Board Lots and Minimum Tradable Quantities

Not every share count is tradable in a normal order. Most exchanges define a board lot (also called a round lot) — a standard bundle size, like 100 shares — and orders below that size, called odd lots, are historically treated as second-class citizens: they may not appear in the consolidated quote, may not be eligible for certain order types, and often get filled only after board-lot orders at the same price.

Why bundles exist at all

Exchanges settled on standard lot sizes decades ago mostly to keep quoting and clearing simple: a market maker posting "100 shares at $50.10" is a clean, comparable unit next to another market maker's "100 shares at $50.11." If everyone could quote arbitrary share counts, comparing depth across price levels would be messier, and clearing systems built around round numbers would need far more bookkeeping for tiny fragments. The convention has stuck even though most trading today is electronic and the original bookkeeping reasons matter far less.

Board lot size isn't universal. In the US it's usually 100 shares (though a stock trading above roughly $250 sometimes uses a 10-share lot, and some very high-priced names use 1 share). Hong Kong sets board lots per stock individually — one name might trade in lots of 500, another in lots of 2,000 — precisely so that a round lot represents a roughly similar notional value across very differently priced stocks. Many other Asian markets follow a similar per-stock approach rather than one flat number.

Worked example

Say a Hong Kong-listed stock has a board lot of 500 shares, trading at HK$8.00. A round lot costs HK$4,000. An investor who wants to buy just 120 shares can't submit that order as a normal round-lot order — 120 is not a multiple of 500. They'd need to either round up to a full lot (500 shares, HK$4,000) or trade in the separate odd-lot market, which typically has thinner liquidity, wider effective spreads, and sometimes a different matching mechanism entirely, so the same 120 shares might cost noticeably more per share than the round-lot quote implies. A backtest that assumes any share count fills at the quoted round-lot price will overstate returns on strategies that trade small, oddly-sized positions in markets with large board lots.

What this means in practice

If you're building execution logic or position sizing for markets outside the US, check the board lot size per instrument rather than assuming 100 shares everywhere — Hong Kong, several other Asian exchanges, and some smaller European venues all vary lot size by stock. For odd-lot orders, expect worse execution: they may be excluded from certain best-price guarantees, may not count toward the displayed quote, and are the segment where retail-sized orders are most likely to get a materially different fill than what the top-of-book quote implies.