What Size Pond Liner Do I Need?

The depth counts twice

The liner has to run down one side, across the bottom, and back up the other. So the sheet you need is the pond's length plus twice the depth — and then the overlap you bury, twice more, once at each edge.

For a 10 × 6 ft pond at 2 ft deep with a foot of overlap:

length:  10 + 2 + 2 + 1 + 1  =  16 ft
width:    6 + 2 + 2 + 1 + 1  =  12 ft

A 10-foot feature needs a 16-foot sheet. That's the single most common surprise in a first pond build, and it's why people come home with a liner that's too small.

Maximum depth for the liner, average depth for the water

These are separate fields because they do different jobs.

Liner Length to buy and Liner width to buy use the maximum depth, because the sheet physically has to reach the deepest point. Size it on the average and it comes up short exactly where you can't fix it.

Water volume uses the average depth, because that's what actually determines how much water the hole holds. A pond with sloping sides, planting shelves and a deep centre can easily average well under half its maximum depth.

Change the maximum depth in the tool and you'll see the liner grow while the volume stays put. That's correct.

The volume is a ceiling, not a measurement

Length × width × average depth describes a box. A real pond has sloping sides, shelves and an uneven bottom, so the true volume is lower — often by a fifth or more.

That's the useful direction to be wrong in. A pump sized for slightly more water than is present still works. A treatment dosed by a volume that's a little high is a much smaller problem than one dosed by a volume that's too low.

If you need a real figure, meter the water going in when you fill it.

The conversion itself is exact: 1 cubic foot = 1728 ÷ 231 = 7.4805 gallons, because a US gallon is defined as exactly 231 cubic inches.

Read the weight figure twice

At the defaults that pond holds 673 gallons. Weight of the water is 5,619 lb — a little over 2.8 tons.

Sitting on undisturbed ground that's usually fine; the ground was carrying soil of a similar weight before you dug it out. But for anything raised, on a deck, on a roof, or over a basement, that's a structural question rather than a gardening one, and it's worth asking someone qualified before filling rather than after.

Water is about 8.345 lb a gallon. Small ponds get heavy faster than people expect.

Buy the overlap generously

It's the cheapest part of the job and the only part you can't add later.

A liner cut flush with the edge creeps as the ground settles and the water works at it, and the repair involves an empty pond. Buried under a course of stone, a paving edge or a turf lip, it stays where you put it.

Trimming a sheet that turned out too big takes a minute with a knife. Making one bigger isn't a thing that exists.

And trim last. Lay the liner loose, fill slowly, and let the water press it into the shape of the hole before cutting anything.

Liner comes in fixed widths

You buy off a roll, so the sheet you purchase is usually bigger than the sheet you need. Offcut from a fixed-width roll is the honest cost of that — 48 sq ft at the defaults.

If it shows a dash, the roll is narrower than your liner, so it can't be cut from that width at all. Two things to try:

• Turn the pond against the roll. Liner is long on one axis and fixed on the other, so a pond that won't fit one way round often fits the other.
• Step up a width. Joining two pieces is possible with the right tape, but it puts a seam in an otherwise seamless sheet — and that's where a leak eventually appears.

Order the underlay at the same time

Liner area needed is also your underlay area. On anything but deep clean sand it isn't optional.

Roots grow, stones work upward as the ground freezes and thaws, and the base settles under several tons of water. All three find a liner eventually. Old carpet, purpose-made fleece or a thick bed of soft sand all do the job.

The reason to bother isn't the cost of the liner. It's that a puncture under a filled pond is the hardest kind of leak to find and the most miserable to fix.