Measure the width of the door. Run a tape measure along your door from the left corner to right corner, and record this number. It is important that you only measure the door. Do not include any other elements, such as weather-stripping.
Determine the height of the door. Run your tape measure along your door from the top corner to the bottom corner, and write this number down. You may need to use a chair and/or ask a friend to help you. Once again, measure only the door itself and no other elements, such as a door sweep.
Figure out the thickness of the door. Hold a tape measure to the edge of the door and record its thickness. Measure this edge on the door frame (known as the jamb), as well. These numbers should be close to the same, but it can be helpful to know both of them.
Measure the height and width of the framed door space. Just to be safe, measure the area where your door will go, too. Record the height and width of the door frame space. This will help ensure that you select the exact right replacement door.