Koi Pond Liner Calculator

Work out exactly what size pond liner to buy. Enter the pond length, width and maximum depth, and the calculator adds the depth on both sides plus your overlap, then matches it to the nearest standard sheet.

Sheet size & area Standard size match Liner weight Always free

📐 Pond Liner Size Calculator

Results update as you type. Use the maximum depth here, not the average — the liner has to reach the bottom of the deepest point.

Measurement units

Feet Metres

Pond dimensions

Maximum length (ft)

Maximum width (ft)

For an irregular or kidney-shaped pond, measure the longest and widest points. The liner has to cover the bounding rectangle, so the shape does not reduce the sheet you need.

Maximum depth (ft)

Overlap per side (ft)

One foot per side (about 30 cm) is the usual minimum for anchoring the liner under coping stones or a turf edge. Allow more on sloping ground, on a pond with a bog filter, or if you are not confident in your measurements — trimming a liner down is easy, adding to one is not.

Liner material Used to estimate the finished weight, which decides whether you can position the sheet yourself. Always buy liner sold as fish safe or pond grade — roofing EPDM often contains algaecides and mildewcides that are toxic to fish.

Common pond sizes

Typical koi pond footprints. Select one to fill in the dimensions, then adjust the depth and overlap to match your build.

How it works

How to Use the Pond Liner Calculator

Four steps to a liner size you can order with confidence.

Measure the excavation

Longest point and widest point of the hole, not the finished water surface. The liner has to cover the whole dig.

Use maximum depth

The deepest part of the pond, measured from ground level. Unlike volume calculations, liner sizing uses the maximum, not the average.

Add your overlap

One foot per side is the usual minimum for anchoring. Add more on sloping ground or where you plan a bog filter or beach edge.

Round up to a standard sheet

Liners are sold in fixed sizes. The calculator shows the nearest one that fits, and how much spare material it leaves.

How to Measure for a Koi Pond Liner

The liner has to run down one side of the pond, across the bottom, up the other side, and then far enough past the edge to be anchored. So the depth is counted twice — once for each side — and the overlap is counted twice as well.

Liner length
pond length + (2 × maximum depth) + (2 × overlap per side)

Liner width
pond width + (2 × maximum depth) + (2 × overlap per side)

A 12 × 8 ft pond that is 4 ft deep with a 1 ft overlap needs 12 + 8 + 2 = 22 ft of length and 8 + 8 + 2 = 18 ft of width, so a 22 × 18 ft sheet. Since liners come in fixed sizes, that becomes a 25 × 20 ft sheet off the shelf.

Three things catch people out:

Depth is doubled. Adding the depth once is the most common liner sizing mistake, and it leaves a sheet that reaches the far edge with nothing to anchor.
Use maximum depth, not average. Average depth is the right figure for volume; the liner has to reach the bottom of the deepest point.
Shelves and slopes need slack. A pond with planting shelves, a stepped profile or steep sides needs the liner to follow more surface than a straight-sided box, so add half a foot on each dimension for a heavily stepped design.

Measure the excavation rather than the intended water surface, and take the measurements after the hole is dug rather than from the plan — few ponds come out exactly as drawn.

Koi Pond Liner Size Chart

Liner sizes for common koi pond dimensions, calculated with a 1 ft overlap on each side and rounded up to the nearest standard sheet.

Weights are for 45 mil fish-grade EPDM at approximately 0.30 lb per square foot, based on the exact sheet rather than the rounded-up standard size.
Pond (L × W × max depth)	Liner needed	Sheet area	Nearest standard size	EPDM weight
6 × 4 × 2 ft	12 × 10 ft	120 ft²	15 × 10 ft	≈ 36 lb
8 × 6 × 3 ft	16 × 14 ft	224 ft²	20 × 15 ft	≈ 67 lb
10 × 8 × 3 ft	18 × 16 ft	288 ft²	20 × 20 ft	≈ 86 lb
12 × 8 × 4 ft	22 × 18 ft	396 ft²	25 × 20 ft	≈ 119 lb
15 × 10 × 4 ft	25 × 20 ft	500 ft²	25 × 20 ft	≈ 150 lb
20 × 10 × 4 ft	30 × 20 ft	600 ft²	30 × 20 ft	≈ 180 lb
20 × 15 × 5 ft	32 × 27 ft	864 ft²	40 × 30 ft	≈ 259 lb

