Partial Fill

What Partial Fill means

A partial fill occurs when the liquidity available at an acceptable price is smaller than the order. The executed portion becomes a position immediately and the treatment of the remainder depends on the time-in-force: an ordinary limit order leaves the balance working, an immediate or cancel order discards it, and a fill or kill order would have prevented the partial fill happening at all. Where the fill is assembled from several price levels, the platform reports a volume-weighted average price.

Partial fills are routine on ECN and multi-bank feeds, where the aggregator walks the available depth from several liquidity providers, and they become more frequent with large size, in thinner crosses, during the Asian session, and around scheduled data when providers withdraw quotes. Brokers using a dealing-desk model more often reject an order outright or requote it instead, because they are quoting a single price rather than passing the order to a book.

The consequences are practical rather than theoretical. A half-filled entry leaves position size, and therefore risk, different from the plan, so any stop distance calculated from an intended notional is now wrong and the trade no longer carries the intended exposure. Automated systems must reconcile the actual filled volume before placing protective orders, since a stop sized for the requested quantity will over-hedge or under-hedge what was actually bought. Traders working by hand should read the executed lot size from the confirmation rather than assume the request was honoured in full.