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Partial Fills: Why Only Part of an Order Executes

For educational purposes only; not investment advice.

Direct answer

Section titled “Direct answer”

A partial fill means an order has executed for fewer shares than its original quantity. If a limit order to buy 1,000 shares receives executions totaling 300, the filled quantity is 300 and the leaves quantity is 700.

What happens to those 700 shares depends on the order’s instructions and the broker or venue. The remainder may continue resting at its limit price, route to another venue, expire at the end of its time in force, or cancel immediately. A partial fill is normally a matching outcome, not evidence that the trading system failed.

Why orders fill in pieces

Section titled “Why orders fill in pieces”

An incoming order can trade only against eligible contra-side interest available under its price and other conditions. A buy limit controls the highest permitted execution price, but does not guarantee quantity. Displayed size can change before the order arrives, hidden or reserve interest may exist, and other orders can have priority.

Venue rules determine ranking, commonly using price and then time among otherwise eligible orders. Routing can divide one parent order across venues, so the broker may report several executions with different timestamps, quantities, and prices.

Time-in-force changes the treatment of the remainder:

  • A day order can rest during the applicable session but generally expires if unfinished when that session ends.
  • A GTC order can remain active subject to broker limits and corporate-action adjustments.
  • An IOC order executes immediately to the extent possible and cancels the unfilled remainder.
  • A FOK order requires the entire quantity to execute immediately or none of it executes.
  • An all-or-none instruction seeks a complete execution but is not necessarily immediate; availability and handling depend on the broker and venue.

Execution example

Section titled “Execution example”

The sell side shows:

Offer price Eligible shares
$50.00 300
$50.03 200
$50.08 900

A limit order to buy 1,000 shares at $50.00 can execute 300 shares and leave 700 unfilled. It cannot pay $50.03 without violating its limit.

If the order is IOC, the remaining 700 cancel. If it is an eligible day order, the remainder may rest at $50.00, but later sellers and queue priority determine whether it fills. If the investor instead uses a market order, the order may sweep all three price levels: 300 at $50.00, 200 at $50.03, and 500 at $50.08, for a volume-weighted average of $50.046 before fees. Full execution is achieved at the cost of price impact.

After the first fill, cash, position size, and exposure have already changed. Any replacement order should be based on the 700 remaining shares, not the original 1,000.

Operational risks and checklist

Section titled “Operational risks and checklist”
  • Check order status, filled quantity, leaves quantity, average execution price, and each execution report before submitting another order.
  • Do not assume displayed depth is reserved for you; it can trade, cancel, update, or have earlier orders ahead of yours.
  • Confirm session eligibility. Extended-hours markets often have less liquidity, wider spreads, and venue-specific order rules.
  • Review commissions and regulatory or venue fees. Multiple fills may or may not create multiple commission charges, depending on the broker’s schedule.
  • Account for odd lots and fractional shares, which can have different display, routing, or execution treatment.
  • Avoid accidental overtrading: cancelling after a fill cancels only the remaining quantity, not completed executions.
  • For a sale, ensure another replacement order does not exceed the shares still available; for a purchase, recalculate cash and margin capacity.
  • Read broker rules for GTC expiration, order modification, corporate actions, and whether changing price or size loses queue priority.

Thinly traded securities, large orders relative to displayed depth, fast markets, news events, and extended hours make partial fills more likely. Breaking an order into smaller pieces can control exposure, but it does not create liquidity and may reveal trading interest.

Common misconceptions

Section titled “Common misconceptions”
  • “A marketable order must fill completely.” It can exhaust immediately available liquidity or be constrained by protections and routing conditions.
  • “A partial fill is slippage.” Partial fill concerns quantity; slippage concerns execution price. Either can occur without the other.
  • “The quote showed enough shares, so they were guaranteed.” Quotes change and may be reached by other orders first.
  • “Cancelling reverses the executed part.” Completed trades remain; cancellation affects only leaves quantity.
  • “A limit order guarantees execution.” It limits price, not whether or how much will trade.
  • “FOK and IOC are the same.” FOK permits no partial execution; IOC permits partial execution and cancels the rest.
Section titled “Related topics”

Authoritative sources

Section titled “Authoritative sources”