Round Lot
A round lot is the standard trading unit for U.S. equity markets, consisting of 100 shares or any multiple of 100 shares, and historically represents the baseline order size that exchanges and market makers have been designed to handle in a standardized way.
The 100-share round lot convention has been embedded in U.S. equity market structure for well over a century. In the floor-trading era, the round lot defined how specialists and market makers posted their quotations and how transactions were matched in the auction process. Regulatory frameworks including the computation of the National Best Bid and Offer (NBBO) were originally designed around round-lot quotes, reflecting an era when institutional activity in standard 100-share (or larger) increments dominated trading activity.
In modern electronic markets, the distinction between round lots and other order sizes has diminished for most practical execution purposes. Exchanges process orders of any share quantity through their electronic matching engines, and the execution mechanics do not meaningfully differ for a 100-share order versus a 47-share order in most large-cap U.S. equities. However, the round-lot convention persists in the regulatory definition of the NBBO, and certain exchange rules and market data calculations continue to be based on round-lot quotes.
For institutional investors, orders are almost always submitted in quantities far exceeding a single round lot — frequently in tens or hundreds of thousands of shares. In this context, the round lot is less a practical constraint and more a historical baseline. Algorithmic execution strategies typically break parent orders into many child orders whose sizes are calibrated to available liquidity, and these child orders may or may not be round-lot multiples depending on the algorithm's design.
The SEC has acknowledged the evolving relevance of the round-lot convention in the context of market structure reform. Proposals in the early 2020s to update the definition of round lots and odd lots for NBBO calculation purposes reflected recognition that the 100-share standard, established for a different era of market structure, may no longer fully capture how prices are formed in contemporary electronic markets where odd-lot activity is significant in high-priced stocks.
For high-priced U.S. stocks — notably those trading at several hundred or thousands of dollars per share — a single 100-share round lot represents a very large dollar commitment, which can influence the depth of the displayed order book at round-lot sizes. This is one reason stock splits remain a routine corporate action: by reducing the share price to a lower level, a company lowers the dollar cost of a round lot, potentially broadening retail participation and improving the liquidity profile of its stock. The relationship between share price, round lot size, and displayed liquidity has been a recurring theme in market structure research.