Round lot

A round lot is the standard trading unit for a stock, traditionally one hundred shares. Since late 2025, SEC rules set smaller round lots of 40, 10, or 1 share for higher priced stocks, though one hundred shares remains the round lot for most stocks. Exchanges and quoting systems are built around this unit, meaning displayed bid and ask sizes, and much of the standard order handling logic, assume orders come in round lot quantities unless specified otherwise.

The one hundred share standard developed because it created a convenient, uniform unit for exchanges to organize trading around, back when transacting in individual shares one at a time would have been impractical for a busy market. An order smaller than a round lot is called an odd lot instead, and historically received different treatment in how it was quoted and executed, though modern rules have removed most of those differences.

For most individual investors today, the round lot distinction matters less than it once did, since fractional share investing and generally higher stock prices have made buying and holding in odd lot or even fractional quantities completely normal. It still matters for understanding displayed market data, since some platforms quote bid size and ask size in round lots, so a displayed size of five for a typical stock represents five hundred shares of available interest at that price, not five individual shares.