Step 1. Check Hinge Size

3.5" x 3.5": typical size for interior doors.

4" x 4": Typical size for 6' 8" (6/8 or ~80") tall exterior steel and fiberglass doors. Also sometimes used on heavier/taller interior doors.

4.5" x 4.5": typically used on 8' (8/0 or ~96") tall exterior doors or shorter exterior doors that are very heavy (e.g., unusually thick or dense wood doors).

Step 2. Check Hinge Shape

Square: Modern angular look.

1/4" Round: Also known as "quarter round" this is a bit of a hybrid between the modern square look and the traditional 5/8" round style. An easy way to identity 1/4" round is by checking if a dime matches the curvature of a corner.

5/8" Round: The traditional and most common hinge shape. An easy way to identify 5/8" round is by checking if a quarter matches the curvature of a corner.