Measure the diameter of the area, and the depth--you'll want to do a diagram, with depth measurements about every 6 inches in a straight line across the pond so you get the shallower areas and the deeper areas. Take that to the store you're buying the liner at, and they will be able to help you get the right amount. You should end up with a bit more than is needed to just cover the area, so you can have the liner extend a bit past the water area.