EPDM liner: calculate and order the right size

In short

• Formula: liner size = pond size + 2 × maximum depth + 2 × edging (30 to 50 cm on each side).
• Calculate the length and the width.
• The EPDM liner is sold by the m² in a given roll width: length obtained = ordered area ÷ width.
• Allow for the protective underlay over the same area.
• Liners and underlays are cut to size: they cannot be returned or exchanged. Check your measurements before ordering.

To order the right EPDM liner, measure the maximum length and width of the pond, add twice the maximum depth, then an edging of 30 to 50 cm on each side. You get the liner dimensions. You then need to choose the appropriate roll width and convert the total into m². Here is the complete method, with a numerical example.

Why the pond area is not enough

The liner must cover the bottom, go up along the walls, and extend around the perimeter to be anchored. A 4 m × 3 m pond covers 12 m² on the ground, but its liner is much larger. The most common mistake is to order the area seen from above: the liner will then be too short on one side.

The calculation formula

• Liner length = maximum pond length + 2 × maximum depth + 2 × edging.
• Liner width = maximum pond width + 2 × maximum depth + 2 × edging.

The edging is used for anchoring: allow 30 to 50 cm on each side. Take the upper end of the range if the ground is uneven or if you want to cover the liner with stones or soil.

Always use the maximum depth, even if part of the pond is shallower. Plant shelves do not reduce the size of the liner.

Numerical example

Pond 4 m long, 3 m wide, 1 m maximum depth. 50 cm edging on each side.

1. Length: 4 + (2 × 1) + (2 × 0.5) = 7 m.
2. Width: 3 + (2 × 1) + (2 × 0.5) = 6 m.
3. Liner required: 7 m × 6 m.

Choosing the width and converting to m²

Our Aquitechnics 1.1 mm EPDM liner is available in several roll widths: 3.05 m, 4.27 m, 4.58 m, 4.88 m, 6.10 m, 7.62 m, 8 m, 9.15 m, 10 m, 12.20 m and 15.25 m.

1. Take the smallest of your two calculated dimensions (here 6 m).
2. Choose the first roll width equal to or greater than (here 6.10 m).
3. The other dimension becomes the length to cut (here 7 m).
4. Area to order = width × length: 6.10 × 7 = 42.7 m².

The quantity is ordered in m². The delivered length corresponds to the ordered area divided by the chosen width. Before confirming, redo this calculation in reverse: 42.7 ÷ 6.10 = 7 m. If the result is shorter than your requirement, increase the area.

What if my pond is wider than the largest roll?

Two solutions: join several sheets with the joining products designed for EPDM, or request a custom-made EPDM liner. For custom-made, send a detailed plan of your pond to support@foudebassin.com.

For very large bodies of water, also see our article on the maximum dimensions of an EPDM liner.

Standard or custom-made pack: how to decide?

Once your two dimensions are calculated, two questions are enough to know if the pack covers your pond. The pack is ordered in roll width × length: its largest width is 15.25 m and its largest length is 25 m. Beyond that, you need to go with custom-made or a joining of sheets.

For our example (7 m × 6 m), the smallest dimension is 6 m, therefore less than 15.25 m, and the largest is 7 m, therefore less than 25 m: the pack is suitable, in 6.10 m wide and 7 m long.

The protective underlay

The underlay is placed under the liner. It protects the EPDM from stones, roots, and pressure points. It is not optional on stony ground.

• Order at least the same area as the liner.
• The Premium 400 g/m² underlay is sold by the m² in a 2 m width. It is laid in strips that slightly overlap.
• The EPDM liner + underlay pack combines both in a single order.

Laying the underlay then the liner: the steps

A well-sized liner is easier to lay: the edging extends everywhere and the folds remain few. Here is the laying order we recommend, once the hole is finished and the measurements confirmed.

