MEMX Tweaks Round Lot Rules to Fit New Regs
Published Date: 12/8/2025
Notice
Summary
MEMX is updating its rules to match a new official definition of a 'round lot'—basically, the standard number of shares in a stock trade. This change affects traders and brokers using MEMX and kicks in right away with no extra costs. It also tweaks some related rules to keep everything neat and clear.
Analyzed Economic Effects
3 provisions identified: 3 benefits, 0 costs, 0 mixed.
New official 'Round Lot' sizes
The Exchange updated its definition of a “Round Lot” to match Rule 600(b)(93) of Regulation NMS. Under that definition, a Round Lot is based on a security's average closing price: $250.00 or less = 100 shares; $250.01–$1,000.00 = 40 shares; $1,000.01–$10,000.00 = 10 shares; $10,000.01 or more = 1 share. For any security that becomes an NMS Stock during an operative period, a Round Lot is 100 shares.
Round lot reassigned twice a year
Round Lot assignments for NMS stocks will be recalculated semiannually based on a one-month Evaluation Period. The March Evaluation Period determines the Round Lot assigned on the first business day of May (operative through the last business day of October), and the September Evaluation Period determines the Round Lot assigned on the first business day of November (operative through the last business day of April).
MEMX rule change effective immediately
MEMX filed the rule change on November 25, 2025 and the Commission waived the 30-day operative delay, making the conforming changes operative upon filing. The Exchange states the changes are implemented immediately, do not amend the operation of the affected rules, and are intended to reduce investor and market participant confusion.
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