Size a flexible pond liner in seconds. Enter the pond's length, width, and deepest point plus how much overlap you want around the edge, and the calculator gives the minimum liner and underlayment size, the nearest stock size to order, the pond volume in US gallons and liters, and an optional cost from your price per square foot or square meter.
For other water volumes, the pool volume calculator handles swimming pools and the aquarium volume calculator covers tanks. Moving soil from the dig? The topsoil calculator estimates how much you will need to fill planting beds around the pond.
Frequently asked questions
How do I calculate what size pond liner I need?
Add twice the maximum depth plus twice the edge overlap to both the length and the width. For a pond 10 ft long, 8 ft wide, and 2 ft deep with 1 ft of overlap, the liner is (10 + 4 + 2) × (8 + 4 + 2) = 16 × 14 ft. Measure the length and width at their widest points and use the deepest point even if most of the pond is shallow — the formula then covers every shelf and slope. Liners stretch very little, so never size them tight.
How much overlap should a pond liner have?
Leave 1 to 2 ft of liner beyond the edge on every side (about 0.3 to 0.6 m). The overlap is buried under soil or coping stones to hold the liner in place, and the extra lets you set the water level after the pond settles. Ponds with waterfalls or streams need more at that end to run up behind the stones. It's easy to trim excess after filling, but impossible to add liner that's too short. Measure twice, with a flexible tape laid into the hole.
Why does the calculator round the liner up to a stock width?
Pond liners (EPDM rubber, PVC, or RPE) are sold in fixed widths — often 10, 15, 20, 25, 30, 40, or 50 ft — and cut to length from the roll. The calculator rounds the narrow side of your liner up to the nearest width your supplier stocks and the long side up to a whole foot (half-meter in metric). Edit the stock-width list to match your supplier. If the liner is wider than the largest stock width, you need a factory-seamed or custom liner.
Do I need underlayment under a pond liner?
Yes. Underlayment is a thick non-woven geotextile that protects the liner from rocks and roots punching through, and it is inexpensive compared with the liner. Cut it to the same size as the liner so it covers every surface the liner does. Some builders also lay a strip on top of the liner where large stones will sit. Old carpet is sometimes used but can rot and may leach dyes, so purpose-made underlayment is the safer choice. On very rocky ground, use a double layer.
How accurate is the pond volume?
For a rectangular pond with straight, vertical sides, length × width × depth × 7.48 gallons per cubic foot is accurate. Real garden ponds have sloping sides and shelves, so they hold less than their bounding box; choosing Irregular multiplies by 0.8 as a rough estimate. Use the volume for sizing pumps and filters, which usually turn the pond over every 1 to 2 hours. For dosing treatments, measure the real volume with a flow meter or by timing a hose of known flow when filling.