Round Lot

What Is a Round Lot?

A round lot is a standard number of shares that serves as a basic unit for trading a stock. In United States equity markets, the usual round lot is 100 shares. An order for 100, 200, 500, or any other whole multiple of 100 shares consists entirely of round lots. Some higher-priced stocks use a smaller round lot under exchange and regulatory rules, so the standard quantity can vary by security.

How Round Lots Work

Round lots are a reference point for much of how a stock exchange organizes trading. Quoted prices and displayed sizes are often based on round lots, and trade reporting has historically followed the same convention. Any quantity smaller than a round lot is called an odd lot. A quantity larger than one round lot that is not an exact multiple, such as 150 shares, combines whole round lots with an odd lot and is sometimes called a mixed lot. Standardizing on round lots has long simplified quoting, record-keeping, and comparisons between trades.

Example

Suppose an investor places a market order to buy 350 shares of a stock with a 100-share round lot. The order contains three round lots and one odd lot of 50 shares. If the same investor instead buys 100 shares, the order is exactly one round lot with no odd-lot portion.

Why It Matters to Investors

Round lot status can affect how an order is priced and handled. Historically, odd-lot orders sometimes received different treatment than round-lot orders, and the quoted bid-ask spread shown to the public has often reflected round-lot sizes. Market structure and reporting rules have changed over time, so the handling of small orders can differ by market and by broker. Investors who trade small quantities may see differences in execution or in any commission charged. Understanding the round lot concept helps explain why small orders can look different from larger ones on a trading screen.