Replacing doors is one of the most dramatic improvements you can make to the appearance and security of your home. Getting proper measurements is essential to purchasing and installing replacement doors. Knowing which measurements matter and how to use them is the key. Height and width are obvious must haves. For exterior doors, you will also need to know the depth of the frame, which is dependent on the framing of the house. You can get all the measurements you need from the original door with a standard tape measure.
Measure the door from top to bottom at the hinge side of the door. Measure only the door slab itself from inside to inside of the frame. In newer homes, building code dictates that all passage doors be at least 80 inches tall, and most are exactly that. Standard replacement doors will all be cut to 80 inches in height. For taller or shorter doors, a custom door and frame will need to be ordered.
Check the width of the door at the top and bottom corners and at the knob. The widest measurement is the original width of the door; smaller measurements have been planed or trimmed over the course of the door's life. Typical door widths are 24, 28, 30, 32 and 36 inches. Most other sizes will require a custom order.
Measure the depth of the door frame. This measurement should be taken from the inside edge of the door casing, or trim, on the inside of the door to the inside of the door casing on the outside of the door casing. Most interior doors will have a frame 5 1/2 inches wide, indicating a two-by-four wall frame. Exterior doors frequently have frames 7 inches wide, indicating a two-by-six frame. Pre-hung frames are available in both sizes.
Assess the height and width of the door threshold, which is the plate at the bottom of the door frame, on exterior doors. The width of the threshold will need to be matched for a proper fit. A perfect match is not typically necessary. Anything within 1/2 inch is sufficient.
Make note of the hinge side of the door as you stand with the door open toward you. This is called the hand of the door. Purchase your replacement doors with the same hand as the original to make installation as simple as possible.