Execution & trading costs

Round lot

Round lot is a standard trading quantity for a security; for many US stocks it has traditionally been 100 shares, but the applicable size can vary by security and market.

What it means

A round lot describes an order quantity in standard-sized units rather than a particular dollar value. For many US stocks, the familiar unit is 100 shares. That is a convention, not a universal rule: the applicable size can differ by security or market, and US quotation rules may use a security-specific round-lot size.

An order can contain several complete round lots. Any shares left over form an odd-lot portion. A quantity smaller than one applicable round lot is an odd lot. These labels describe share counts; they do not say whether an order is likely to receive a favorable price or be filled.

How it is calculated

First, identify the applicable round-lot size for the security and market. Then divide the proposed share quantity by that size:

Q = L × R + O

  • Q = total order quantity, in whole shares.
  • L = number of complete round lots, a nonnegative whole number.
  • R = applicable round-lot size, in shares per lot.
  • O = remaining shares, where 0 ≤ O < R.

Equivalently, L = floor(Q / R) and O = Q − (L × R). The floor function rounds down to the nearest whole number. If O = 0, the whole-share quantity is an exact multiple of the round-lot size. A round lot counts shares, not US dollars: multiplying shares by a share price gives an order's approximate value, not its lot classification.

Worked example

Assume a hypothetical stock has an applicable round-lot size of 100 shares and a hypothetical price of US$20 per share. These numbers are only for illustration.

  1. A proposed order for 250 shares has R = 100 and Q = 250.
  2. Dividing 250 by 100 gives two complete lots: L = 2.
  3. The remainder is O = 250 − (2 × 100) = 50 shares. The order contains two round lots plus a 50-share odd-lot portion.
  4. At US$20 per share, its approximate share value is 250 × US$20 = US$5,000, excluding execution costs.

For comparison, 300 shares would be three complete 100-share lots with no remainder. Changing the hypothetical price would change the order value, but not those share-count calculations.

Why it matters before you trade

Lot size helps you read quotes and order-book quantities correctly. A displayed quantity expressed in lots must be converted to shares using the applicable lot size; assuming every lot means 100 shares can misstate the displayed depth. Quote-display conventions and order-handling rules can also differ for round-lot and odd-lot quantities.

Check the proposed quantity against the security's lot convention and your broker's order-entry rules. An odd-lot portion is not automatically invalid or more expensive, but a visible quote is not a promise that the full order will fill at one price. The bid-ask spread, available depth, order type and participation in trading volume are separate execution considerations.

How PreTrAIde uses it

PreTrAIde's published product facts do not state that it classifies orders as round lots or applies a round-lot rule. Its pre-trade analysis instead starts with the quantity and execution conditions the user enters for each stock in a basket. Its quantitative layer estimates the bid-ask spread, participation in daily volume and market impact, among other measures. Those checks relate to the size and potential execution costs of a proposed order, whether its quantity is an exact multiple of a round lot or not.

PreTrAIde returns a GO, NO-GO or MODIFY verdict as decision support before an order is placed. It does not receive, transmit or execute orders; users trade through their own brokers.

Common mistakes

  • Treating 100 shares as the round-lot size for every security and market.
  • Confusing a round lot with US$100 worth of stock; it is a share-quantity unit, not a monetary unit.
  • Calling an entire 250-share order an odd lot when the applicable size is 100 shares: only its 50-share remainder is the odd-lot portion.
  • Assuming an exact round-lot quantity guarantees a fill or a particular execution price.
  • Using lot classification as a substitute for checking spread, market depth, market impact and broker order rules.

Frequently asked questions

Is a round lot always 100 shares?

No. Although 100 shares is a familiar convention for many US stocks, the applicable round-lot size can depend on the security and market. Check the convention relevant to the quote or order.

Is 250 shares a round lot or an odd lot?

If the applicable round-lot size is 100 shares, 250 shares consists of two complete round lots and a 50-share odd-lot portion. The answer changes if the applicable lot size is different.

Can an odd-lot stock order be executed?

Yes, an odd-lot quantity is not inherently unexecutable. Whether an order is accepted and how it fills depend on the broker, market conditions and order instructions.

Related terms

Odd lot Odd lot is a stock trade or order for fewer shares than the applicable round-lot size, commonly fewer than 100 shares for a US-listed stock. Fractional shares Fractional shares are holdings of less than one whole share of stock, typically created through a broker's dollar-based purchases, dividend reinvestment, or corporate actions. Order book Order book is a record of currently available buy and sell orders for a security, organized by price and quantity, that shows displayed liquidity at a particular venue or across specified venues. Market depth Market depth is the quantity of buy and sell orders available at different prices in an order book at a particular moment. Bid-ask spread Bid-ask spread is the difference between the lowest displayed price to sell a stock (ask) and the highest displayed price to buy it (bid), measured per share. Partial fill Partial fill is the execution of fewer shares than an order requests, leaving a remaining quantity that may stay open or be canceled under the order’s conditions.

Execution & trading costs

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Educational content, not investment advice. PreTrAIde is analysis software, not a broker: it never executes orders, and its verdicts are decision support, not recommendations to buy or sell.