These flexible, rugged, long-lasting liners are made of 40mil EPDM rubber especially for water garden applications, and produced to the strictest specifications. Liners carry a 20-year manufacturer's warranty. How to Calculate Liner Size: To find the dimensions of the liner you need, take the desired length and width of your pond and add two times the desired maximum depth. You'll need some overlap on the pond banks to secure the liner in place, so add an additional 1-1/2 to 2 feet to these figures. For example: For a finished pond that measures 10 ft long x 15 ft wide x 1-1/2 ft deep, you'll need a 15' x 20' liner. The following Pond Sizes are based on 2' depth with 1' overlap for securing: Liner Size Pond Size 15' x 20' 9' x 14' 10' x 10' 4' x 4' 15' x 25' 9' x 19' 10' x 15' 4' x 9' 20' x 20' 14' x 14' 20' x 25' 14' x 19'