Glossary
Odd lot
An order or trade of fewer than 100 shares, smaller than a standard round lot.
An odd lot is a quantity of stock smaller than a round lot, which in the US has traditionally meant fewer than 100 shares. An order for 37 shares is an odd lot; an order for 250 shares is a mixed lot, two round lots plus a 50-share odd lot. Odd lots used to be a niche of small investors. Today, with high share prices, fractional investing and algorithms that split orders into small pieces, they make up a large share of all US stock trades.
Why the distinction matters
For most investors, odd lots trade like any other order and fill at normal prices. The differences are in market data. Odd-lot trades are reported to the consolidated tape and count in volume, but odd-lot quotes have historically been left out of the national best bid and offer, which is built from round-lot quotes.
For very expensive stocks, a 100-share round lot can be worth tens of thousands of dollars, so a large fraction of the real liquidity sits in odd lots. SEC changes to Regulation NMS (adopted in 2020 and revised in 2024) shrink the round lot for higher-priced stocks so that more of these quotes count; check the current implementation status, since the timetable has been pushed back before.
Odd lots in practice
Brokers that offer fractional shares often aggregate customer orders and trade the odd-lot remainder internally. Some older trading rules and options strategies refer to 100-share blocks: an options contract still covers 100 shares, so writing a covered call requires a full round lot of stock.