A unit pack (lot) of shares the size of which varies according to the price of a share. It is a standard trading quantity (e.g., 500 or 1000 shares) for a particular security. On exchanges, trading takes place in multiples of the round lot. The cost of trading in round lots is usually lower than for trading in quantities less than a round lot (such quantities are referred to as odd lots). In the United States, a round lot is 100 shares on all the stock exchanges. For thinly traded stocks, it consists of 10 shares. In countries where exchanges list a high number of of low-priced stocks, round lots are often larger or set at levels proportionate to price ranges.

Round lots are also referred to as board lots.