What size pond liner do I need?

Add twice the maximum depth and 2 ft to the pond's length, and the same to its width. A 12 × 8 ft pond that is 3 ft deep needs a 20 × 16 ft liner: 12 + 6 + 2 by 8 + 6 + 2.

How much overlap should a pond liner have?

Firestone's formula allows 1 ft on each side. Some suppliers suggest up to 2 ft per side, especially for irregular ponds or edges where the liner tucks under heavy stones. More overlap costs a little more liner but can be trimmed; too little can't be fixed.

Do I use the average or the maximum depth for a liner?

The maximum depth. The liner has to reach the deepest point, so shelves and slopes don't reduce the size you need.

How do I size a liner for an irregular pond?

Measure the longest length and the widest width across the pond, even if they are in different places, and use the deepest point. Allow a bit more overlap, for example 1.5 ft per side, because folds around curves use up liner.

Do I need underlay under a pond liner?

Yes, on soil. Firestone's instructions call for a geotextile underlay before the liner goes in. Cut it to the same size as the liner.

Pond Liner Calculator: What Size Liner Do I Need?

JavaScript is disabled in your browser, so the interactive calculator below won't run. The math, formula, and worked example are all written out further down this page under “How this is calculated.” Enable JS (or open this page in another browser) to use the live calculator.

Whether you compute by hand or in the calculator, the same caveat applies: verify against your test kit and the linked primary source before dosing. reefcalcs is reference information, not veterinary or engineering advice.

Pond liner size

Pond length (ft)

The longest length at the water line, not including shelves outside it.

Pond width (ft)

The widest point.

Maximum depth (ft)

The deepest point, not the average: the liner has to reach the bottom.

Overlap per side (ft)

Liner left over the edge to anchor under stones. Firestone's formula allows 1 ft (2 ft in total).

Buy a liner of

16 × 12ft

Rounded up from 16 × 12 ft

Liner area

192ft²

Underlay

16 × 12ft

Cut to the same size as the liner.

Liner length = pond length + 2 × maximum depth + 2 × overlap, and the same for width (Firestone PondGard). Shelves don't change it: the liner follows them down to the deepest point.

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Liner and underlay to match:

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45 mil EPDM pond liner

EPDM rubber liner, 45 mil thick. Buy the size the calculator gives.

See options on Amazon

Geotextile pond underlayment

Goes under the liner to protect it from stones and roots. Same size as the liner.

See options on Amazon

How this is calculated

liner length = pond length + (2 × maximum depth) + (2 × overlap)
liner width  = pond width  + (2 × maximum depth) + (2 × overlap)
overlap      = 1 ft (0.3 m) per side by default
buy size     = rounded up to the next whole foot (or 0.5 m)
underlay     = the same size as the liner

Firestone's PondGard brochure gives the liner size as "Length + (2 x Depth) + 2 Extra Feet" and "Width + (2 x Depth) + 2 Extra Feet", which is 1 ft of overlap on each side. Some suppliers allow more: All Pond Solutions adds 2 ft (0.6 m) per side. Change the overlap if your edging needs more liner to tuck under, or if the pond is an irregular shape.

Use the maximum depth

The liner has to reach the deepest point, so the formula uses the maximum depth, not the average. Plant shelves and sloped sides don't change it: the liner follows them on the way down. That's different from the pond volume calculator, which uses the average depth because shelves hold less water.

Underlay

Firestone says to "Place a geotextile fabric as underlay before installing the liner", after clearing stones and roots and spreading sand over the base. It protects the liner from punctures, and it is cut to the same size.

FAQ

What size pond liner do I need?: Add twice the maximum depth and 2 ft to the pond's length, and the same to its width. A 12 × 8 ft pond that is 3 ft deep needs a 20 × 16 ft liner: 12 + 6 + 2 by 8 + 6 + 2.
How much overlap should a pond liner have?: Firestone's formula allows 1 ft on each side. Some suppliers suggest up to 2 ft per side, especially for irregular ponds or edges where the liner tucks under heavy stones. More overlap costs a little more liner but can be trimmed; too little can't be fixed.
Do I use the average or the maximum depth for a liner?: The maximum depth. The liner has to reach the deepest point, so shelves and slopes don't reduce the size you need.
How do I size a liner for an irregular pond?: Measure the longest length and the widest width across the pond, even if they are in different places, and use the deepest point. Allow a bit more overlap, for example 1.5 ft per side, because folds around curves use up liner.
Do I need underlay under a pond liner?: Yes, on soil. Firestone's instructions call for a geotextile underlay before the liner goes in. Cut it to the same size as the liner.

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Sources & references

1.
Firestone PondGard EPDM pond liner brochure
https://dev.holcimelevate.com/content/dam/elevateamericas/migrated-document/Documents/us/lining/pondgard-brochure.pdf
2.
All Pond Solutions — Calculating the size of your pond liner
https://www.allpondsolutions.co.uk/fishkeeping-advice/calculating-the-size-of-your-pond-liner.html
3.
Pond Pro Direct — Pond liner sizing
https://pondprodirect.com/blogs/pond-building-installation/pond-liner-sizing

Last reviewed 2026-10-29