Odd Lots, Round Lots And Mixed Lots

Most U.S. equity exchanges define a round lot as the standard trading unit — historically 100 shares for most stocks, though the rules now scale it down to 40, 10, or even 1 share for very high-priced names. An odd lot is any order smaller than a full round lot: 37 shares of a $100 stock, say. A mixed lot combines the two — an order for 137 shares is one round lot plus a 37-share odd lot tacked on.

The distinction matters because for decades, odd-lot orders and trades were excluded from the consolidated tape (the public feed showing the National Best Bid and Offer) and from official trade-and-quote statistics, on the theory that they were too small to matter for price discovery. That created a real blind spot: a large institutional order can be sliced into hundreds of odd-lot child orders specifically to stay under the radar, meaning a meaningful chunk of real trading activity was historically invisible in the public data reporting stream. Regulation NMS amendments have since required more odd-lot activity to be reported, but odd lots still don't count toward setting the official NBBO the way round lots do.

For retail traders this rarely matters — a broker fills an odd-lot order just like any other. For anyone building signals off public tape data, though, ignoring the odd-lot reporting gap can mean systematically underestimating true volume and misreading how much real interest sits at a given price.