Size the liner before you dig
A flexible liner is the easiest way to build a garden pond of any shape. The liner must cover the floor, climb both walls and leave enough edge to anchor under soil or stones. Measure the pond at its longest and widest points, as if it were a rectangle, and add twice the maximum depth plus the overlap on each side:
Liner length = pond length + 2 × max depth + 2 × overlap
Liner width = pond width + 2 × max depth + 2 × overlap
The formula works for irregular and kidney-shaped ponds too, because the liner is a rectangle folded into the hole.
Worked example
A pond 10 ft long and 6 ft wide, 2 ft deep at its deepest, with 1 ft of overlap all around:
- Liner length: 10 + 4 + 2 = 16 ft.
- Liner width: 6 + 4 + 2 = 12 ft.
- Liner area: 192 sq ft, about $192 at $1 per sq ft for 45-mil EPDM.
- With an oval shape and an average depth of 1.5 ft, the pond holds about 528 gallons.
- A pump moving about 528 gallons per hour circulates the whole volume once an hour.
Liners come in standard sizes, so round up to the next available size. Buy the same size of underlayment fabric to protect the liner.
Estimating the volume
The water volume is length × width × average depth, multiplied by about 0.785 for oval or round ponds. Each cubic foot holds 7.48 gallons. Knowing the volume helps you size the pump, filter and UV clarifier, and dose water treatments correctly.
Building tips
- Shelves: a planting shelf 8 to 12 inches deep around the edge holds marginal plants and makes the pond safer.
- Depth for fish: at least 2 ft, and 3 ft or more in cold climates so fish can overwinter below the ice. Koi need 3 ft or more.
- Level edges: check with a long level or a water level; the water line shows every mistake.
- Remove stones and roots, then lay 2 inches of sand or an underlayment fabric before the liner.
- Fill slowly while pulling out wrinkles and folding the excess into neat pleats.
- Local rules: some towns treat ponds deeper than 18 to 24 inches like swimming pools and require a fence. Check before digging, and call 811 to locate buried utilities.