Execution & trading costs

Smart order routing

Smart order routing is a broker or trading system's automated process for directing an order among eligible execution venues according to available prices, liquidity, order instructions and routing rules.

What it means

A stock may be available for execution at more than one venue, including exchanges and other eligible trading venues. A smart order router (SOR) decides where to send an order or its separate portions. It may compare displayed quotes, available share quantities, fees, order instructions and the likelihood of a fill. The broker or trading system controls the router; the investor generally does not see every routing decision as it happens.

Routing is part of execution, not a judgment about whether the stock is worth owning. Different routers can use different rules, and the lowest displayed price is not necessarily the lowest total execution cost if too few shares are available there.

How it is calculated

There is no universal smart order routing formula. One way to compare possible routes for a buy order is to estimate an all-in cost per share:

Estimated cost per share = (Σ(qᵢ × pᵢ) + ΣFᵢ + estimated impact) / Q

  • Q is the total order quantity, in shares.
  • qᵢ is the quantity expected to fill at venue i, in shares; the expected quantities must add up to Q for this comparison.
  • pᵢ is the expected execution price at venue i, in US$ per share.
  • Fᵢ is the net fee for that portion, in US$; a rebate is treated as a negative fee.
  • Σ means add the amounts across the venues used.
  • Estimated impact is the estimated extra cost, in US$, from the order affecting prices. It must not be counted twice if it is already included in pᵢ.

The formula produces US$ per share. It is an illustrative comparison, not a description of every router's algorithm. A real router must also account for uncertain fills, changing quotes and applicable order instructions. For a sell order, the comparison instead focuses on expected net proceeds.

Worked example

Suppose, hypothetically, an investor submits a buy order for 200 shares. Venue A displays 100 shares at US$50.00, while Venue B displays 200 shares at US$50.03. Assume all displayed shares remain available and both venues charge US$0.01 per executed share; ignore additional market impact for this example.

  1. Sending all 200 shares to B would cost 200 × US$50.03 + 200 × US$0.01 = US$10,008, or US$50.04 per share.
  2. Routing 100 shares to A and 100 to B would cost 100 × US$50.00 + 100 × US$50.03 + 200 × US$0.01 = US$10,005, or US$50.025 per share.
  3. The split route is US$3 cheaper in this simplified scenario.

The example assumes complete fills at the displayed prices. If A's quote disappears before execution, the actual result can differ.

Why it matters before you trade

Routing can affect the price paid or received, whether an order fills fully, and how quickly it fills. These trade-offs can matter more when an order is large relative to available liquidity or when quotes move rapidly. A limit price and other execution conditions constrain what may be executed, but they do not guarantee a fill.

Before placing an order, estimates of spread, market impact and participation in daily volume can provide context for execution costs. They cannot identify in advance the venue that will produce the best actual fill.

How PreTrAIde uses it

PreTrAIde does not route, receive, transmit or execute orders. Its Pre-trade analysis evaluates a stock basket before an order is placed and provides a suggested execution strategy, alongside estimated spread, market impact, liquidity measures and participation in daily volume. These estimates can inform a user's assessment of execution conditions; they are not venue-by-venue routing instructions. Users place trades themselves with their own broker. PreTrAIde's verdicts are decision support, not recommendations.

Common mistakes

  • Assuming a router guarantees the best price. Quotes and available quantities can change before an order reaches a venue.
  • Looking only at a quoted price. Share availability, fees, impact and incomplete fills can change the overall result.
  • Confusing routing with order type. A market or limit order sets execution instructions; routing determines where an eligible order is sent.
  • Assuming a pre-trade estimate is an actual fill. Estimated costs and execution results are different measurements.

Frequently asked questions

Does smart order routing guarantee the best execution price?

No. Prices and available shares can change, and routing decisions may balance price with fill probability, speed, fees and order instructions.

Can a smart order router split one stock order across venues?

Yes. A router may send portions to different eligible venues when their available liquidity and prices make splitting appropriate under its rules and the order's instructions.

Does PreTrAIde use smart order routing?

PreTrAIde does not route or execute orders. Its Pre-trade analysis provides execution-related estimates and a suggested execution strategy; users submit any orders through their own broker.

Related terms

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. Order book Order book is a record of currently available buy and sell orders for a security, organized by price and quantity, that shows displayed liquidity at a particular venue or across specified venues. 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. 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. 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. Broker Broker is a firm or individual that handles a customer's securities orders, connecting the customer to trading venues or other parties that can execute them.

Execution & trading costs

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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.