1. Prepare the bottom. Remove stones, roots, and sharp edges, then compact the bottom, the walls, and the shelves.
2. Unroll the underlay. Lay the 2 m strips side by side with a slight overlap, from the bottom to the edging, without leaving any bare earth area.
3. Present the liner. Unfold it next to the hole. In mild weather, EPDM is more flexible and settles more easily.
4. Center it with several people. Place it over the hole and check that the edging extends on all sides before letting it down. Avoid walking on it with shoes with lugged soles.
5. Form the folds. In the corners and on the shelves, prefer a few large folds to many small ones.
6. Fill gradually. The water presses the liner against the underlay: adjust the folds as the filling progresses.
7. Cut back and anchor the edging only when the pond is full, then cover it with stones, soil, or a border.

To go further on this point, read our guide to avoid folds in an EPDM liner.

Before ordering: the checklist

1. Dig first, then measure. The final dimensions of the hole are often different from the plan.
2. Measure the length and width at the furthest points, with a tape measure placed at the bottom of the hole.
3. Measure the actual maximum depth.
4. Apply the formula, then round up.
5. Check the conversion in m² with the chosen width.
6. Add the underlay.
7. In case of doubt, send us your measurements before placing the order.

Note before opening the pack sheet:

• Maximum length of the hole: ……… m
• Maximum width of the hole: ……… m
• Actual maximum depth: ……… m
• Retained edging on each side: ……… cm
• Calculated liner dimensions: ……… m × ……… m
• Chosen roll width and length rounded to the next half-meter: ……… m × ……… m

Important: a cut liner cannot be returned or exchanged

EPDM and PVC liners, as well as protective underlays, are cut to your measurements. They cannot be returned or exchanged. This is why we advise you to have your calculation checked before ordering. A slightly oversized liner is easily trimmed during installation. A liner that is too small cannot be fixed.

Delivery of large liners

Large-dimension liners and underlays are delivered to your home by a specific carrier. Delivery is free in Belgium; to other European countries, the shipping costs are calculated automatically in the cart according to the ordered dimensions. In case of doubt, contact support@foudebassin.com or call us at +32 10 86 03 54. The shipping time is indicated on the product sheet.

Edge finishing and equipment: what else to plan

The liner ensures the waterproofing, but it is the edge that makes the visual success of the pond. The anchoring edging provided in the calculation must be covered to protect the EPDM and make it invisible: flat stones, planted soil, pebbles, or flexible edging.

• Pond edging: for a clean edge between the lawn and the pond, see the Ecolat edging, in a roll with its stakes.
• Pump, filtration, lighting: equip your entire pond at once based on its dimensions.
• Any doubt about the calculation or the finishing? Call an expert at +32 10 86 03 54.

Our ideas for hiding the edge are gathered in the article How to hide the liner of my pond?

Need advice?

Send us the length, width, and maximum depth of your pond, or a dimensioned sketch. We will check the calculation with you.

Phone: +32 10 86 03 54 – E-mail: support@foudebassin.com. Response in business days.

See also: all our EPDM liners and our guide to avoid folds in an EPDM liner.

Frequently asked questions

Should I count the depth twice?

Yes. The liner goes down one wall and up the opposite wall. We therefore add twice the maximum depth, in both the length and the width.

My pond has shelves: can I reduce the size of the liner?

No. The shelves lengthen the path of the liner rather than shortening it. Keep the formula with the maximum depth.

Can I flip a liner if I got the wrong dimension?

No. The EPDM and PVC liners and underlays, cut to size, cannot be returned or exchanged. Have your measurements checked before ordering.

Should the underlay have the same width as the liner?

No. What matters is the covered area. The 2 m wide underlay is laid in several strips, side by side, with a slight overlap.

What dimensions to choose in the pack for a 4 m × 3 m pond?

With 1 m maximum depth and 50 cm edging on each side, the liner must measure 7 m × 6 m. In the pack, choose the roll width of 6.10 m and a length of 7 m.

Up to what size can you order the liner + underlay pack?

The largest roll width offered is 15.25 m and the length goes from 1 to 25 m, in 0.5 m increments. Beyond that, request a custom-made EPDM liner or contact us for a joining of sheets.

My calculated length falls between two values: what should I do?

Always round up to the next half-metre. A slightly oversized liner can be trimmed during installation, but a liner that is too short cannot be made up.

How do I measure a freeform pond?

Measure the length and width at the points furthest from the hole, with the tape measure laid on the bottom, then apply the formula as you would for a rectangular pond.