It is rather exceptional, but it can certainly happen occasionally: an order is partially filled and/or filled over multiple days.
How does it happen?
Essential in the pricing of instruments listed on the stock exchange is the principle of supply and demand. If you wish to buy, there must be a seller willing to sell their shares at an agreed price. If you wish to sell, there must be a buyer who wants to take over your shares at a desired price. This pricing process is continuously taking place and can be followed in the order book through the bid and ask prices.
Suppose you wish to buy 500 shares at a maximum of 52.60 EUR. In the image below you will see an order book. You will immediately get execution for the 330 shares for which someone has a sell limit of 52.60 EUR. The remaining 170 shares you wish to buy will remain on hold on the bid side. If you do not get execution that day, and the duration of your order is longer than one day, it may be that you get execution for your remaining requested shares tomorrow, later, or even not at all.
Below you see an example of such an order book, which you can also see when you have a level 2 subscription for live prices.
I get a transaction fee for every day my order is (partially) filled. Is that correct?
You indeed pay transaction fees for each day that your order gets (partially) filled. If your order is executed in different parts on the same day, there will be one transaction fee.
How can I avoid partial fills over several days?
For many large stocks, partial execution will rarely occur. The less liquid a stock is, the greater the chance that an order may take several days, and sometimes even weeks or months, to be executed. Naturally, it can be helpful to check the bid and ask prices and take into account some tips & tricks to avoid partial executions.
1. Change the limit of your order during the day
When you work with a limit order, you can change the limit within your order for many exchanges. Based on what you see in the order book, you might choose to adjust your limit to get an order executed within the day. When you adjust your limit, of course, you pay more for the shares (when buying) or receive less (when selling). It is always up to you to judge whether adjusting a limit is desirable. In the example below, we want to adjust our limit from 50 euros to a higher limit from the order screen and the 'open orders'.
2. Choose a market order
With a market order, you will (usually) get an immediate execution for all your shares at the price that is then available in the order book. It is important to note that with a market order, you agree to the price offered or asked by the counterparty. Even with a market order, especially with less liquid instruments, there is no guarantee the order will be fully executed at once and it is always advisable to check what price you would pay/receive to avoid surprises.
3. Choose a day order
A day order speaks for itself: it is valid for one day. To avoid multiple fees for orders on different days, you can also opt for a day order. A day order can still be partially executed. For instance, if there are only a few sellers who want to sell their securities at your limit, then only the part for which a corresponding price was found will be executed.
You can also cancel your order if you see a partial fill and do not want to wait for the remaining order to be filled.