When measuring for a storm door with screen replacement, follow these steps to ensure a perfect fit: Width Measurement: Measure the opening width horizontally across the top, middle, and bottom of the door (points A, B, and C). Record the narrowest dimension among these measurements. Height Measurement: Measure the opening height vertically on the left, middle, and right sides of the existing door. Record the shortest dimension among these measurements. Mounting Surface Requirements: Ensure you have a 1-inch wide x 1-inch deep mounting surface for the storm door. Hinge-side shimming may be necessary to fit the existing opening. You can use materials like paint stir sticks or wooden yardsticks as shims. Standard vs. Custom Sizes: Most standard storm doors are 32 inches and 36 inches wide, but some models are also available in 30 inches and 34 inches. Custom storm doors can be ordered in various models, including heights up to 8 feet. Remember to take accurate measurements and consult your nearest LARSON dealer with your recorded dimensions before purchasing your new storm door.