Pond liner size & volume calculator

Enter your pond's length, width and depth to get the liner size to buy and the gallons it holds, with the pond drawn on its liner sheet.

Inputs

Pond shape

For a free-form pond, measure the longest and widest points.

Sets the liner size.

Sets the volume. Plant shelves make it less than the deepest point; leave blank for a flat floor.

Liner lying flat around the rim. 1 ft is usual; allow more under edging stones.

Pond volume

1,317gal

≈ 4,984 L. You need a liner at least 17 × 14 ft.

Volume assumes the whole pond is 2 ft deep. Plant shelves and sloped sides hold less, so enter an average depth for a truer figure.

Liner size
17 × 14 ft
Stock size to buy
20 × 15 ft
Liner area
238 sq ft
Litres
4,984 L
Surface area
88 sq ft
Depth used for volume
2 ft
Your pond on its liner, from above
Liner 17 × 14 ftoverlapwalls11 ft8 ft

A rectangular pond 11 ft by 8 ft, 2 ft at its deepest, needs a liner at least 17 × 14 ft (238 sq ft), allowing 1 ft of overlap on each side. At an average depth of 2 ft it holds about 1,317 gal (4,984 L). Liner comes in 5 ft steps, so buy 20 × 15 ft.

How it works

A flexible liner has to cover the floor, climb every wall and lie flat past the edge where rocks hold it down:

liner length = pond length + 2 × deepest depth + 2 × overlap
liner width  = pond width  + 2 × deepest depth + 2 × overlap
gallons      = surface area (sq ft) × average depth (ft) × 7.4805
  • Deepest depth sets the liner, because the liner has to reach the lowest point.
  • Average depth sets the volume. If you leave it blank, the calculator assumes a flat floor at the deepest depth and says so.
  • Surface area is length × width for a rectangle, and 0.785 × length × width for a round or oval pond. A free-form pond is estimated as an oval of its longest length and widest width.

In the drawing, the inner outline is your pond, the dashed band around it is the liner that climbs the walls, and the outer band is the overlap.

Worked example

Russell Water Gardens’ water garden: 11 ft long, 8 ft wide and 2 ft at its deepest, with 1 ft of overlap a side.

  • Liner length: 11 + 2 × 2 + 2 × 1 = 17 ft
  • Liner width: 8 + 2 × 2 + 2 × 1 = 14 ft, or 238 sq ft. Buy a 20 × 15 ft liner and the same size of underlayment.
  • If its average depth is 1.5 ft, it holds 11 × 8 × 1.5 × 7.4805 = 987 gallons (3,738 litres). If it really were 2 ft deep all over, it would hold 1,317.

Tips from the pond

  • Dig first, then measure. Run a tape down one wall, across the floor and up the other in both directions. That, plus the overlap, is the most exact liner size there is.
  • Shelves save liner nothing. The liner still has to reach the deepest point, so the deepest depth goes in the sum no matter how many shelves there are.
  • Be cautious with the volume. Russell Water Gardens notes that most people overestimate their pond. For treatments and salt, start with a lower figure and dose up.
  • Add the extras. Water in a waterfall basin, stream or filter counts too when you size a pump or dose a treatment.

FAQ

How do I work out what size pond liner I need?

Add twice the deepest depth, plus an overlap on both sides, to the pond's longest length and widest width. For an 11 × 8 ft pond 2 ft deep with 1 ft of overlap a side, that's 11 + 4 + 2 = 17 ft by 8 + 4 + 2 = 14 ft. Liner comes in 5 ft steps, so buy 20 × 15.

Why twice the depth?

The liner goes down one wall, across the floor and back up the other wall. Down and up is two depths, in each direction. Use the deepest point even if most of the pond is shallower; the liner has to reach it.

How much overlap should I leave?

Webb's and Russell Water Gardens allow 1 ft on each side (2 ft added to each dimension); Hozelock allows 0.5 m a side. Leave more if the edge will be buried under stones or folded into a trench, or if the ground around the pond isn't level.

How many gallons does my pond hold?

Multiply the surface area by the average depth, then by 7.48 for gallons. Use the average depth, not the deepest point; plant shelves and sloping sides make most ponds hold much less than their deepest point suggests. For a pond with shelves, a rough average is the depth of the shelves and the floor weighted by how much of the pond each covers.

Does the shape change the liner size?

No. A liner is a rectangle, so a round, oval or free-form pond needs the same size liner as the rectangle it fits in. The extra material folds into the curves. Shape does change the volume; an oval holds about 79% of the rectangle around it.

Sources

  1. What Size Pond Liner Do I Need?, Russell Water Gardens. Length + 2 × depth + 2 ft safety overlap; a 17 × 12 ft pond 2 ft deep needs 23 × 18 ft (bought as 25 × 20, since liner comes in 5 ft steps); an 11 × 8 × 2 ft pond needs 17 × 14 ft.
  2. How to Size a Pond Liner, Webb's Water Gardens. Pond length + (2 × max depth) + 2 = liner length, the same for width; the 2 ft gives 1 ft of overhang each side. Works for round, kidney and natural shapes.
  3. Calculating Gallons, Webb's Water Gardens. Length × width × average depth × 7.5; a 10 × 10 × 2 ft pond is 200 cubic feet, about 1,500 gallons.
  4. Building a Garden Pond, Hozelock. Max length + (depth × 2) + 1 m overlap; a 2.5 × 2 m pond 0.5 m deep needs a 4.5 × 4 m liner. Volume in m³ × 1000 = litres.
  5. How Much Water Volume is in My Pond?, Russell Water Gardens. Length × width × average depth × 7.48 as a first estimate; most people overestimate how much their pond holds.

Formula and sources last checked September 30, 2026. How we test formulas.

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