When measuring your doorway you need to measure from the top of the doorway to the floor, including the width as shown in the image.

This is the measurements used to calculate the correct door size to cover the opening properly.

If your opening has a jamb with door stop, include this in your measurement as if it is part of the opening.

It is important to support the measurements with a photo, to ensure there is no obstructions when the door is installed.