Pond liners keep water from seeping into the soil. Even in heavy clay soils, a liner is necessary. You can buy rigid pond liners in a variety of shapes. These are durable and may include built-in waterfalls. Many are quite small. If you want a larger pool or would like to design your own shape, consider using a polyvinyl chloride (PVC) liner. Use a liner specifically designed for pools. While other plastics initially may be cheaper, many are not resistant to ultraviolet light and will break down quickly. Some plastics may also be toxic to fish. Liners also come in different thicknesses. A thicker liner tends to be more resistant to punctures. While expensive and requiring more expertise to install, cement is also an option as a pool liner.
If you use PVC, you will need to get a liner large enough for your pool. To determine how large a piece you will need, determine the maximum width, length, and depth of your pond. Multiply the maximum depth by 3. Then add this number to both the length and width. This will allow enough plastic to be securely held down around all pond edges.
Courtesy of the US Department of Agriculture, Natural Resources Conservation Service.