Why the formula uses depth twice
A flexible pond liner is a single flat sheet draped into the excavation, so in each direction it must be big enough to travel down one side, across the bottom and up the other side — with spare material at every edge to anchor under stone, turf or an edging trench. That geometry is exactly why depth appears twice in the formula: the sheet descends on one side and climbs back up on the other, so each direction needs two full depths added, not one.
The rule deliberately uses the maximum depth even though the sheet drapes over shallower shelves, because the liner must reach the deepest point without stretching; folds simply take up the slack elsewhere over shallower zones. Liner manufacturers publish this same formula in their sizing guides, typically recommending an overlap of around 30 cm (about a foot) per edge for anchoring, with more where the surround is uneven or an edging detail demands it.
The formula
In each direction the liner must cover the pond dimension plus a full depth of descent on each of the two opposite sides, plus the overlap allowance at each edge. The liner area is simply the product of the two computed dimensions, which is how liner is priced per square meter.
- Liner length = Pond length + 2 × Max depth + 2 × Overlap
- Liner width = Pond width + 2 × Max depth + 2 × Overlap
- Liner area = Liner length × Liner width
Worked example: a 4 m × 3 m pond, 1 m deep
With a 30 cm (0.3 m) overlap per edge: liner length = 4 + 2×1 + 2×0.3 = 6.6 m, and liner width = 3 + 2×1 + 2×0.3 = 5.6 m. That gives a total liner area of 6.6 × 5.6 = 36.96 m² — round up to the next stock sheet size the supplier cuts, since the computed dimensions are minimums, never a figure to round down from.
| Dimension | Calculation | Result |
|---|---|---|
| Liner length | 4 + 2(1) + 2(0.3) | 6.6 m |
| Liner width | 3 + 2(1) + 2(0.3) | 5.6 m |
| Liner area | 6.6 × 5.6 | 36.96 m² |
Getting the sizing wrong
The two most common sizing mistakes both leave the liner short: using average depth instead of maximum depth (the liner must reach the deepest point, and average-depth sizing comes up short on both sides), and adding depth once instead of twice per direction (the sheet descends and climbs back up, so each direction genuinely needs two full depths). Skipping the overlap allowance is the third — it leaves nothing to anchor under the edging stones, letting the liner slip into the pond over time.
Standard installation practice is to order underlay (geotextile or manufacturer-specified padding) at the same dimensions as the liner — it protects the sheet from stones and roots and is required by most liner warranties — and to fill the pond first, letting the liner settle into the excavation, before trimming the edges to their final size.
자주 묻는 질문
What size liner do I need for a 4 by 3 meter pond, 1 meter deep?
With a standard 30 cm overlap per edge: (4 + 2 + 0.6) × (3 + 2 + 0.6) = 6.6 m × 5.6 m, an area of about 37 m². Round up to the nearest stock sheet size the supplier offers.
How much overlap should a pond liner have at the edges?
Liner manufacturers commonly recommend around 30 cm (about 12 in) per edge as the anchoring allowance, tucked under edging stones, turf or into an anchor trench. Uneven surrounds or deep anchor trenches justify more.
Why is liner size based on maximum depth, not average depth?
The sheet must physically reach the deepest point of the excavation without stretching. Over shelves and shallower zones the surplus material simply folds — flexible liners are installed with folds as a matter of course — so sizing on anything less than maximum depth leaves the sheet short.
Do I need underlay beneath a pond liner?
Yes in almost all installations. A geotextile underlay protects the liner from stones, roots and settlement damage, and most EPDM and PVC liner warranties require it. Order it at the same dimensions as the liner.
참고 자료
- Firestone (Holcim Elevate) PondGard EPDM liner installation and sizing guides — the dimension + 2×depth + overlap sizing formula and underlay requirements.
- Pond liner supplier technical guidance (e.g., OASE, Gordon Low/SealEco) — stock sheet sizes, overlap recommendations and fill-before-trim installation practice.
- Geosynthetic industry practice — geotextile underlay use beneath flexible geomembranes to prevent puncture.