Partial Fill
Definition
A partial fill occurs when the broker can only execute part of the requested order volume — for example, requesting 5 lots but only filling 3 at the requested price because liquidity is thin. Common during fast markets, news events, and on illiquid instruments. EAs must handle partial fills explicitly or risk inconsistent position state.
In-depth: Partial Fill
Partial fills occur when the requested order size exceeds the available liquidity at the requested price. Causes: 1. Fast markets — by the time the order arrives, only some liquidity is available at the original price; remaining liquidity is at worse prices 2. Low-liquidity windows — Asian session opens, around major news, on exotic instruments 3. Large orders relative to typical instrument liquidity — a 50-lot order on EURUSD is unlikely to partial-fill; the same order on USDZAR (less liquid) almost certainly will 4. Broker-specific position-sizing limits — some brokers enforce maximum order sizes per request
Broker-model behaviour: - ECN brokers: partial fills are common and explicit. The order is filled to the depth of book available at the requested price, with the remainder either filled at the next-best price (slippage) or rejected (depending on order type — Fill or Kill vs Immediate or Cancel vs Limit). - Dealing-desk brokers: behaviour varies. Some honour partial fills similar to ECN; others reject the entire order rather than partial-fill.