Two rows are worth noticing. The 15 × 10 ft pond lands exactly on a standard 25 × 20 sheet with no waste, while the 20 × 15 ft pond needs 32 × 27 and has to be bought as 40 × 30 — nearly 300 square feet of spare material. If you are still designing the pond, checking your dimensions against standard liner sizes before you dig can save a meaningful amount of money.

EPDM, PVC or RPE for a Koi Pond?

All three are used for koi ponds. They differ in flexibility, lifespan, weight and how easy they are to seam on site.

Typical characteristics of pond liner materials. Specific products vary by manufacturer and thickness.
Material	Weight	Flexibility	Typical service life	Notes
45 mil EPDM	≈ 0.30 lb/ft²	Very flexible	20–30 years	The default for koi ponds. Folds easily into complex shapes; heavy in large sheets.
60 mil EPDM	≈ 0.40 lb/ft²	Flexible	25–35 years	Tougher underfoot, noticeably heavier to handle and position.
30 mil PVC	≈ 0.19 lb/ft²	Moderate	10–15 years	Cheaper and lighter, can be solvent welded. Stiffens and becomes brittle with UV exposure over time.
45 mil RPE	≈ 0.16 lb/ft²	Stiffer	20–40 years	Reinforced, very puncture resistant and light for its strength, but harder to fold into tight contours.

The one non-negotiable

Buy liner sold specifically as fish safe or pond grade. Roofing EPDM looks identical and is often cheaper, but many roofing grades contain algaecides, mildewcides or plasticisers that leach into the water and are toxic to fish. The saving is not worth the risk, and the damage is not always obvious until the pond is stocked.

Do I need underlay?

Yes, on any pond you intend to keep. Underlay is a non-woven geotextile that sits between the ground and the liner and protects it from stones, roots and settlement. A liner puncture in a filled, stocked koi pond is a serious problem — draining, finding the hole and patching it means moving the fish — and underlay costs a small fraction of the liner. Buy the same sheet size as the liner, and on stony or clay ground consider a double layer or a sand blinding underneath as well.

Handling the sheet

Liner weight scales quickly. A 25 × 20 ft EPDM sheet is around 150 lb, and a 40 × 30 is nearly 360 lb as delivered — beyond what two people can carry and position, particularly through a garden gate. Check the weight before ordering, plan the access route, and unfold the liner in place rather than carrying it opened. Laying it on a warm day helps: EPDM is much more compliant in the sun than at 5 °C.

FAQ

Frequently Asked Questions

Liner sizing, overlap and material questions, answered.

How do I calculate what size pond liner I need?

Liner length is pond length plus twice the maximum depth plus twice the overlap per side, and liner width follows the same formula. A 12 × 8 ft pond at 4 ft deep with a 1 ft overlap needs 12 + 8 + 2 = 22 ft by 8 + 8 + 2 = 18 ft, so a 22 × 18 ft sheet, bought as a standard 25 × 20. The depth is added twice because the liner goes down one side and up the other.

How much overlap does a pond liner need?

One foot per side is the usual minimum, which is enough to anchor the liner under coping stones, paving or a turf edge. Allow more on sloping ground, where the edge detail is complex, where a bog filter or beach edge is planned, or if you are not fully confident in the measurements. Trimming excess liner takes minutes; a sheet that finishes short of the edge cannot be fixed.

Should I use maximum or average depth for liner sizing?

Maximum depth. Average depth is the correct figure for calculating water volume, but the liner physically has to reach the bottom of the deepest point and come back up, so it is sized on the deepest measurement. Using average depth is a common mistake and produces a sheet that runs out before it reaches the far edge.

