To work out the size of the pond liner you will need to fit your excavated pond, you need to know the following dimensions of the pond in metres;

• The length of the pond at it longest point (L),

• The width of the pond at its widest (W) and

• The depth of the pond at the deepest point (max. D).

If you have not measured your pond dimensions in metres, it is usually easier to convert the taken measurements before you attempt to work out the liner size as all the liners are sold in metre quantities.If the pond is not perfectly square, or is an odd shape, you should use the largest measurements for length and width to ensure complete coverage. The liners length and width are calculated independently of each other.The length of the liner needed should be the length of the pond (L), plus twice the depth (D), plus 1 metre (to give a suitable overlap of 50cm on each side of the pond). The same calculation is used for the width of the liner, substituting the length of the pond for the width (W).

The formula for calculating liner size is;

Liner length = L + (2 x D) + 1Liner width = W + (2 x D) + 1

E.g. a pond is 5 metres long, 3 metres wide and has a maximum depth of 1 metre;Liner length = L + (2 x D) + 1

= 5 (2 x 1) + 1

= 5 + 2 + 1

= 8 metresLiner width = W + (2 x D) + 1

= 3 + (2 x 1) + 1

= 3 + 2 + 1= 6 metresTherefore, the pond will need an 8m x 6m liner.For working out the amount of underlay required, in metres squared, you need to have first figured out your required liner size. You simply multiply the liner’s length by its width to calculate the underlay.E.g. the liner size is calculated at 8m x 6m;Underlay required = L x W= 8 x 6= 48 m²Therefore, the pond will require 48 m² of underlay.