Why partial fills happen to retail traders

[Shadow Trader]

A market order for 5 lots is sent. The broker fills 3.2 lots at one price and 1.8 at another, or only fills 3.2 and cancels the rest.

Partial fills occur when the available liquidity at the best price is smaller than the order. For small retail sizes, 0.1 to 1 lot, this rarely happens in majors. On exotic pairs, in thin hours or during news, it becomes more common.

The effect on a scalper is double. The average entry price shifts from the intended level, and the risk now applies to a different size than planned, so stops and targets calculated on the full size are mismatched.

Brokers handle it differently. Some offer fill-or-kill options, which cancel the order completely if it cannot be filled in full. Others default to partial execution.

Knowing the setting on the account avoids unpleasant surprises, particularly when scaling position size up.

Has anyone encountered partial fills, and what settings helped?