Lot Size

What Is Lot Size?

Lot size defines the standardized number of shares or units in a single trading increment. In US stock markets, the traditional round lot is 100 shares. Orders for exactly 100 shares (or multiples of 100) are round lots, orders for fewer than 100 shares are odd lots, and orders combining both (like 250 shares) are mixed lots.

The round lot convention dates back to the physical trading floor era when standardized quantities simplified the matching process. While electronic trading has eliminated most practical reasons for the distinction, the concept persists in market structure rules and data reporting.

Round Lots vs. Odd Lots

Round lots have historically received preferential treatment in market microstructure. The National Best Bid and Offer (NBBO), the benchmark for best execution, is calculated using only round lot quotes. Odd lot quotes at better prices exist in the market but are not reflected in the NBBO. This means the true best available price may be better than the displayed NBBO.

Odd lot trading has increased dramatically as retail participation has grown. Many retail traders purchase fewer than 100 shares, particularly in high-priced stocks. The SEC has proposed rules that would include odd lot quotes in the NBBO calculation, which would improve transparency and potentially tighten displayed spreads.

The distinction between round and odd lots also affects options exercise. Each option contract represents 100 shares (a round lot). When options are exercised, they create round lot positions. This connection between options and the 100-share lot maintains the practical relevance of the round lot concept for options traders.

Lot Sizes in Other Markets

Forex uses its own lot system: standard (100,000 units), mini (10,000), micro (1,000), and nano (100) lots. The lot size determines the value of a pip movement and directly affects position sizing and risk calculation.

Futures contracts have fixed lot sizes determined by the contract specification. One E-mini S&P 500 contract represents $50 times the index value. One gold futures contract represents 100 troy ounces. These fixed sizes mean futures traders adjust their number of contracts rather than the lot size.