A liner has to run down one side, across the bottom and up the other, then lie flat on the ground around the edge so stones can hold it. That is why a pond that looks small on the ground needs a surprisingly large sheet.
How to use this calculator
- Lay out the shape with a garden hose and measure the longest length and the widest width, even for a round or kidney-shaped pond.
- Enter the depth at the deepest point, not the shelves.
- Under More options, change the edge to bury on each side if you want a wider rim.
- Round the result up to the next size the liner is sold in.
Worked examples
A small garden pond
Pond: 8 × 6 ft · Deepest point: 1.5 ft
- Liner size
- 13 × 11 ft
- Liner area
- 143 sq ft
A water garden with fish
Pond: 12 × 10 ft · Deepest point: 2 ft
- Liner size
- 18 × 16 ft
- Liner area
- 288 sq ft
A koi pond
Pond: 16 × 12 ft · Deepest point: 3.5 ft
- Liner size
- 25 × 21 ft
- Liner area
- 525 sq ft
A wider edge for stone coping
Pond: 10 × 8 ft · Deepest point: 2 ft · Edge to bury, each side: 2 ft
- Liner size
- 18 × 16 ft
- Liner area
- 288 sq ft
How the liner size is worked out
Kansas State University's rule is the pond's length at its widest spot, plus twice the depth, plus 2 ft for the edges, and the same for the width. The twice-the-depth part covers the liner going down and coming back up; the edge allowance leaves about a foot on each side to bury or cover with stone.
K-State also suggests laying out the pond with a margin of 18 to 24 inches around the edge, so the liner can overlap it and be held down with stone. Add more for a waterfall or a stream: their liner runs up the slope too.
Choosing a liner
- Flexible rubber (EPDM) liners for water gardens are 40 to 45 mil thick and resist the sun's UV.
- Texas A&M expects about 30 years from a fish-grade rubber or butyl liner and 7 to 15 years from fish-grade PVC. Other guides give shorter or longer lives, so treat these as rough figures.
- If you keep fish or plants, buy a liner sold for ponds and labeled fish-safe: roofing rubber can carry treatments that harm them.
What goes under the liner
Texas A&M suggests a layer of sand about 2 inches deep under any flexible liner, to cushion it from stones and roots, so dig the hole that much deeper. Over the sand, K-State lays a sheet of underlayment fabric up the walls. Buy underlayment about the size of the liner.
How deep to dig
A backyard water garden is usually 18 to 24 inches deep, with a plant shelf around the edge about 10 inches deep and 18 inches wide. Ponds that keep fish outdoors all year often have a section 3 to 4 ft deep that does not freeze, and koi need at least 3 ft.
In many cities, a pond deeper than 18 inches has to be fenced like a pool. Check your local rules before you dig.
Common questions
How do I calculate the size of a pond liner?
Add twice the deepest depth and 2 ft to the pond's longest length, and the same to its widest width. The calculator does it and gives the area too.
How much overlap does a pond liner need?
About a foot on each side, which is what K-State's rule allows. A wider rim, 18 to 24 inches, gives room to anchor the liner under stones.
What thickness of pond liner do I need?
Rubber water garden liners are 40 to 45 mil thick, which suits most backyard ponds.
Is a rubber or PVC pond liner better?
Rubber (EPDM) usually lasts longer: Texas A&M gives about 30 years for a fish-grade rubber liner, against 7 to 15 years for fish-grade PVC. PVC costs less, so it suits a pond you may change.
Do I need underlayment under a pond liner?
Yes, it is cheap insurance. A bed of sand and a sheet of underlayment protect the liner from roots and sharp stones that would puncture it.