What is the standard lot size for most stock trades on U.S. exchanges?

The standard lot size for most stock trades on U.S. exchanges is 100 shares.

This traditional trading unit is called a round lot, or sometimes a board lot. An order for fewer than 100 shares is generally called an odd-lot order. The convention helped exchanges organize trading and calculate commissions when stock transactions were handled in larger standardized blocks.

The rule is a convention, not a universal requirement. Exchange rules can vary by market and security, and modern brokerages commonly allow investors to buy single shares or fractional shares. Some stocks may also have exchange-specific lot sizes, particularly when prices or trading conditions make a 100-share unit impractical.

A common mix-up is confusing a round lot with the minimum number of shares an investor may buy. Investors can usually purchase fewer than 100 shares; 100 shares is simply the customary standard lot. The value of a round lot therefore depends on the stock price: 100 shares of a $20 stock cost $2,000 before fees, while 100 shares of a $400 stock cost $40,000.