Ethylene Propylene Diene Monomer, EPDM, is a flexible material that can easily conform to any configurations you choose. When selecting a liner, make sure to get one that is meant for ponds. Unfortunately, pool liners or roofing material often get experimented with and pond owners learn a hard lesson. These materials often are constructed with chemicals to reduce algae or mold that can be harmful to fish and they may tear easily. Instead, stick with LifeGuard 45 mil EPDM Pond Liner. It is safe for fish, plants, and wildlife and is the ideal product to give your feature a natural look. Start by getting the measurements for the longest, widest, and deepest points of your pond or pond to be. Next punch these numbers into the calculator: (Length + (2 x Depth + 1)) by (Width + (2 x Depth + 1)).