These black Aqualast - Firestone rubber liners carry a lifetime guarantee and are 1.00 mm thick and are very flexible easily fitting round those unusual shapes. They come in standard EVEN NUMBERED widths from 10 feet to 30 feet and then 36. 40 and 50 feet. The length can be up to 100 feet. Once you have decided on the measurements (This is decided by adding twice the depth to the length and width) i.e. a pond 10 feet by 16 feet with a depth of 2 feet will need a liner 14 (10+4) X 20 (16+4). You then multiply these two figures together to get the area in square feet. In this case 14 x 20. = 280 square feet.Enter dimensions of the liner (14 x 20 for our example) in the top box then the number of square feet (280 for our example) in the " Quantity " box below it.