What is a "Round Lot" in stocks?

Simply stated, an "Odd Lot" is a stock order comprised of fewer shares than a “Round Lot”. The size of a round lot is determined using tiering based on the share price of the stock.  The tiering is as follows: | Share price (average closing price) |…

What has changed with the round lot size?

On November 3, 2025, the SEC implemented new "Round Lot" definitions under the Market Data Infrastructure (MDI) Rules to enhance market transparency. The new "Round Lot" definitions changed how quote information is displayed based on stock price…

By default, the BasketTrader rounds mixed lots to the nearest hundred. Odd lots for U.S. Stocks and Warrants will be rounded up to 100, and odd lots for non-U.S. Stocks will not be rounded. When you upload orders to BasketTrader from a csv file there will be a pop-up window that asks whether you would like to round order sizes with the option to "Remember my settings."…

An order for an amount of shares greater than a round lot, but not a multiple of the round lot is a "Mixed Lot" order (aka a Partial Round Lot or PRL order)

• A mixed lot/PRL (i.e. 257 shares) direct-routed to NYSE/AMEX will be submitted in whole to the exchange (applies to both market and limit orders)…