Effective Spread vs. Quoted Spread
The Formal Definition
The quoted spread is the difference between the displayed National Best Bid and Offer (NBBO) at order entry. The effective spread measures the actual execution price relative to the spread midpoint, reflecting real price improvement or execution slippage.
Effective Spread = 2 × |Execution Price - NBBO Midpoint Price at Order Arrival|
Cole Barrett's Reality Check
The Unvarnished Bottom Line"Quoted spreads are the price tag on the shelf; effective spreads are what you actually pay at the register. If a stock is quoted at $10.00 Bid / $10.10 Ask, the quoted spread is 10 cents. If your broker fills your buy order at $10.03, the effective spread is 6 cents. Effective spread proves whether your broker provides real price improvement or skims your fills."
Interactive Simulator: Test the Math
Real-World Example: Scenario Breakdown
Examining the real numbers for: Executing a market order to buy 1,000 shares (NBBO: $50.00 Bid / $50.10 Ask | Midpoint: $50.05)
| Execution Metric | Smart-Routed Direct Lit Broker (Price Improvement) | Internalized PFOF Broker |
|---|---|---|
| Fee / Rate | $1.00 fee | $0.00 'free' |
| Spread / Buffer | Quoted Spread: $0.10 | Order filled inside the spread at $50.02 | Quoted Spread: $0.10 | Order filled at the full Ask at $50.10 |
| Execution / Status | Effective Spread = 2 × |$50.02 - $50.05| = $0.06 | Effective Spread = 2 × |$50.10 - $50.05| = $0.10 |
| Total Cost / Result | Delivered true execution quality superior to quoted spread | Wholesaler internalized the spread and paid a kickback to the broker |
How Brokers Weaponize This Term
Brokers advertise 'Zero Commission' trading while public SEC Rule 605 filings reveal their effective-to-quoted spread ratios are significantly worse than brokers routing directly to lit market centers.
Broker Evaluation Matrix
Cole Approves
Interactive Brokers: SmartRouting engine independently benchmarks and reports real-time effective spread performance and price improvement statistics on every trade confirmation.
Read Audit →Cole Flags / Avoids
Zero-Fee PFOF Desks: Routes client market orders to internalizers where effective spreads consistently match the full quoted spread with minimal price improvement.
View Trap Details →Frequently Asked Questions
What does it mean if the effective spread is lower than the quoted spread?
It means the trader received price improvement, executing inside the prevailing National Best Bid and Offer.
Where can you look up a broker's average effective spread?
Under SEC Rule 605, all US broker-dealers and market centers must publish monthly public execution quality reports detailing effective-to-quoted spread ratios.