Pond Liner Calculator

The liner is not the size of the pond. It has to go down one side, across the bottom, up the other side, and far enough over the bank to be held down — so the length is the pond's length plus twice the depth plus twice the overlap. Buy the pond's dimensions and the liner stops halfway up the far bank.

Pond length in metres

The longest dimension of the hole.

Pond width in metres

Across the hole at its widest.

Depth in metres

At the deepest point.

Overlap at the edge, in metres

How far the liner goes over the bank. 0.3 is usual.

Shape

Pump turnovers an hour

Once an hour is the usual guidance for a garden pond.

4.8m × 3.8m of liner

18.2m² in all. The pond holds about 2700 litres — 3600 would be the figure if the sides were vertical — so the pump needs to move 2700 litres an hour.

✓ The liner reduces back to the pond it covers — 4.8 by 3.8m of liner, less two depths of 0.6m and two overlaps of 0.3m, gives back 3 by 2m
✓ The liner is bigger than the pond in both directions — it has to go down one side, across the bottom, up the other and over both edges
✓ The volume allows for the shape — 2700 litres, which is 75% of the 3600 litres the box would hold — sloping sides hold less than a tank of the same size
✓ The volume never exceeds the box the pond sits in — a pond dug to 3 by 2 by 0.6m cannot hold more than 3600 litres
✓ The pump moves the whole pond as often as you asked — 2700 litres an hour turns 2700 litres over 1 times

What this generator does

Adds twice the depth and twice the overlap to each of the pond's dimensions for the liner, and applies a shape factor to the box volume for the water it really holds.

How to use this tool

Measure the hole: length, width and the depth at the deepest point.
Choose how far the liner should go over the edge.
Pick the shape, which decides how much water it really holds.
Read the liner size and the litres an hour the pump must move.

Understanding the controls

Pond length and width: The hole at its longest and widest, in metres. An irregular pond is measured at its extremes, since the liner is a rectangle.
Depth: At the deepest point. It is added twice to each dimension, because the liner goes down one side and up the other.
Overlap at the edge: How far the liner extends over the bank to be weighted down or trimmed in. Thirty centimetres is usual; less and it can pull in as the pond fills.
Shape: How the sides are cut. A rectangular pond with steep sides holds nearly its full box; sloping sides, planting shelves and a kidney shape all hold considerably less.
Pump turnovers an hour: How many times an hour the whole pond should pass through the pump. Once an hour is the usual guidance for a garden pond, more with fish.

Common use cases

Buying a liner for a pond you have just dug
Working out the real volume before choosing a pump
Seeing how much bigger the liner gets with a deeper pond
Checking a supplier's suggested size against the arithmetic
Sizing underlay, which is the same size as the liner

How this generator works

The liner is the pond's length plus two depths plus two overlaps, and likewise across, which is checked by taking it back apart: subtracting the depths and overlaps must give back the pond you typed. The volume is the box the pond sits in, reduced by a factor for the shape — a quarter less for sloping sides, and more again for a kidney shape or planting shelves — because pretending a pond is a tank overstates what it holds and oversizes the pump. That is checked too: the volume can never exceed the box, and the pump flow must be the volume times the turnovers you asked for.

Randomness and fairness

Nothing is random. The same hole always needs the same liner.

For how randomness is produced across the whole site, see how Generate Random works.

Limitations and good to know

Shape factors are rules of thumb, and a pond dug by hand rarely matches any of them exactly.
Liner is sold in fixed widths, so you will buy the next size up from the figure here.
Underlay is the same size as the liner, and a stony hole wants it.
Pump sizing also depends on head height and on any filter or waterfall, which are not modelled.

Privacy and your data

The liner size is worked out in your browser. No pond measurements are transmitted or stored.