Open the door and go outside. Measure the width inside the doorjamb from side to side including the trim on each side. Take one width measurement at the top, one in the middle and one at the bottom.
Write all three measurements down on paper with a pen marking which measurements they are clearly. The top, middle and bottom measurements are not always the same on a door width.
Measure the width of the door opening from the inside of the house in the same manner. Write down all three measurements accordingly.
Measure the height of the door opening from the bottom of the doorsill to the header at the top of the door opening.
Measure the thickness of the actual old door.
Things You Will Need
- Measuring tape
- Older doors that need replacing may have changes in their original size from weathering. Doors that warp or do not have adequate sealant for protection can either shrink or swell in size.
- If the replacement door will be a pre-hung door in a casing, remove the trim around the door on the inside and measure horizontally and vertically from the framing on each side to the other side. This process includes measurements for the door casing so that it will fit in the area provided with little additional framing.