Pond Liner Size Calculator

How this pond liner size calculator works

A flexible pond liner must be large enough to cover the pond’s footprint and travel down the side walls to the deepest point and back up the other side. That’s why depth is counted twice. We also add an overlap allowance on every edge so you can tuck the liner under coping stones, soil, or edging for a secure, watertight finish.

This calculator outputs the recommended liner length and width based on four inputs: pond length, pond width, maximum pond depth, and overlap per edge (added on both sides of each dimension).

Formulas used (liner dimensions)

Let:

• L = pond length
• W = pond width
• D = maximum pond depth
• O = overlap on each edge (same unit as the other inputs)

Then the liner size is:

• Liner length = L + 2D + 2O
• Liner width = W + 2D + 2O

MathML version of the same formulas:

Where LL is liner length and LW is liner width.

Interpreting your results

The calculator returns a recommended liner rectangle (length × width). When purchasing, you typically choose a liner sheet that is at least as large as these dimensions. If a supplier sells standard sizes, round up to the next available size rather than down.

• If your pond is roughly rectangular: the result is usually a good estimate.
• If your pond has curves (oval/kidney): use the maximum length and maximum width across the pond as inputs. The result is an approximation; curvy shapes may need extra slack.
• If you have shelves/steps: shelves can increase the amount of liner required because the liner must follow additional contours. Consider increasing overlap or adding extra allowance beyond the calculator’s output.

Worked example

Suppose you’re planning a pond with:

• Length L = 10 ft
• Width W = 7 ft
• Max depth D = 2.5 ft
• Overlap each edge O = 1 ft

Compute liner dimensions:

• Liner length = 10 + 2(2.5) + 2(1) = 10 + 5 + 2 = 17 ft
• Liner width = 7 + 2(2.5) + 2(1) = 7 + 5 + 2 = 14 ft

Result: you’d shop for a liner that is at least 17 ft × 14 ft. If liners are sold in common increments (e.g., 15×20), you would choose the next size up that meets or exceeds both dimensions.

Typical overlap guidance

Overlap is the extra liner beyond the pond edge used to secure and hide the liner. Many DIY builds use 1.0–1.5 ft per edge. Consider more (for example, 1.5–2.0 ft) if you plan heavy rock edging, have soft/erodible soil, want more room to reposition during installation, or expect settling.

Liner material comparison (quick reference)

Limitations & assumptions (read before buying)

• Shape approximation: The formula is best for rectangular ponds and provides an estimate for oval/curved ponds using max length/width.
• Depth input is maximum depth: If your pond has deep pockets or steep transitions, you may need extra slack beyond the estimate.
• Shelves/steps/irregular contours: Benches for plants, multiple tiers, or highly irregular edges can increase liner needs.
• Measurements must use the same units: This page labels feet (ft). Don’t mix feet and inches (or meters) in the same calculation.
• Purchase sizing: Manufacturers and retailers may sell fixed sizes; always round up and verify available roll widths and seam options.
• Site preparation matters: Underlayment, padding, and how the liner is folded into corners can affect final fit.

Before-you-buy checklist

• Measure length/width at the widest points (and re-check after excavation if possible).
• Use the deepest point for depth.
• Decide your edging method (rocks/coping/turf) and choose overlap accordingly.
• Plan for underlayment and remove sharp rocks/roots to reduce puncture risk.
• Round up to the next available liner size if between sizes.