The round lot is now a price-tiered quantity, and correcting that is why this page exists. Until late 2025 the 100-share unit was an exchange convention rather than a federal rule. The Cboe filing puts it plainly: "Prior to this change, a 'round lot' was not defined in the Act or Regulation NMS. The definition of a 'round lot' was included in the rules of the individual exchanges, including Exchange Rule 11.10, which defined a 'round lot' as one hundred shares." The SEC adopted a federal definition in its 2020 market data infrastructure rules, revised it in 2024 and accelerated its implementation, and the revised definition took effect on November 3, 2025, the first business day of that November.
Rule 600(b)(93) assigns the size from the stock's average closing price on its primary listing exchange during the preceding Evaluation Period:
A security that becomes an NMS stock partway through an operative period is assigned a round lot of 100 shares. Note the highest price band. Where the round lot is a single share, a one-share order is a round lot, so no whole-share order in such a stock can be an odd lot. For reporting purposes sub-share orders are counted separately rather than folded in, because Rule 600(b)(18) splits every dollar bucket three ways, into "less than a share", "odd-lot" and "at least a round lot".
The threshold moves twice a year without anything about the stock changing. The Evaluation Period is all the trading days in March for the round lot assigned on the first business day of May, and all the trading days in September for the size assigned on the first business day of November. Each assignment stays operative until the next one: the May size runs through the last business day of October, and the November size through the last business day of the following April. A stock that drifts across a band boundary between one September and the next therefore has a different odd-lot threshold in successive years with no split, no offering and no corporate action behind it.
A quotation is round-lot sized by definition, and that is the structural reason odd lots sat outside the visible market. Rule 600(b)(16) defines a bid or offer as "the bid price or the offer price communicated by a member of a national securities exchange or member of a national securities association to any broker or dealer, or to any customer, at which it is willing to buy or sell one or more round lots of an NMS security." An order too small to be a round lot could not, as a matter of definition, form part of the displayed quotation. That is not a market failing to notice small orders; it is what the word quotation was defined to mean.
That is being addressed, in stages. Rule 600(b)(26) now lists odd-lot information as one of the elements of core data, and Rule 600(b)(69) defines odd-lot information in three parts: odd-lot transaction data disseminated under the effective national market system plan as of April 9, 2021; odd lots priced at or inside the national best bid and offer, aggregated at each price level at each exchange and association; and the best odd-lot order to buy and to sell, meaning the highest-priced odd-lot buy order above the national best bid and the lowest-priced odd-lot sell order below the national best offer. The SEC recorded that a subsequent release set the compliance date for every exchange and association to make the necessary data available to the exclusive securities information processors, and for those processors to collect, consolidate and disseminate odd-lot information including the best odd-lot orders, to the first business day of May 2026.
The execution-quality rules count odd lots separately, on a schedule worth knowing. Rule 605 requires the detailed monthly report to be "categorized by security, order type, and order size", and Rule 600(b)(18) is the definition that supplies the size categories, splitting every dollar bucket into less than a share, odd lot, and at least a round lot. So odd-lot fills are counted in their own bucket rather than blended into everything larger. The shorter summary report the same rule requires is broken out by dollar size alone, so the odd-lot detail sits in the detailed report. The SEC moved the compliance date for those amendments from December 14, 2025 to August 1, 2026, and reporting entities have until the end of September 2026 to make their August 2026 reports publicly available. One further piece arrives later: the price improvement statistics measured against the best available displayed price, a reference price that Rule 600(b)(14) builds from the best odd-lot order as well as the national best bid and offer, are required beginning November 2026.
"Odd lot" does not mean the same thing in every rule, and one place still says one hundred. Regulation SHO's circuit-breaker rule adopts the Rule 600 definition by cross-reference, so it moves with the tiers. Regulation M excepts "transactions in odd-lots" from its restrictions on distribution participants, and also excepts transactions to offset odd lots in connection with an odd-lot tender offer under Rule 13e-4(h)(5). That exemption, however, is written for "offers to purchase from security holders who own an aggregate of not more than a specified number of shares that is less than one hundred." The issuer picks the number, but the rule caps what it may pick below one hundred shares, and that ceiling was not re-pointed at the tiered round lot. So an issuer's odd-lot tender offer is still bounded by a sub-100-share figure whatever that issuer's round lot happens to be.
