When placing a market order, you are guaranteed an order fill as long as there is a bid price (greater than 0.00) when selling or an ask price when buying. This rule applies to equities, equity options, futures, and options on futures. However, there are pros and cons to market orders.
When selecting Market as your order type, MKT will appear in the price field, indicating that your order will fill at the market. Additionally, Market orders are not available for any multi-leg options orders (verticals, iron condors, calendars, etc.).
To learn more about limit orders instead, click here.
To learn how to set a default order type (market or limit) for single-leg options orders or stock, please click here.
Pros: When routing a market order you are guaranteed that your order will fill in its entirety or partially (when there is no market). For example, if you route an order to buy 500 shares of XYZ @ MKT and the current ask price is $20, then your entire order will fill.
Cons: When routing a market order you have no control over what price you are filled at, whether you are buying or selling. When routing a market order, you are essentially stating that you are willing to get filled at any price. Using the same example, routing an order to buy 500 shares of XYZ @ MKT although the ask price at the time of order entry was $20 does not mean entire the order fills at $20, although it is possible. As a result, your order may partially fill at $20, but the remaining lot can fill at a price above or below $20 depending on market movement.
However, there is one exception to market orders when trading futures. To learn more, click here.