A replacement hinge is easiest to match before the old one is removed. Small differences in corner shape, size, screw spacing, and finish can keep a new hinge from sitting flat or aligning with the existing mortise.
Record the important dimensions
Measure the hinge leaf from top to bottom and from the barrel to the outside edge. Note the screw-hole pattern, the number of holes, the thickness of each leaf, and whether the hinge is a standard residential size.
Check the corner and barrel
Look at the leaf corners. They may be square or rounded, and the radius matters when the hinge sits in a routed mortise. Count the knuckles and compare the barrel diameter with the replacement listing.
Match handing and finish
Most butt hinges are not handed, but special hinges can be. Photograph the old hinge, record the door thickness, and match the finish if appearance matters. Use the same screw length unless the replacement instructions specify otherwise.
Test before final tightening
Hold the new hinge in the mortise and confirm that the leaves sit flush. Drive one screw in each leaf first, test the door swing, and then install the remaining screws. Stop if the hinge binds or the wood is splitting.