How to measure interior door

1. Remove the door slab from its hinges and gently lay it on a pair of sawhorses or the floor.

2. Sketch a tall rectangle on a piece of paper to represent the door. Add a circle along one long edge representing the lockset -- the term for the handle or knob -- and three small rectangles to indicate the hinge mortises along the other long edge.

3. Measure and mark the length and width of the door beside the respective lines representing these dimensions on the piece of paper; continue to mark all measurements down as you take them.

5. Determine the distance from the top of the door to the top of each hinge. Gauge the hinge size, typically either 3 1/2, 4 or 4 1/2 inches, as measured including the overall size of both hinges while lying flat. Mark down these measurements as well. Measure the door thickness and the hinge reveal, which is the distance between the end of the hinge and the edge of the door. Record the wall thickness, typically around 4 1/2 inches if you have 2-by-4 studs covered by drywall or 6 1/2 inches if you have 2-by-6 studs and drywall.

6. Pry off with a pry bar the door casing, gently inserting the bar between the casing and the wall, to expose the framing on the sides and top of the door, which are called the jack studs and the header respectively.

7. Record the smallest width measurement from jack stud to jack stud as taken at the top, middle and bottom of the door. Record the smallest of two measurements from the threshold, carpet or flooring to the header, taken near the left and the right jack studs. These are the dimensions of the rough opening, which a lumberyard can use to order a prehung door with its built-in frame to fit.