Door hinges are sized according to the size and weight of the door. When renovating your home and exchanging old rusted hinges for new hinges, determining the size of the existing hinge eases the installation of new hinges. A correctly sized replacement hinge will fit into the recess (mortise) created in the door jamb and on the door for the original hinge. Three dimensions are needed to ensure the replacement hinge fits into an existing mortise.
Open the door. Remove the screws from the middle hinge of the door with a Phillips screwdriver. If the hinge sticks due to paint buildup, score along the outside of the hinge with a utility knife and gently pry the hinge from the mortise with a flat-head screwdriver.
Lay the open hinge on a flat surface. Measure the distance from one side of the hinge, across the hinge, to the other side of the hinge with a tape measure to determine the hinge width. The ordering width of a replacement hinge refers to the total open width of the hinge.
Measure the height of the hinge with a tape measure. Standard hinge lengths are 3 1/2, 4, 4 1/2, 5 and 6 inches.
Determine the size of the hinge's corner radius, if the corner is not square. Rest the straightedge along the top edge of the hinge. Do not rest one side of the straightedge on the head of the hinge pin. Measure the distance from the bottom of the straightedge to the bottom of the radius. The measurement will either be 1/4 or 5/8 inch.