A partial fill occurs when your stockbroker cannot buy or sell the full number of shares you listed on an order. This can happen if you are a big money investor, trading large numbers of shares, or if you like to dabble in thinly traded stocks for which there may not be many buyers and sellers. If your broker can't move the requested number of shares on the first attempt, the trade will stay open until the full amount has been reached.
Number of Days to Complete the Transaction
If your stock order is only partially filled, and then later in the day the broker is able to move the rest of the shares, you will only be charged one commission. You pay the full stock trade rate on the first amount of shares purchased or sold, and you will not be charged for the later completion of the order as long as it happens on the same day. If the unfilled portion of the order is completed on a different market day, you will be charged another commission.
You Make Changes to an Order
This rule -- charging you one commission during one trading day for a partially filled order -- can be disrupted if you make changes to your order between partial fills. If you get a partial fill and then you change the parameters of the remaining unfilled order, such as the limit price or number of shares, the broker will charge another commission when the rest of the order is completed. From the broker's point of view, changing the order is the same as submitting a new one, subjecting you to another commission charge.
Order Management Techniques
For individual investors, a common cause of partial fills is the use of limit prices to buy thinly traded stocks. In this case a limit price is usually a good idea, since the bid-ask spread of lightly traded stocks can be much greater than the cost of an extra commission. If you want to avoid the second commission for having your order completed a day later, set up your order as a day limit instead of a good till cancelled order.
Don't Be Fooled Twice
You probably learned about your broker's partial fill commission policy because it happened to you. The broker will not refund a commission because you did not know the policy. Consider this a learning experience about limit orders for certain types of stocks: They can lead to a partial fill and extra commission costs. You can then decide whether you are willing to accept the extra costs or decide to cancel the rest of the order if you do get partially filled on any future order.
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