Most Accurate Way to Determine Size: Look at Number of Screws on Top of Flap 6 Screws-Extra Large Flap 5 Screws-Large Flap 4 Screws-Medium Flap 3 Screws-Small Flap Replacement flaps for your pet doors are a quick way to increase energy efficiency and update a well-worn pet door. Its easy to replace the flaps-simply take out the screws from the inside frame of the pet door-remove the bar and existing flap-center the new flap-put the bar back in and tighten the screws. Single flap pet door catching in the strong winds? Add this as a second flap. It adds weight eliminating the problem. For the Extreme Weather Door: Uses 2 flaps-one on either side of the yellow insulated flap. If you need to replace the middle flap, you will need to purchase an Extreme Weather Door Flap Kit. The Wall Entry Aluminum Pet Door uses 2 flaps on either side of the tunnel for extra energy efficiency. One flap is in the inner opening and one is for the outer opening. This Flap Fits These Doors Freedom Aluminum Pet Door Plastic Pet Door Extreme Weather Door (yellow insulated flap sold separately) Freedom Patio Panel Pet Door Wall Entry Aluminum Pet Door (new model) see picture at bottom Freedom Patio Panel Pet Doors Larson Storm Door with Freedom Aluminum Pet Door (new model) Features Soft, tinted vinyl single flap with magnetic strip to provide a seal against weather and bugs Easy installation that you can do yourself Box Includes One Replacement Door Flap