How Liner Sizing Works
A pond liner is one continuous sheet, and it has to make the whole journey: down the near wall, across the bottom, up the far wall — in both directions — and still leave material at the rim to anchor. So each dimension of the sheet is the pond's dimension plus twice the depth plus twice the overlap. Depth here means the maximum depth, because the sheet doesn't know where the deep end is; it has to be able to reach the deepest point from either side.
Measure the pond at its widest points in both directions, taking in any shelves, marginal planting terraces and beach edges you've planned. The calculator models the pond as a simple box with vertical sides, which real ponds never are: shelves step in and out, and banks slope. That geometry takes up slack in the sheet — the step over a shelf edge runs the liner a little further than the straight-line depth suggests — which is one reason the underlay carries 10% extra and why sizing up a roll width never hurts. For a heavily shelfed or freeform design, add a margin on purpose rather than by accident.
The overlap is the anchor. A typical minimum is 30 cm (12 in) per side, run out flat beyond the pond edge and fixed — under edging stones, into a turf-covered trench, or beneath a concrete collar. Go wider, 45 cm (18 in) or more, when the edging is heavy stone, when you want a gentle beach exit for wildlife, or when the surrounding ground is soft and likely to settle. Under-sized overlap is a common failure: the liner slips back into the pond under water pressure and the level starts dropping.
EPDM and PVC Liners
Two materials dominate flexible pond liners. EPDM rubber is typically the more durable choice: it stays flexible in cold weather, resists UV and root attack, and commonly carries long service-life expectations from manufacturers. PVC is lighter and easier to handle, which matters on small ponds and solo installs, but it typically stiffens with age and cold, and its service life is generally shorter. Thickness matters in both families — commonly 0.5 mm (20 mil) PVC at the light end and 1.0–1.5 mm EPDM for garden ponds, heavier for big or rocky sites. Whatever the material, the manufacturer's spec for your pond size and fish load outranks any general guidance; the pond liner guide compares them properly.
Why Underlay Matters
Underlay is the geotextile layer that goes between the subsoil and the liner, and skipping it is the classic false economy — a punctured liner means draining the pond and starting again. Stones, sharp gravel, roots and even old brick fragments in the subsoil all work on the liner from below, quietly, for years. Underlay also runs up the sides of the hole, not just under the base, because roots attack from any angle. The calculator sizes it at liner area + 10%: the extra allows for wrapping the shelf edges and overlaps where sections join.
What the Results Mean
- Liner Length / Width: The sheet dimensions to order — rounded up to the nearest standard roll width or cut length your supplier sells.
- Liner Area: Length × width of the sheet. Useful for comparing roll widths and for offcut planning.
- Underlay Area: Liner area + 10%, the usual shaping and joining allowance.
Liners are commonly cut from rolls of standard widths, so you buy the length at the next roll width up from your calculated width — and widths vary by supplier. Phone before you dig, not after: the roll widths on offer may shape how you size the hole, and an extra half-metre of width is less grief than a seam in the middle of a pond.