What it means
A marketable limit order combines an immediately executable price with a firm price boundary. A buy limit is marketable when its limit price is at or above an available ask; a sell limit is marketable when its limit price is at or below an available bid. Whether it fills immediately depends on available shares, routing, and changes in the quotes before the order reaches the market.
Unlike a market order, it cannot execute beyond its limit price. Unlike a limit order priced away from the current opposite-side quote, it is intended to take available liquidity rather than wait for the market to reach its price. A marketable order can still fill only partly, or not at all, if accessible liquidity disappears. Any unfilled portion may remain open unless its time-in-force or broker instructions cancel it.
How it is calculated
Marketability is a comparison, not a cost formula. For a buy, buy limit price >= available ask price; for a sell, sell limit price <= available bid price.
- Buy limit price: the most the buyer will pay per share, in US$ per share.
- Sell limit price: the least the seller will accept per share, in US$ per share.
- Available ask price: a displayed or otherwise accessible offer to sell, in US$ per share.
- Available bid price: a displayed or otherwise accessible offer to buy, in US$ per share.
The inequality tells you whether a quote could be matched at those prices. It does not tell you how many shares are available, whether a broker can access that quote, or what the final average fill price will be. For a buy, the limit is a ceiling, not a target: execution may occur at a lower price. For a sell, it is a floor.
Worked example
Suppose, hypothetically, a stock has a best displayed bid of US$49.98 and ask of US$50.00. An investor enters a limit order to buy 300 shares at US$50.05, with no immediate-or-cancel instruction.
- Check marketability: US$50.05 is above the US$50.00 ask, so the buy is marketable against that ask.
- Check available size: Assume 100 shares are accessible at US$50.00 and another 100 at US$50.03. If those quotes remain available, 200 shares could fill for
100 × US$50.00 + 100 × US$50.03 = US$10,003, an average of US$50.015 per share before fees. - Account for the remainder: The other 100 shares cannot fill above US$50.05. If no further eligible shares are available, they remain unfilled and may rest at the limit, subject to the broker’s handling and time-in-force rules.
The US$50.05 limit does not mean all 300 shares will fill, or that filled shares must cost US$50.05.
Why it matters before you trade
A marketable limit sets a worst permitted execution price per share while seeking a prompt fill. The trade-off is completion risk: a tight limit may leave shares unfilled if quotes move, while a wider limit permits execution across more expensive ask levels on a buy or lower bid levels on a sell.
Before placing an order, distinguish the quoted spread from order-book depth. A small bid-ask spread does not establish that enough shares are available at the best quote. Order size relative to daily volume, potential market impact, trading hours, and time-in-force also affect how an order may execute. The limit controls eligible fill prices; it does not cap commissions or guarantee a particular total transaction cost.
How PreTrAIde uses it
PreTrAIde does not claim to place marketable limit orders or calculate their fills. Its pre-trade analysis lets a user specify a buy or sell side, quantity, and execution conditions for each stock in a basket before placing any order. It estimates bid-ask spread, participation in daily volume, and market impact, and provides an execution score and suggested execution strategy as decision support. Those estimates can help frame questions about an order’s size and potential costs, but they are not a live order-book check or a promise of execution. Users place trades themselves with their own broker.
Common mistakes
- Treating the limit as the expected fill price. A buy can fill below its limit; a sell can fill above it.
- Assuming an immediate full fill. Available quantity and changing quotes can produce partial fills or no fill.
- Ignoring the unfilled balance. Without a cancellation instruction, the remainder may stay active under the order’s time-in-force.
- Confusing the limit with a total-cost cap. Fees and other transaction costs are separate from the per-share price boundary.