Sub-Penny Internalization Price Improvement Erosion
The Formal Definition
The progressive long-term reduction in average price improvement delivered to retail investors by wholesale internalizers, where brokerages market 'shares improved' metrics while delivering nominal fractions of a cent ($0.0001) that capture less than 5% of the prevailing bid-ask spread.
Realized Price Improvement Ratio = Total Realized Price Improvement Dollars / Total Quoted Spread Dollar Value
Cole Barrett's Reality Check
The Unvarnished Bottom Line"Brokers love putting big numbers on their marketing pages: 'We delivered $500 million in price improvement this year!' What they don't tell you is that they improved your fill by a hundredth of a penny on a ten-cent spread. They took 99.9% of the spread for themselves, gave you a fraction of a cent so they can check the regulatory box, and called it a customer benefit."
Interactive Simulator: Test the Math
Real-World Example: Scenario Breakdown
Examining the real numbers for: A retail trader purchasing 1,000 shares of an equity with an NBBO of $40.00 Bid / $40.10 Ask ($0.10 spread)
| Execution Metric | Lit Exchange Midpoint Match User | PFOF Sub-Penny Internalized Client |
|---|---|---|
| Fee / Rate | $0.0035/share DMA rate ($3.50 total) | $0 advertised commission |
| Spread / Buffer | Order routed directly to a lit exchange order book with midpoint routing instructions | Order routed to an off-exchange wholesale market maker under a payment for order flow agreement |
| Execution / Status | Matched against an independent institutional seller at the true $40.05 midpoint ($0.05 price improvement per share) | Wholesaler provided an advertised 'price improvement' of $0.0001 per share, filling the order at $40.0999 |
| Total Cost / Result | Captured genuine midpoint price improvement on a transparent lit exchange | Received token sub-penny price improvement while wholesaler monetized the spread |
How Brokers Weaponize This Term
Review your broker's SEC Rule 605 execution quality reports. Look at the 'Price Improvement Percentage'. If your average price improvement on liquid stocks is less than 10% of the prevailing quoted spread, your broker's wholesalers are using sub-penny skims to capture spread revenue.
Broker Evaluation Matrix
Cole Approves
Interactive Brokers: Routes client orders through its SmartRouting engine to actively hunt for true midpoint executions across lit exchanges rather than sub-penny wholesale internalizers.
Read Audit →Cole Flags / Avoids
Zero-Commission PFOF Apps: Routes 100% of retail market orders to wholesale internalizers that use token sub-penny price improvement to monetize customer flow.
View Trap Details →Frequently Asked Questions
What is SEC Rule 612 (The Sub-Penny Rule)?
Rule 612 prohibits market participants from displaying quotes in increments smaller than $0.01 for stocks priced over $1.00, though wholesale internalizers are legally permitted to execute trades in sub-pennies off-exchange.
Why is the SEC proposing changes to sub-penny rules?
Regulators have proposed reducing the minimum tick size to half-pennies ($0.005) or smaller for liquid stocks, allowing public exchanges to compete directly with off-exchange wholesale internalizers.