Liner planning2026-09-12

What size pond liner do I need?

For a pond 16 feet long, 10 feet wide and 3 feet deep, allowing one foot beyond the edge on every side gives a starting liner size of 24 by 18 feet. Measure the finished excavation and check the edge detail before ordering.

Cross-section showing a 16-foot pond span, two 3-foot depth allowances and one foot at each edge, totaling 24 feet — Length calculation for the worked example. The drawing is schematic; the liner follows the excavated profile. Open the diagram for a larger view.

Include both sides of the pond

A sheet must run down one side, across the basin and up the other. Add twice the maximum depth to each horizontal dimension, then add the allowance needed beyond both edges. Aquascape’s installation checklist uses maximum depth twice in its basic pond-liner calculation. The Atlantic Water Garden System guide also includes two feet of total edge allowance.

Example with a one-foot allowance at each edge
Dimension	Calculation
Liner length	16 + (2 × 3) + (2 × 1) = 24 ft
Liner width	10 + (2 × 3) + (2 × 1) = 18 ft

That is 432 square feet of material before any additional allowance. Area alone cannot tell you whether a sheet fits: both its length and width must cover the required dimensions.

How shelves affect the measurement

On a simple stepped profile, the horizontal shelf runs are already part of the overall span. The vertical drops add up to the maximum depth. A shelf at 18 inches and a floor at 36 inches therefore give a total depth of 36 inches, not 54.

Do not add every layer’s depth together. Instead, measure across the finished excavation with a flexible tape following the actual route the liner will take, including the shelves and both edges. Check more than one route if the pond bends or has a deep section off to one side. Complex shapes and edge details may need more material than the basic rectangular allowance suggests.

Set the allowances explicitly

The one-foot edge allowance in this example is an assumption. Your anchoring, coping and supplier’s installation instructions determine what the finished pond needs. Waterfalls and streams also need their own dimensions and connection details.

In the Pond Tools liner calculator, enter the maximum length, width and depth, then set the overhang. The optional fold allowance is added once to each calculated sheet dimension. Selecting eight inches would change this example to 24 feet 8 inches by 18 feet 8 inches.

Choose a sheet that meets or exceeds both calculated dimensions, including the allowances you need. Check the supplier’s available sizes and confirm the excavation measurements before cutting.

Keep the liner and volume calculations separate

Liner sizing uses the deepest part because the sheet must reach it. A simple volume calculation uses average water depth. For a worked comparison of a rectangular basin and a pond with shelves, see How big is a 3,000-gallon pond?