Cboe BZX Aligns Trading Definitions with New Round Lot Rules
Published Date: 11/20/2025
Notice
Summary
Cboe BZX Exchange is updating its trading rules to match a new official definition of a 'round lot'—basically, the standard number of shares in a trade. This change affects traders and brokers using the exchange and takes effect immediately, with no extra costs involved. It’s all about keeping things clear and consistent so trading runs smoothly!
Analyzed Economic Effects
2 provisions identified: 2 benefits, 0 costs, 0 mixed.
Round Lot Becomes Price-Based
If you trade on Cboe BZX, the definition of a 'round lot' will change from a fixed 100 shares to a price-based size effective November 3, 2025. Under Rule 600 of Regulation NMS a round lot will be: 100 shares for stocks with average closing price $250.00 or less; 40 shares for $250.01–$1,000.00; 10 shares for $1,000.01–$10,000.00; and 1 share for $10,000.01 or more. The round lot assignment is recalculated semiannually using a one-month Evaluation Period (March for May assignments; September for November assignments).
Rulebook Language Updated for Clarity
Cboe BZX will update related Exchange rules to remove fixed share references and instead reference the new round lot definition on November 3, 2025. For example, phrases like 'one hundred (100) shares' and the reference to 'five round lots' usually being '500 shares' will be replaced so 'normal unit of trading' means the round lot defined in Exchange Rule 11.10. The change is intended to reduce confusion for Exchange Members and the public.
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