Pond Liners and Underlay

Choose your pond liner and calculate the sheet size

Use our pond liner calculator to estimate the flat sheet needed for your pond, then compare PVC and EPDM liners and add suitable protective underlay. For a simple rectangular pond where neat corners matter, you can also compare a made-to-measure box-weld liner.

Pond liner calculator: what size sheet do I need?

Enter the maximum dimensions of your pond in millimetres. For example, 5m = 5000mm. This estimates a minimum flat-sheet size, including an allowance beyond every edge.

Use the deepest point, measured to the intended anchoring edge—not just the water level. The default 500mm allowance is added at both ends of each sheet dimension. Adjust it to suit your actual edging.

Your estimated sheet size and matching listed options will appear here.

This is a planning estimate, not a guarantee of fit. Check the finished pond with a flexible tape or string along the full profile, including shelves, slopes and the finished edge, in both directions. Use the larger measurement if the actual profile needs more material. Do not stretch an undersized liner or trim it before it has settled.

Browse 0.75mm EPDM sheets · Browse 1mm EPDM sheets · Price a made-to-measure box-weld liner

A box-weld liner is measured differently and includes a 100mm top flap, not this flat-sheet edge allowance. For a complex pond, send us a fully dimensioned plan.

Start with the right lining for your pond

The liner holds the water; the underlay protects it from the surface beneath. Choose a flexible sheet for a pond with curves and shelves, or a made-to-measure box-weld liner for a neat, shaped fit. Then add the protection and fittings your build needs.

Flexible pond liners

Compare 0.5mm PVC, 0.75mm EPDM and 1mm EPDM for garden, wildlife and koi ponds. A flat sheet is shaped and folded into the excavation.

Made-to-measure box weld

Price a custom EPDM liner for a simple rectangular pond. Choose 0.75mm or 1mm, with a 100mm top flap included.

Protective underlay

Add white 150gsm underlay or black heavy-duty 250gsm underlay. Cover the base and sides, allowing for overlaps and the actual pond shape.

Need help choosing? Read our liner and underlay guide below, or send us your measurements and a sketch for a more complicated pond.

Choosing between PVC, EPDM and a box-weld liner

Start with the pond’s shape and the finish you want. A flexible sheet works around curves, planting shelves and gently sloping sides, but needs folds at corners. A box-weld liner is made to fit a measured shape, so it is particularly useful when you want clean lines inside a formal or raised pond.

PVC sheet liner

0.5mm PVC is the budget-conscious option in this range. It is lighter to handle and can be a practical choice for a smaller garden or wildlife pond. Allow for careful preparation, underlay and proper edge finishing.

EPDM rubber liner

EPDM is a flexible rubber lining. Compare 0.75mm EPDM with the thicker 1mm EPDM option when choosing the material for your build. Both still need a smooth, supported surface and protective underlay.

Shaped EPDM liner

Our custom box-weld liners have their floor and walls formed to order. They avoid the bulky corner folds of a flat sheet. Accurate finished internal dimensions are essential.

The liner is the waterproof layer, not a self-supporting tank. It needs continuous support beneath the floor and behind the walls. Extra liner thickness does not compensate for sharp ground, an unstable wall or poor preparation.

How to measure for a flat sheet liner

For a straightforward pond, a useful starting calculation is:

Sheet length = maximum pond length + twice the maximum depth + edge allowance at both ends

Sheet width = maximum pond width + twice the maximum depth + edge allowance at both sides

For example, a pond 4m long × 2m wide × 1m deep needs a starting sheet size of 6.6m × 4.6m if you allow 300mm at each edge. That is an example allowance, not a guarantee of fit: shelves, slopes, the route over the finished contours and your chosen edging can require more material.

Measure the actual excavation and check the edge detail before ordering. Do not stretch a liner to make an undersized sheet fit, and do not trim away the surplus before the liner has settled and the edge levels are confirmed.

