This black PVC Pond liner carries a lifetime guarantee. It is more flexible than the 20 year type and will mould to your pond well. It is .05mm thick. It comes in standard widths of 12 16 20 and 26 foot. For custom sizes choose one of the standard widths above (12, 16, 20 and 26 foot wide) and multiply by the length you need. e.g. if you want a 12ft by 15 ft liner,( Enter 12 x 17 in the Size Box ).You now need the square feet (12x17=204) The 204 would then go in the Quantity Box.