Can I use roofing EPDM for a koi pond?

No. Roofing EPDM looks the same as pond liner and is often cheaper, but many roofing grades contain algaecides, mildewcides and plasticisers that leach into the water and are toxic to fish. Buy liner that is explicitly sold as fish safe or pond grade. This is the one specification on a pond build that should never be substituted.

Do I need underlay under a pond liner?

Yes on any pond you plan to keep long term. Underlay is a non-woven geotextile that protects the liner from stones, roots and ground settlement, and it costs a small fraction of the liner it protects. Buy the same sheet size as the liner. On stony or heavy clay ground, a double layer of underlay or a sand blinding beneath it is worth the extra effort.

How heavy is a pond liner?

Around 0.30 lb per square foot for 45 mil EPDM, so a 25 × 20 ft sheet is about 150 lb and a 40 × 30 ft sheet close to 360 lb. RPE is lighter for its strength at roughly 0.16 lb per square foot, and 30 mil PVC is about 0.19. Check the weight before ordering: large EPDM sheets are beyond what two people can carry, and the liner should be unfolded in position rather than moved once open.

What size liner do I need for a 1,000 gallon pond?

It depends on the shape, not just the volume. A 1,000 US gallon pond at 3 ft deep might be 10 × 4.5 ft, which needs a 18 × 12.5 ft liner with a 1 ft overlap, or 8 × 5.5 ft at 3 ft deep, which needs 16 × 13.5. Measure the actual excavation and use the formula rather than working back from gallons, because ponds of the same volume can have very different footprints.

Can I join two pond liners together?

EPDM can be seamed on site with proper primer and seam tape, and PVC can be solvent welded, but a factory-welded single sheet is more reliable for a koi pond. Every field seam is a potential leak in a pond that is difficult and stressful to drain once stocked. If the pond is larger than any standard sheet, most suppliers will make a single custom-welded liner to your dimensions, which is the better answer.

Free tools

Related Koi Pond Tools

The liner is one of several numbers that come out of the same three measurements.

Koi Pond Calculator

Turn the same length, width and depth into water volume in gallons and litres.

Koi Pond Pump Size Calculator

Size the pump and pipework for the pond you are about to line.

Koi Pond Stocking Calculator

Check how many koi the finished pond will support before you buy fish.

Substrate Calculator

Working on an indoor tank too? Estimate gravel, sand or soil by volume and weight.

The koi pond calculator turns the same dimensions into water volume, the pump calculator sizes the circulation for a new build, and the koi fish care guide covers what to do after the pond is filled.

Methodology and accuracy

Liner size is calculated as pond dimension + (2 × maximum depth) + (2 × overlap per side), applied to both length and width. This is the standard pond liner formula and assumes the liner runs down one side, across the base and up the other side with the stated overlap anchoring each edge.

The sheet area is the product of the two calculated dimensions and is exact for the figures you enter. It does not add slack for planting shelves, stepped profiles or steeply sloping sides, which need the liner to follow more surface than a straight-sided pond — add roughly half a foot to each dimension for a heavily stepped design.

The nearest standard size is chosen from a list of sizes commonly stocked by liner suppliers, allowing the sheet to be rotated. Ranges vary by supplier and by country, so treat it as a guide to what to look for rather than a catalogue entry, and confirm availability before finalising a pond design around a particular size.

Weight estimates use typical values of 0.30 lb/ft² for 45 mil EPDM, 0.40 for 60 mil EPDM, 0.19 for 30 mil PVC, 0.13 for 20 mil PVC and 0.16 for 45 mil RPE. Actual weights vary between manufacturers, and the delivered roll includes packaging.

Unit conversions are exact: 1 ft = 0.3048 m, 1 ft² = 0.09290304 m², 1 lb = 0.45359237 kg. Material service lives quoted on this page are typical industry figures, not warranties.