Lot Size

A lot size is the standardized quantity of a financial instrument in a single transaction. In the U.S. stock market, a standard lot (or round lot) is 100 shares. Orders for fewer than 100 shares are called odd lots, while orders for multiples of 100 are round lots, and mixed orders (e.g., 350 shares) are mixed lots. Historically, odd lots carried higher transaction costs and were harder to execute, but modern electronic trading has largely eliminated these disadvantages. In futures and options markets, lot sizes are defined by the contract specification—one E-mini S&P 500 futures contract represents $50 × the index level, while one options contract controls 100 shares of the underlying stock. In forex, a standard lot is 100,000 units of currency, a mini lot is 10,000, and a micro lot is 1,000. Understanding lot sizes is essential for position sizing, risk management, and calculating the dollar value of price moves.