A neater finish with a custom box-weld liner

For a simple rectangular pond with vertical sides and one flat floor level, our box-weld liner calculator gives an instant price. Enter the finished internal length, width and depth in millimetres, then choose 0.75mm or 1mm EPDM.

A 100mm top flap is included for the planned coping detail. This is different from the edge allowance used to estimate a flat sheet liner. The usual manufacturing lead time is around 2–3 weeks, depending on factory workload.

Shelves, steps, sloping floors, curved walls and other shapes need a bespoke quotation. Send a fully dimensioned plan through our contact form, including every change of depth and the intended edge finish. Agree any window, drain or pipe connection before manufacture; do not assume openings will be factory cut.

Choose the underlay as well as the liner

Our white 150gsm underlay and black heavy-duty 250gsm underlay sit beneath the liner to help protect it from abrasion and puncture. The gsm figure is the material’s weight per square metre, not its thickness in millimetres.

The black 250gsm option is the heavier-duty choice. Both need a properly prepared surface: remove stones, sharp roots and debris first. Underlay is not a structural layer or a guaranteed barrier against aggressive roots.

Cover the floor, sides and shelves, with sufficient overlap between pieces. Where gravel or stone will sit above the liner, plan an appropriate protective layer there too. Large rocks and heavy features need their own stable support.

The box-weld calculator can add suitable packs of black underlay. These are standard flat packs to cut and overlap, not a shaped second liner. If you select that option, you do not need to order the same underlay again separately.

Plan the fittings before you line the pond

A bottom drain, wall skimmer or viewing window needs the correct support and sealing connection. Decide where these will sit before ordering a shaped liner or cutting a flat sheet.

Choose a liner adhesive, seam or repair system specifically suited to the material and job. A spray bonding adhesive, waterproof seam system and underwater repair sealant are not interchangeable. Check compatibility and the manufacturer’s surface-preparation and curing instructions before use.

Installation and care: common questions

Can I fit a pond liner myself?

Many smaller ponds are manageable with careful preparation. Larger sheets can be heavy, so plan enough help to position them without dragging. Lay the liner relaxed into the base and shelves, arrange broad folds and fill progressively while checking the edges and connections. Follow any adhesive curing instructions before filling. For a large, deep or structurally complex pond, get experienced installation advice.

Can I use sand, carpet or an old liner instead of underlay?

We recommend purpose-made pond underlay on a properly prepared base. Sand can move, old carpet may contain fixings or unsuitable materials, and an old liner can conceal debris or damage. None removes the need to check the surface beneath the new liner.

How should I finish the edges and reduce folds?

Let the liner settle into the pond rather than pulling it tight. Keep enough surplus for your coping, paving, turf or other edge detail, and keep low spots above the intended water level. Arrange flat-sheet folds neatly rather than cutting them out. A box-weld liner offers a shaped alternative when the pond dimensions and construction suit it.

Can I join or repair a liner?

Use the patch, primer and seam system specified for the exact material. Simply overlapping two sheets does not create a waterproof joint. A PVC repair kit is not automatically suitable for EPDM, and a general-purpose sealant is not a universal replacement for a seam system. For large tears, failed seams or ageing material, ask whether replacement is the better option.

What if my pond is losing water?

Check edges, waterfalls, pipework and fittings as well as the liner: not every drop in level is a puncture. Keep fish at a safe water level and maintain filtration and aeration while investigating. Once the leak is identified, follow the relevant repair system’s preparation and curing instructions or arrange a professional repair.

How long will the liner last, and how do I care for it?

Life expectancy depends on the particular product, installation and exposure. Check the individual listing for its specification and guarantee rather than assuming one lifespan applies to every liner. Avoid sharp tools, inspect exposed edges and fittings, and use a repair system matched to the material if damage occurs. Keep a new liner clean and protected before installation.

Can I use these liners for a swimming pond, lake or another project?

Please send the design and intended use first. A large lake, swimming pond, tank or steep-sided installation can need different drainage, anchoring, protection and material specifications. We can help identify an appropriate lining solution rather than choosing solely by sheet thickness.