fills all 2,000 shares 560 milliseconds sooner, and prompt execution is what a market order asks for.
How brokers review best execution under FINRA Rule 5310
A broker can evaluate the best market order by order. FINRA’s regular-and-rigorous-review framework also covers firms that use automated, nondiscretionary routing arrangements or internalize orders without order-by-order review.
That review occurs at least quarterly and examines securities and order types separately. It compares current routing results with competing markets, assesses material differences and supports routing changes or the reasons for retaining the arrangement.
Relevant outcomes include price improvement, speed, fill likelihood, size and transaction costs. The broker retains its duty when routing through another firm and must account for payment arrangements in evaluating execution quality.
Best execution for options orders: what to record
Record the NBBO, midpoint, order limit, fill price, filled contracts and completion time. For a multi-leg trade, compare the strategy’s net price and the completed leg quantities.
Review completed trades by ticker, size and order type. Put the execution conditions that consistently serve your trading objectives into your next order plan. The options profit calculator shows how entry prices across the spread change breakevens and payoff.
Limitations of best execution
- The duty is reasonable diligence in seeking the most favorable terms, so judge a broker on its record across many orders.
- Match the route to the order’s objective. Both example routes cost $60,080 and average $30.04, and Route A completes 560 milliseconds sooner, the edge a market order wants.
- Name the benchmark before comparing costs. Arrival midpoint, the NBBO and other documented references give different numbers; the example’s $40 cost is measured against the $30.02 arrival midpoint.
- Regular and rigorous review runs at least quarterly by security and order type, so it judges routing in aggregate. Record your own fills to judge each order.
- Give the broker only the instructions the order needs. A limit price or a venue choice narrows the choices available to it.
Bottom line
Best execution is the broker-dealer’s duty to seek the most favorable result for each customer order, judged on price together with size, speed, fill likelihood and costs. Judge your own fills the same way: record the NBBO, midpoint, fill price, filled quantity and completion time, and compare complete orders rather than single prints.
Clear answers
Frequently Asked Questions
Sources and Further Reading
- FINRA Rule 5310: Best Execution and Interpositioning
- SEC: Trade Execution
- FINRA Regulatory Notice 15-46: Best Execution Guidance
How entries are sourced, calculated and updated: editorial methodology.