Glossary
Round lot
The standard trading unit for a security, traditionally 100 shares for US stocks; orders smaller than a round lot are odd lots.
On this page
A round lot is the standard unit in which a security trades. For US stocks it has long been 100 shares, so an order for 300 shares is three round lots and an order for 50 shares is an Odd lot. The definition matters less for what you can buy, since brokers fill odd-lot and fractional orders routinely, and more for how quotes are built: under US market rules, only round-lot orders count toward the official best bid and offer.
How it works
Exchanges display bids and offers, and the securities information processors combine the best of them into the national best bid and offer (National best bid and offer (NBBO)). Historically, an order had to be at least one round lot to be included. Odd-lot orders, even at better prices, did not set the official quote.
That gap grew as share prices climbed and as more trading came from small and fractional orders. For a $900 stock, one round lot is a $90,000 commitment, so much of the real liquidity sits in odd lots.
Recent rule changes
In 2020 and again in 2024 the SEC adopted changes to US market data rules that shrink the round-lot size for higher-priced stocks, assigning smaller round lots as the price rises, and add odd-lot information to the consolidated data. Implementation has been staggered and parts of it postponed, so check current SEC and exchange notices for which pieces are in force at a given date.
Outside the US, round lots vary: many markets set board lots by price band, and some Asian exchanges use lots of 100, 500 or 1,000 shares. For options, the equivalent unit is one contract, usually covering 100 shares.