If the average desk height is between 28 - 30 inches then that would give you around 27 - 29 inches of height under the desk most likely. However, you may need to account for a keyboard tray if the desk has one. You should also be sure you are getting a desk that is right for your height. If you are tall then a desk of that height may not be good to start with and you might need to look into a desk that is height adjustable.