Flexible liners — EPDM rubber, butyl and PVC — make it possible to build a pond of almost any shape. The one thing they can’t do is stretch. Order a liner that is too small and you either dig a smaller pond or buy a second liner; order one that is generously sized and you trim the excess. This guide explains the standard formula and the decisions behind it.
The formula
liner length = maximum length + (2 × maximum depth) + (2 × overlap)liner width = maximum width + (2 × maximum depth) + (2 × overlap)
The liner crosses the pond, runs down one side and up the other — that’s the 2 × depth — and lies flat beyond the edge on both sides, which is the 2 × overlap. Do the length and width separately.
Worked examples
| Pond | Depth | Overlap | Liner |
|---|---|---|---|
| 8 × 6 ft | 2 ft | 1 ft | 14 × 12 ft |
| 10 × 8 ft | 3 ft | 1 ft | 18 × 16 ft |
| 15 × 10 ft | 3 ft | 1.5 ft | 24 × 19 ft |
| 3 × 2 m | 0.8 m | 0.3 m | 5.2 × 4.2 m |
| 4 × 3 m | 1.2 m | 0.5 m | 7.4 × 6.4 m |
Check one: 10 + (2 × 3) + (2 × 1) = 18 ft long; 8 + 6 + 2 = 16 ft wide. The pond liner size chart covers more sizes and depths.
How much overlap?
The overlap anchors the liner. It sits under turf, paving or coping stones and is the main thing that stops the liner slipping into the pond as it fills. Common recommendations range from about 30 cm (1 ft) to 60 cm (2 ft) per edge.
- 1 ft (30 cm): firm clay soil, turf edge or light edging.
- 1.5–2 ft (45–60 cm): sandy or loose soil, heavy edging stones, or a capillary break (the liner is turned up behind the edging to stop the soil wicking water out).
Overlap is the one place to be generous. Trimming 6 inches off a finished edge takes a minute; finding 6 inches you don’t have is impossible.
Measuring the hole
Size the liner from the hole, not from your plan — ponds always end up a little different once you start digging. Measure the longest length and widest width at the top of the hole, and the deepest point. For a curved or irregular pond, still use the longest and widest straight-line measurements: the liner is a rectangle, and the parts beyond a curved edge become folds or trimmings.
Shelves don’t add liner
Marginal shelves are ledges, usually 9–12 in (23–30 cm) deep and wide, for baskets of marginal plants. It is tempting to add their surfaces to the calculation, but you don’t need to: going down a stepped side the liner travels the same total vertical distance as a straight side (the full depth), plus the same horizontal distance (the width of the pond). The formula already counts both. What shelves do need is careful folding at corners.
Sloped sides need slightly less
A pond with evenly sloping sides — a bowl — has a slanted side longer than the depth but a bottom smaller than the top. The precise liner length is bottom length + 2 × slant length + 2 × overlap, where slant length = √(depth² + ((top − bottom) ÷ 2)²). This is always a little smaller than the basic formula, so the basic formula is safe. The Sloped sides option in the pond volume calculator works it out exactly.
Underlay
Liners puncture from below — stones, roots, the corner of a brick. A geotextile underlay protects the liner and is cheap insurance. Buy it the same size as the liner (some builders also put a layer on top of the liner where rocks will sit). On stony ground, add an inch of soft sand on the floor of the hole under the underlay.
Choosing the material
- EPDM rubber: flexible, UV-stable, fish-safe grades widely available; the default choice for most ponds.
- Butyl rubber: similar to EPDM, long-lived, common in the UK.
- PVC: cheaper and lighter, stiffer in cold weather and shorter-lived in sunlight. Best for small, shaded ponds.
- RPE / reinforced polyethylene: very light and strong for its weight, but less flexible to fold into tight shapes.
Whatever you choose, check it is labelled fish-safe. Roofing membranes can contain additives that harm fish.
Rounding to stock sizes
Liners are sold in fixed widths (for example 5, 10, 15, 20 ft or 2, 4, 6, 8 m) and cut to length from the roll. Round your width up to the next stock width and your length up to the next foot or half-metre. A slightly larger liner gives you more overlap — which, as above, is never a problem.
A quick volume check
While you have the measurements, work out the volume too: liner size tells you what to buy, volume tells you which pump, filter and UV to buy. The pond volume calculator does both in one go.