Measure the opening width (horizontally) as shown below. Take three measurements, one across the top, middle and bottom of the door (A, B, C). Record the narrowest dimension. Measure the opening height (vertically). Record the shortest dimension.
Doorway Opening Size Guide
1-in. wide x 1-in. deep mounting surface is required. Hinge-side shimming may be required to fit existing opening. Use an appropriate shim thickness to reduce the opening to size noted. Shims are not included - household shim materials may include paint stir sticks or wooden yardsticks.
Most standard storm door sizes are 32" and 36" wide, some models are also available in 30" and 34". Custom storm doors are available in many models, up to 8' high.