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Key Takeaways
- Effective spread equals twice the direction-adjusted difference between execution price and the order-receipt midpoint.
- The factor of two puts the measure on the same scale as the full quoted spread.
- A buy below the arrival midpoint or a sell above it produces a negative effective spread.
- A fill exactly at that midpoint produces zero.
- The worked buy has a two-cent effective spread and $5 of price improvement on 500 shares.
What is effective spread?
Effective spread is twice the direction-adjusted difference between execution price and the NBBO midpoint at order receipt. The direction is +1 for a buy and −1 for a sell. The quoted spread is ask minus bid, and doubling the fill’s midpoint difference puts effective spread on that same full-spread scale.
Formula
Effective spread = 2 × D × (execution price − NBBO midpoint at order receipt); E/Q ratio = effective spread / quoted spread- D is +1 for a buy and −1 for a sell.
- The benchmark is the NBBO midpoint when the order arrives: $20.02 in the example.
- Effective spread is what you paid versus the middle of the quote, doubled so it compares with the full spread.
- Divide by the quoted spread for E/Q when that quoted spread is positive.
- Percentage effective spread = effective spread / midpoint × 100.
How to calculate effective spread: a 500-share example
A market center receives a market order to buy 500 shares of XYZ while the NBBO is $20.00 bid and $20.04 ask. The arrival midpoint is $20.02 and the quoted spread is $0.04. The order fills at $20.03.
- Arrival midpoint = ($20.00 + $20.04) / 2 = $20.02
- Effective spread = 2 × (+1) × ($20.03 − $20.02) = $0.02 per share
- E/Q = $0.02 / $0.04 = 0.50
- Purchase cost = $20.03 × 500 = $10,015
- Price improvement versus the $20.04 arrival ask = $0.01 × 500 = $5
- Cost above the $20.02 arrival midpoint = $0.01 × 500 = $5
The order buys 500 shares for $10,015, saving $5 relative to the arrival ask. Its effective spread is two cents per share, half the four-cent quoted spread.
Book the trade at its real cost, $10,015 plus any fees, and use the two-cent effective spread to compare this fill with other fills and routes.
Effective spread sign, partial fills and Rule 605
Against that same quote, a $20.01 buy has a negative two-cent effective spread. A $20.02 buy has zero. A $20.04 buy has a four-cent effective spread, equal to the quoted spread.
For partial fills of this market order, weight each execution by its filled quantity against the same receipt benchmark, the midpoint when the order arrived.
How traders use effective spread
Compare the spread measures with filled quantity, completion time and explicit fees. Review orders by ticker, size and order type to understand the results your routing delivers.
In options flow, each print’s position relative to its associated quote shows where contracts trade inside the spread, and the options profit calculator shows how entry prices change breakevens and payoff.
Limitations of effective spread
- Fix the benchmark before comparing fills. The statistic uses the $20.02 midpoint at order receipt, so a later quote or a different timestamp moves the result.
- Track the rest of execution quality beside it. The two-cent spread scores price, while fill rate, speed, completed quantity and explicit fees each need their own record.
- Use E/Q when the quote has width. The ratio divides by the $0.04 quoted spread in the example, and a locked quote with equal bid and ask has no width to divide by.
- Judge option fills against the option’s market. Rule 605 averages cover NMS-stock orders, so read a listed-option execution against the option’s bid, ask and midpoint at entry.
- Weight averages by filled shares and compare orders with matching size, type and ticker. A simple average gives a small order the same weight as a large one.
Bottom line
Effective spread shows what an order paid relative to the midpoint when it arrived, on the same scale as the quoted spread. Use it with the E/Q ratio to compare executions across stocks and routes, weight it by filled shares, and read it alongside completion, speed and fees.
Clear answers
Frequently Asked Questions
Sources and Further Reading
- FINRA: SEC Rule 605 Execution Quality Reporting
- CFA Institute: Trading Costs and Electronic Markets
- Fidelity: Commitment to Execution Quality
How entries are sourced, calculated and updated: editorial methodology.