Execution & trading costs

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.

What it means

An order for a stated number of shares does not always execute all at once. If some shares execute but the full requested quantity does not, the order has a partial fill. The executed shares become part of the position or sale; the unfilled shares remain a separate question.

What happens to that remainder depends on the order and its time-in-force instructions. It may remain open, expire, or be canceled. A partially filled order should therefore not be treated as either wholly completed or wholly unexecuted. Each execution can also have a different price.

How it is calculated

Fill rate (%) = (filled shares ÷ ordered shares) × 100

  • Filled shares is the number of shares that have executed, measured in shares.
  • Ordered shares is the total number of shares requested in the order, measured in shares.
  • Fill rate is the executed portion of the order, expressed as a percentage. It describes quantity, not whether the execution price was favorable.

To track what has not executed, use unfilled shares = ordered shares − filled shares. This quantity is measured in shares. If an order receives multiple executions, add their share quantities to obtain filled shares; divide total execution value by total filled shares to calculate the share-weighted average fill price.

Worked example

Suppose, hypothetically, an investor submits an order to buy 100 shares. The broker reports two executions: 30 shares at US$20.00 and 20 shares at US$20.10. No other shares have executed.

  1. Add the executions: 30 + 20 = 50 filled shares.
  2. Calculate the fill rate: (50 ÷ 100) × 100 = 50%.
  3. Find the remainder: 100 − 50 = 50 unfilled shares. Whether those shares can still execute depends on the order’s conditions and status.
  4. Calculate the executed amount: (30 × US$20.00) + (20 × US$20.10) = US$1,002.00, excluding fees.
  5. Find the average fill price: US$1,002.00 ÷ 50 = US$20.04 per filled share.

The investor has bought 50 shares, not 100. US$20.04 describes the shares already executed; it does not set a price for the remaining 50.

Why it matters before you trade

A partial fill can leave a position smaller than planned or, for a sale, leave more shares held than intended. In a multi-stock basket, different lines may fill to different degrees, changing the basket’s weights and exposures. Checking share quantities, available liquidity, and execution conditions beforehand helps make those possible outcomes visible, but cannot guarantee a full fill.

Order instructions affect the outcome. A limit order controls the worst permitted execution price but may fill only partly—or not at all. An immediate-or-cancel order permits an immediate partial fill and cancels its remainder; a fill-or-kill order requires the entire specified quantity to execute immediately or none of it. Verify the instructions offered by the broker and the order status after submission rather than assuming a remainder is still active.

How PreTrAIde uses it

PreTrAIde performs pre-trade analysis of a stock basket before an order is placed. The user specifies each line’s side, quantity, and execution conditions. Its analysis includes liquidity scoring, estimated spread, participation in daily volume, and estimated market impact; it returns a basket-level and line-level verdict, risk assessment, and suggested execution strategy with plain-language reasoning.

Those estimates can inform a check of execution risk, including the possibility that an order may not fill in full. They are not a prediction of the exact filled quantity. PreTrAIde does not receive, transmit, or execute orders; users trade through their own brokers. Users can later log whether a trade was executed, partially executed, or skipped, along with real fill prices, in PreTrAIde Memory.

Common mistakes

  • Counting ordered shares as owned or sold: Only executed shares change the position.
  • Assuming the remainder will execute: Check its status and time-in-force instructions.
  • Confusing fill rate with execution quality: A 100% fill can still occur at an unfavorable price; a partial fill can involve several prices.
  • Ignoring basket effects: Uneven fills can produce different position weights from those originally planned.

Frequently asked questions

What happens to the rest of a partially filled order?

The unfilled quantity may remain open, expire, or be canceled, depending on the order’s instructions and status. Check the broker’s order record before assuming it can still execute.

Can a limit order be partially filled?

Yes. Available shares may execute at the limit price or better without enough eligible shares to complete the order; the remainder’s status depends on its instructions.

How do you calculate the average price of a partial fill?

Multiply each execution price by its filled share count, add those amounts, then divide by the total filled shares. Do not include unfilled shares in the denominator.

Related terms

Limit order Limit order is an instruction to trade at a specified price or better: a buy cannot execute above its limit, and a sell cannot execute below its limit. Time in force Time in force is an order instruction that specifies how long an order remains eligible for execution before it expires or is canceled. Immediate or cancel (IOC) Immediate or cancel (IOC) is a time-in-force instruction to execute an order immediately to the extent possible and cancel any unfilled quantity rather than leave it open. Fill or kill (FOK) Fill or kill (FOK) is a time-in-force instruction requiring an order to execute in full immediately at its permitted price or better; otherwise, the entire order is canceled. Liquidity Liquidity is the ability to buy or sell an asset promptly in the desired quantity without substantially moving its price or incurring excessive trading costs. Basket trading Basket trading is the execution of trades in multiple stocks as a coordinated group, with each stock specified as a separate line and the combined costs and exposures assessed alongside individual trades.

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.