This is the size of the gap but not the size of the extra piece you are going to need.
Extending the length of a door.
Make sure the door is closed and take out the measuring tape.
For that you need to subtract at least a 1 8th of an inch for clearance.
Measure the distance between the bottom of the door and the floor.
How to fix a too short door.
The lines should extend from the floor to the ceiling.
Check all the way across in case one area is uneven in measurement.
The filler piece will be 1 1 2 in.
Up from the floor at the high spot.
Measure where the door meets the floor when it is closed.
To seal out weather insects and dirt it is important to properly size your door.
Make a light pencil mark on the door 2 in.
Clearance under the door.
When you can see daylight under your door it is not the right size and needs to be.
Any way to extend the length of the door replacing all the doors in the house would be to expensive.
Carefully cut off the door at the pencil mark to get a clean square edge.
Remove the door by pulling out the hinge pins and lay it on a pair of padded sawhorses.
If there is carpet flooring remove a 1 4 inch space from the total measurement for easement from the carpet to the door bottom.
Place it in the center of the door at the bottom.
Years ago we removed all the carpeting in the house.