How the liner size is worked out
The liner length is the pond length, plus the depth twice, plus the overlap twice. The width works the same way.
A pond 3 metres by 2 metres and 0.6 metres deep, with a 0.3 metre overlap, needs a liner 4.8 metres by 3.8 metres.
The overlap is not slack. It lets the liner run under the edging and be held down, and it allows for a slight misjudgement in the shape.
Worked example
A 3 by 2 metre pond at 0.6 metres deep with a 0.3 metre overlap needs 4.8 by 3.8 metres, which is 18.24 square metres of liner.
Add a shelf and the depth figure has to be the deepest point, not the average, because the liner still has to reach the bottom.
Choosing and fitting a liner
Butyl rubber lasts longest and tolerates movement; flexible PVC is cheaper and shorter lived. Both are sold in fixed widths, so a size that fits a standard roll avoids a joint.
A joint in a liner is a leak waiting to happen, so it is worth choosing a pond shape that fits a stock width rather than buying two pieces.
Underlay goes beneath the liner to protect it from stones and roots. It is cheaper than the liner and it is the difference between a ten year pond and a thirty year one.
Limits of this calculator
It assumes a simple shape with vertical or gently sloping sides and a flat bottom. A stepped or irregular pond needs the liner measured around the profile.
It does not include underlay, edging, adhesive or the cost of a jointed liner.
It does not calculate the volume, which is a separate figure and the one you need for treatment doses.
For how much water the pond holds, use the pond volume calculator.
Inputs for the pond liner calculator
Pond length
Length of the pond at its longest point, in metres.
Pond width
Width at the widest point, in metres.
Maximum depth
Deepest point in metres. The liner needs this at both sides.
Overlap at each edge
Extra liner to tuck under the edging, 0.3 m at each edge is usual.
CALCULATOR QUESTIONS
Pond liner calculator questions
How much overlap do I need?
About 0.3 m at each edge, enough to run under the edging and be held down.
Should I use the average or the deepest depth?
The deepest point. The liner has to reach the bottom everywhere.
Butyl or PVC?
Butyl lasts longer and tolerates movement; PVC is cheaper and thinner. Underlay matters more than the choice.
Can I join two pieces?
You can, but a joint is the most likely place to leak. Choose a shape that fits a stock width if you can.