What Happens When an Order Only Partly Fills?

You sent a limit order to buy 500 shares at 23.40. The confirmation says 300 filled. The other 200 are still sitting there, marked open.

A partial fill happens whenever there is less size available at your price than you asked for. Sellers took 300 of your 500 at 23.40, then the price moved away, and nobody else was willing to sell at 23.40 for the time being.

What the remainder does

It keeps working. The unfilled 200 shares stay in the book at your limit price, in the same place in the queue, until one of three things happens: more sellers arrive and fill it, you cancel it, or its time in force expires, which for a day order means the end of the regular session and for a good-till-canceled order could mean weeks later.

That last case catches people. A remainder left open for days can fill long after the reason for the trade has gone. It fills quietly, too.

Market orders partly fill less often, because they take whatever prices are available. In a thin stock they can still execute in several pieces at different prices, which shows up as several lines on the execution report.

Your position then carries an average price. Say a market order for 300 fills 100 at 23.40 and 200 at 23.46: the cost is 2,340 + 4,692 = 7,032, and 7,032 / 300 gives an average of 23.44. Use that average, and the true share count, when you measure the distance to your stop.

What it does to attached orders

This is where platforms differ most. If the entry was a bracket order or had a stop and target attached, your platform will size those exits one of two ways:

• To the filled quantity. The stop covers 300 shares now and grows as more fill.
• To the whole order. The stop is for 500 shares even though you hold 300.

The second arrangement can go wrong. If the stop triggers while you hold only 300 shares, a sell order for 500 could, depending on how your account and broker handle it, try to sell more than you own, which in a margin account can open a short position you never intended to take.

Your broker’s help pages for bracket and attached orders should say which rule applies. Test it once with a small order if they do not.

Costs and leftovers

Some brokers charge a commission per execution, others per order, and many charge nothing on stock trades. Where it is per execution, one order filled in three pieces over two days can cost three times what you expected. Check your fee schedule.

Leftover sizes are the other nuisance. Say you cancel with 137 shares filled. Your position is 137 shares.

Your plan was probably built around 500. The risk on 137 shares is about a quarter of what you sized for, so either the trade is now too small to matter or you top it up later at a different price, which changes your average cost and your stop math.

What to do right after a partial fill

Look first. Then decide on purpose.

1. Open the order detail. How many filled, how many remain, what is the time in force?
2. Check the position. Does the share count match the fills?
3. Check the attached exits. Are they sized to what you hold?
4. Decide on the remainder. Still want it? Leave it. The setup changed? Cancel it.
5. Rerun the risk. Put the actual share count and price into the trade risk worksheet so the stop you are holding matches the position you really have.

If the remainder never filled at all, the reasons are the same ones behind any unfilled limit, covered in why a limit order does not fill when the price touches it.