Pond Liner Size Calculator

Enter the longest and widest points of your pond and its deepest point. You get the size of liner to buy, with the overlap for the edges already added.

Units
For a round pond, enter the distance across for both length and width.
Measure the hole, from ground level to the lowest point. Here you need the deepest point, not the average.
Liner left on the ground around the edge to hold it down. Firestone allows 1 ft (30 cm) a side; some sellers advise 2 ft (60 cm).
Liner to buy (rounded up)17 × 13 ft5.18 × 3.96 m
Liner area221 sq ft20.5 m²

A pond 10 ft (3.05 m) long, 6 ft (1.83 m) wide and 2.5 ft (0.76 m) deep at its deepest needs a liner of at least 17 × 13 ft, with 1 ft of overlap on each side. Rounded up to the sizes liner is sold in, buy 17 × 13 ft.

Exact size from the formula
17 × 13 ft (5.18 × 3.96 m)
Liner length
10 ft + 2 × 2.5 ft + 2 × 1 ft
Liner width
6 ft + 2 × 2.5 ft + 2 × 1 ft
Underlay
The same size as the liner
  • Do not trim the liner until the pond is full. The weight of the water pulls it down into the hole.
  • Plant shelves and steps need no extra liner. The sum already allows for the full drop to the deepest point on both sides.
  • A stream or waterfall needs its own piece of liner.

What to work out next

Pond volumeHow much water the pond holdsUnit converterFeet, meters, gallons, liters

How to work out pond liner size

The liner has to cross the pond, go down one side to the deepest point, come up the other side, and still leave a strip on the ground at each edge to hold it in place.

Liner length = pond length + 2 × deepest point + 2 × overlap
Liner width = pond width + 2 × deepest point + 2 × overlap

This is the method the liner maker Firestone prints for its PondGard liner, with 1 ft (30 cm) of overlap on each side.

Worked example. The pond is 12 ft long and 8 ft wide, and 3 ft deep at the deepest point. Overlap is 1 ft a side.

  • Length: 12 + (2 × 3) + (2 × 1) = 20 ft
  • Width: 8 + (2 × 3) + (2 × 1) = 16 ft

Buy a liner of 20 × 16 ft.

What to measure

  • Length and width: the longest and the widest points of the hole, even if the pond is round, oval or kidney-shaped. Liner is sold as a rectangle, so think of the box the pond fits in.
  • Depth: the deepest point of the hole, from ground level. This is different from the volume sum, which uses the average depth.
  • Overlap: the strip left flat on the ground around the edge, later hidden under stones or turf.

How much overlap?

Firestone allows 2 ft in total, which is 1 ft (30 cm) on each side. Some sellers, such as All Pond Solutions, advise 2 ft (60 cm) on each side. More overlap makes the edge easier to finish and costs a little more liner. Less than 1 ft a side is risky: the liner can slip in as the pond fills.

Shelves, steps and slopes

You do not need to add anything for plant shelves or steps. A liner that goes down in steps covers the same total drop as one that goes straight down, and the formula already allows for the full drop on both sides. Sloping sides need a little less liner than the formula gives, so you will have some to spare.

Round up, then trim last

Liner is sold in set sizes, often in whole feet or half meters. Always round up to the next size. A liner that is slightly too big is trimmed in minutes; one that is too small cannot be fixed.

Do not trim until the pond is full. The weight of the water pulls the liner down into every corner, and the edge moves in as it does.

Underlay goes under the liner to protect it from stones and roots. Buy it the same size as the liner.

Common mistakes

  • Using the average depth. The liner must reach the deepest point, so use the deepest.
  • Adding the depth once. The liner goes down one side and up the other: add it twice.
  • Forgetting the overlap. With none, there is nothing to hold the liner at the edge.
  • Measuring the water instead of the hole. Measure from ground level.
  • Trimming the liner before the pond is full. It pulls in as the water settles it.

Questions and answers

What size pond liner do I need?

Add twice the deepest point and twice the overlap to both the length and the width. For a pond 12 ft by 8 ft and 3 ft deep with 1 ft of overlap a side, that is a 20 ft by 16 ft liner.

How much overlap should a pond liner have?

At least 1 ft (30 cm) on each side, which is what Firestone allows for its PondGard liner. Some sellers advise 2 ft (60 cm) a side for an easier edge.

How do I measure a liner for a round or kidney-shaped pond?

Measure the longest and widest points, as if the pond sat in a box, and use the same formula. Liner comes as a rectangle, so the corners are trimmed off after the pond is filled.

Do plant shelves need extra liner?

No. A liner that drops in steps covers the same total depth as one that drops straight down, and the formula already includes the full depth on both sides.

What size underlay do I need?

The same size as the liner. It lies under the whole liner, sides and overlap included.

Numbers for standard tanks

Have a standard tank? Its size, weight, heater and stocking are already worked out.

Sources

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