pond liner calculator
Work out the exact liner sheet you need, plus underlay area, water volume and the right thickness. Metres or feet, rectangular, circular or irregular ponds. If you already know your size, go straight to HDPE pond liner supply.
Calculate your liner size
Pond liner calculator
Liner size, underlay, water volume and recommended thickness
How to calculate pond liner size
The formula is the same everywhere, and it has three parts: the pond itself, the depth on both sides, and an overlap allowance at the top.
Liner length = pond length + (2 × max depth) + (2 × overlap)
Liner width = pond width + (2 × max depth) + (2 × overlap)
The depth is counted twice because the liner has to travel down one side and up the other. The overlap is what you bury in the anchor trench or tuck under the edging — without it the liner finishes flush with the water line and has nothing to hold it.
Use 0.3m (12in) minimum, 0.5m (18in) for anything larger than a garden pond. For a lined reservoir or a pond that will be walked around, more is better than less. Trimming excess costs you nothing; being 200mm short means a new sheet.
A worked example
A pond 4m long, 3m wide and 1m deep, with 0.5m of overlap:
Length: 4 + (2 × 1) + (2 × 0.5) = 7m Width: 3 + (2 × 1) + (2 × 0.5) = 6m
So you need a 7m × 6m sheet — 42m². Note this is more than double the pond’s own footprint of 12m². That surprises most people, and it’s the single most common reason a liner order comes up short.
That surprises most people, and it’s the single most common reason a liner order comes up short. Our HDPE sheet specifications show roll widths and lengths so you can check what your calculated size means in rolls.
Sloped sides
For slopes gentler than about 45°, add 10–15% to the depth figure before you calculate. Our installation method statement covers slope preparation and anchor trench detailing in full.
How to measure your pond
Measure at the widest points
Not the average, and not the water line — the widest and longest points of the excavation, measured across the top edge.
Measure depth at the deepest point
If the pond has shelves or a deep zone, use the deepest measurement. The liner is one continuous sheet and has to reach the bottom of the deepest part.
Measure before you dig, or after?
After. Excavations rarely finish at the dimensions on the drawing. Measure the hole you actually have, not the one you planned.
Irregular shapes
A liner is a rectangular sheet regardless of the shape of the hole. For larger irregular water bodies, see lake liner supply and installation.
Pond liner size chart
Common pond sizes with 0.5m overlap. All figures in metres.
| Pond size | Depth | Liner needed | Sheet area |
|---|---|---|---|
| 2 × 1.5 | 0.6 | 4.2 × 3.7 | 15.5 m² |
| 3 × 2 | 0.8 | 5.6 × 4.6 | 25.8 m² |
| 4 × 3 | 1.0 | 7.0 × 6.0 | 42.0 m² |
| 5 × 3 | 1.2 | 8.4 × 6.4 | 53.8 m² |
| 6 × 4 | 1.5 | 10.0 × 8.0 | 80.0 m² |
| 8 × 5 | 1.5 | 12.0 × 9.0 | 108.0 m² |
| 10 × 6 | 2.0 | 15.0 × 11.0 | 165.0 m² |
| 15 × 10 | 2.0 | 20.0 × 15.0 | 300.0 m² |
| 20 × 15 | 2.5 | 26.0 × 21.0 | 546.0 m² |
In feet, with 18in overlap:
| Pond size | Depth | Liner needed | Sheet area |
|---|---|---|---|
| 6 × 5 | 2 | 13 × 12 | 156 ft² |
| 10 × 6 | 3 | 19 × 15 | 285 ft² |
| 12 × 8 | 3 | 21 × 17 | 357 ft² |
| 16 × 10 | 4 | 27 × 21 | 567 ft² |
| 20 × 12 | 5 | 33 × 25 | 825 ft² |
| 30 × 20 | 6 | 45 × 35 | 1,575 ft² |
Sheets wider than 7m need welded panels. See geomembrane welding machines and seam methods for how panels are joined.
What thickness pond liner do you need?
