How this calculation works
A liner must follow the sloping excavation, not merely span its top opening. The diagonal side length is greater than vertical depth and can differ between the length and width directions. This page calculates those paths for a simple rectangular bottom.
Blank dimension = bottom dimension + 2 × depth × sqrt(1 + slope²) + 2 × anchorage; strips cover width with shared overlapsA worked example
A four-by-three-metre bottom, one-metre depth and one-to-one slopes need 2√2 metres of side path across each dimension. Adding half a metre anchorage per edge gives about 7.828 m by 6.828 m. Two-metre roll strips with 0.1 m shared overlap need four strips across that width. Each is the full required liner length, so offcuts and corner folds remain part of the material budget.
Good to know
Straight uniform slopes and rectangular bottom only; no ledges, irregular excavation, tailored corners, liner stretch or seam approval. Anchorage and joining allowances must be appropriate to the actual liner and site. This geometric budget does not certify pond structure or waterproof installation.
Reference: OpenStax — right-triangle geometry (opens in a new tab) ↗
What the model answers
A liner must follow the sloping excavation, not merely span its top opening. The diagonal side length is greater than vertical depth and can differ between the length and width directions. This page calculates those paths for a simple rectangular bottom.
The result is a bounding rectangular blank suitable for a folded-corner concept, not a tailored three-dimensional panel pattern. It deliberately keeps anchorage as extra material beyond the modeled side slopes.
The Rectangular pond section capacity calculator estimates water volume at a level. Liner dimensions follow surface paths and anchorage, so capacity alone cannot determine the required sheet dimensions.
Measure and enter consistently
Enter bottom dimensions, vertical depth and horizontal run per unit vertical rise. A slope value of one means one metre of horizontal run for each metre of rise. Zero represents an ideal vertical side.
Anchorage is added at both edges of each dimension. Supply a roll width and a shared overlap only if comparing full-length parallel strips. The overlap must remain smaller than roll width so each added strip contributes positive coverage.
Interpret the result
The unseamed area is the rectangular blank’s length times width, not the exact installed surface area. Folded or trimmed corner material is included in that blank and can make purchased area larger than the excavation’s wetted area.
The strip count assumes seams parallel to the liner length. It does not search the alternate roll orientation or select a welding system. The purchased strip area can exceed blank area because full roll widths are retained.
Work through the numbers
A four-by-three-metre bottom, one-metre depth and one-to-one slopes need 2√2 metres of side path across each dimension. Adding half a metre anchorage per edge gives about 7.828 m by 6.828 m.
Two-metre roll strips with 0.1 m shared overlap need four strips across that width. Each is the full required liner length, so offcuts and corner folds remain part of the material budget.
Where this model stops
Straight uniform slopes and rectangular bottom only; no ledges, irregular excavation, tailored corners, liner stretch or seam approval.
Anchorage and joining allowances must be appropriate to the actual liner and site. This geometric budget does not certify pond structure or waterproof installation.
A few questions, answered
Why not use top length plus twice depth?
That ignores the true diagonal path along a sloped side. The slope ratio determines the longer path.
Does the strip count approve a seam?
No. It only counts geometric coverage under an entered overlap; seam construction is separate.