Stapeley Water Gardens

The simplest method, assuming that the pool is constructed with inward sloping sides (7.5cm (3 inches) for every 23 cm (9 inches) down), is to add twice the pools maximum depth to the pools maximum length to arrive at the pond liner length, and add twice the pools maximum depth to the maximum width to arrive at the pond liners width. The will guarantee adequate overlap.
If the pool is dug, you could also measure a cord that follows the shell of the pool along its longest length and its widest width, but remember to add at least 30cm (12 inches) to both measurements to include overlap.

For example, if the proposed pool, with sloping sides, were to measure a maximum of 2.75 metres x 1.5 meters x 0.5 meters (in any shape) the pond liner required would be 3.75 metres by 2.5 metres.