Thickness depends on what the pond does and how deep it is. Full technical values for each thickness are on our HDPE geomembrane specification.
| Application | Depth | Thickness | Mil |
|---|---|---|---|
| Small garden pond, water feature | Under 1m | 0.75mm | 30 mil |
| Standard garden pond, small irrigation | Up to 1.2m | 1mm | 40 mil |
| Koi pond, fish pond | Any | 1mm minimum, 1.5mm preferred | 40–60 mil |
| Irrigation reservoir, farm pond | 1–2.5m | 1.5mm | 60 mil |
| Ornamental lake, canal | 1–2.5m | 1.5mm | 60 mil |
| Dam, large reservoir, landfill | Over 2.5m | 2mm+ | 80 mil+ |
Why koi ponds need more
Koi are large, and koi ponds get cleaned, netted and walked into. Anything under 1mm punctures too easily under a boot or a dropped tool. 1.5mm is the sensible choice if the pond will be maintained regularly.
Exposed vs covered
If the liner sits under water permanently it’s protected from UV. Exposed edges above the water line take the most punishment — one more reason the overlap allowance matters, since buried edges last longer than exposed ones.
How much underlay do you need?
The same area as the liner. Underlay goes under the whole sheet, so if you need 42m² of liner you need 42m² of non-woven geotextile.
Do you always need it?
If your excavation is clean, graded sand with no stones, roots, glass or debris, you can lay directly. In practice almost nobody has that. Anything with gravel, construction debris or sharp subsoil needs a cushion layer — and the cost of geotextile is a fraction of the cost of draining a full pond to patch a puncture.
What weight?
200–300 g/m² covers most garden and irrigation ponds. Go heavier for stony subgrades or where the liner will carry point loads.
Calculating liner for other shapes
Circular ponds
Treat the diameter as both length and width. A round pond 4m across and 1m deep with 0.5m overlap needs a 7m × 7m sheet. You will have four large corner offcuts — unavoidable, since liner is supplied as a rectangle.
Oval and kidney-shaped ponds
Use the maximum length and maximum width. Same as irregular shapes: enclose the pond in the smallest rectangle that fits it.
L-shaped ponds
Calculate as a rectangle enclosing the whole L. Splitting it into two rectangles and welding a seam is usually more expensive than accepting the offcut, unless the pond is very large.
Box-welded liners
A box-welded liner is prefabricated to the exact shape of the pond, with welded corners so it sits flush without folds. It costs more and needs precise measurements, but it looks considerably better on formal or raised ponds where folds would be visible.
Frequently asked questions
Liner length = pond length + twice the maximum depth + twice your overlap allowance. Liner width uses the same formula with the pond width. Depth counts twice because the liner goes down one side and up the other.
For a 4m × 3m pond 1m deep with 0.5m overlap, you need a 7m × 6m sheet — 42m². More than three times the pond’s footprint, because of the depth and overlap allowances.
Because a flat sheet has to line a three-dimensional hole. A 1m deep pond consumes 2m of liner in each direction before you’ve covered any of the base.
0.3m (12in) minimum for small garden ponds, 0.5m (18in) for anything larger. If you’re building an anchor trench, follow the trench detail on your drawing instead.
Yes — switch the units to feet at the top. Results show in both feet and metres, and volume in litres, US gallons and UK gallons.
Length × width × average depth gives cubic metres; multiply by 1,000 for litres. The calculator does this automatically and adjusts for sloped sides, which typically hold about two-thirds of the volume the full box would suggest.
0.75mm to 1mm for garden ponds, 1mm to 1.5mm for koi and fish ponds, 1.5mm for irrigation reservoirs and lakes, 2mm and above for dams and heavy containment.
The same area as the liner, since it goes underneath the entire sheet.
The geometry is the same, so the figure will be close. Swimming pools usually use different liner materials and a fitted rather than draped installation, so treat the result as a material estimate rather than a specification.
Yes. We supply HDPE geomembrane in 0.75mm to 3mm on 7-metre rolls, with welding and installation across the UAE and material supply for export.