The moment catches most traders off guard: you place an order for 1,000 shares, the platform confirms the trade, and your position shows 600. Not 1,000. Six hundred. The fill — the actual execution of your order — came back incomplete. Suddenly your average entry price, your risk size, and your profit target are all slightly wrong.

A fill is simply the completion of a buy or sell order in the market. When your broker sends your order to an exchange or liquidity pool, a counterparty on the other side has to agree to take it. If enough volume exists at your price, you get a full fill. If not, you get whatever is available — a partial fill — and the rest of your order either waits, cancels, or re-queues depending on your order type.

CONCEPTA fill is the confirmed execution of your order — the moment risk transfers from intention to reality.
WARNINGPartial fills leave you with an unintended position size — your stop loss may now risk too much or too little capital.
KEY IDEAYour P&L is always calculated on filled size, not intended size — these two numbers are rarely the same in thin markets.

Here is where it gets mechanical. Suppose you want to buy 1,000 shares of a stock at $10.00 using a limit order — meaning you will only pay $10.00 or less. At that moment, only 600 shares are being offered at $10.00 by sellers. You receive a partial fill: 600 shares at $10.00. The remaining 400 sit as an open order. Then the price ticks up to $10.05, and your limit order is never filled. Your total cost is $6,000 for a 600-share position, not the $10,000 you had mentally committed.

1,000 Share Order — Partial Fill at $10.00 600 FILLED @ $10.00 400 UNFILLED price moved up Intended: 1,000 shares P&L calculated on 600 shares only Risk exposure: $6,000 not $10,000

The P&L distortion is real and compounding. If the stock rises to $10.50, your profit on 600 shares is $300 — not the $500 your original plan assumed. Your stop loss, if set at $9.80 to risk $200 on 1,000 shares, now risks only $120. That sounds safer, but it also means your reward-to-risk ratio has shifted without you touching a single setting. Traders who do not account for partial fills often find their system performance diverges from their backtested results over time. Understanding exactly what a fill means — and how partial execution cascades through position sizing — is foundational knowledge covered well across resources like Investopedia's explanation of fills, the mechanics of exchange order types on Wikipedia, and Investopedia's deeper breakdown of partial order execution.

Experienced algorithmic traders build fill-rate assumptions directly into their position-sizing formulas, testing how often partial fills occur in their target instruments before committing real capital.

Your order and your fill are two different things — never confuse the trade you intended with the trade you actually got.

This content is for educational purposes only and does not constitute financial product advice. Past performance is not indicative of future results. Profit Logic Ltd (ACN 688 669 936) accepts no responsibility for errors or omissions in this content or anywhere on this website. Always seek advice from a licensed financial adviser before making investment decisions.