Glossary term
Effective Spread
Effective spread is a trade execution cost measure that compares the execution price with the midpoint of the quoted bid-ask spread at the time of the order or trade.
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What Is Effective Spread?
Effective spread is a trade execution cost measure that compares the execution price with the midpoint of the quoted bid-ask spread at the relevant time. It is commonly used in execution quality analysis because it looks at where the trade actually occurred relative to the market's quoted midpoint.
The quoted spread shows the visible difference between the best bid and best offer. Effective spread goes one step further by asking how costly the actual execution was compared with the midpoint of those quotes.
Key Takeaways
- Effective spread measures execution cost relative to the quote midpoint.
- It is commonly calculated as two times the absolute difference between the execution price and the midpoint.
- A lower effective spread generally indicates a better execution cost for a marketable order.
- Effective spread can differ from quoted spread because trades may execute inside, at, or outside the displayed quotes.
- The metric is used in Rule 605 execution quality reporting and market-structure analysis.
How Effective Spread Works
For a simple stock example, suppose the best bid is $20.00 and the best offer is $20.10. The midpoint is $20.05. If a buy order executes at $20.08, the execution is three cents above the midpoint. The effective spread is commonly described as twice that distance, or six cents.
The doubling convention reflects the round-trip cost implied by trading away from the midpoint. For a buy, paying above the midpoint is a cost. For a sell, receiving below the midpoint is a cost. Using the absolute difference makes the measure comparable across buys and sells.
Effective Spread Versus Quoted Spread
Measure | What it captures |
|---|---|
Quoted spread | Difference between the displayed best bid and best offer. |
Effective spread | Actual execution cost relative to the midpoint. |
Realized spread | Execution economics after later price movement. |
Price improvement | Execution better than the quoted bid or offer. |
How to Interpret It
Effective spread helps compare execution quality across venues, brokers, and market conditions. A trade that receives price improvement may have a lower effective spread than a trade filled at the quoted offer or bid. A trade executed outside the quote may have a higher effective spread.
The metric has limits. It depends on the benchmark quote time, market data quality, order type, and whether the order is marketable. It also does not capture every dimension of execution quality, such as fill probability for nonmarketable limit orders.
Effective spread is especially useful when quoted spreads are narrow but executions differ meaningfully across venues. Two brokers may display the same commission schedule, but one may consistently obtain better midpoint-relative executions. The difference is small per share, yet meaningful across frequent trading or large order flow.
The Bottom Line
Effective spread is a practical measure of the cost embedded in a trade execution. It compares the actual fill with the quote midpoint, helping investors and regulators evaluate whether an order received competitive execution.