Free pond planner
Pond Liner Size Calculator & Complete Buying Guide UK
Work out your liner and underlay size (or pond sealer for a concrete pond), pond volume, pump and filter in one go, matched to the sizes and prices we stock today. Metric or imperial.
Sealing a rendered concrete pond without a liner? Work out how much pond sealer you need →
Pond calculator questions
How do I calculate what size pond liner I need?
Liner length = pond length + (2 × maximum depth) + 0.6m, and liner width = pond width + (2 × maximum depth) + 0.6m. The 0.6m gives a 30cm edge on every side. A 3m × 2m pond 0.6m deep needs at least 4.8m × 3.8m, so choose 5m × 4m.
How much edge overlap do I need?
30cm on each edge is standard. Allow 50cm if you are finishing with paving or rocks, and add the depth and width of an anchor trench if you use one.
Do pond shelves need extra liner?
No. Size from the maximum depth and the shelves are covered. Shelves do reduce the water volume, which the planner allows for.
What size pump do I need?
Turn the pond volume over about every 2 hours for ornamental and goldfish ponds and every hour for koi, then allow extra for pipework and any waterfall height. Wildlife ponds do not need a pump.
What size filter do I need?
Choose a filter rated for more than your pond volume: about 1 × for plant ponds, 1.5 × for goldfish and 2 × for koi, and size up again for a pond in full sun.
How much pond sealer do I need for a concrete pond?
Work out the inside area (floor plus walls) and multiply by 0.55kg per m² for three coats of clear pond sealer. A 3m × 2m × 1m pond has 16m² inside, so it needs about 8.8kg.
How much underlay do I need?
Order underlay in the same size as the liner. If you join pieces, overlap them by at least 300mm.
How to Calculate Your Pond Liner Size — Complete UK Guide
Getting the right pond liner size is the single most critical step in building a successful garden pond. Too small and you'll face leaks at the edges; too large and you'll waste money on unused material. This guide explains exactly how to calculate the liner you need, and which type suits your specific pond.
The Pond Liner Size Formula Explained
The formula is straightforward once you understand the logic. A pond liner must cover the full base of your pond, travel up every side to ground level, and then fold over the edge to be buried or concealed under edging stones. This is why depth matters so much — a 1m deep pond requires the liner to travel 1m down each side twice (once per side), adding 2m to each dimension. The edge allowance (typically 0.3m each side) ensures you have enough material to anchor the liner securely.
Liner Length = Pond Length + (2 × Depth) + (2 × Overlap)
Liner Width = Pond Width + (2 × Depth) + (2 × Overlap)
For a pond that is 4m long, 3m wide, and 0.75m deep with the standard 0.3m edge allowance, you would need a liner of at least 6.1m × 5.1m. Always round up to the next available panel size — cutting down is simple; a liner that's 10cm too short means starting from scratch.
Accounting for Pond Shelves
Many garden ponds include planting shelves — shallow steps around the perimeter that allow marginal plants to be positioned at varying depths. Shelves do not need extra liner when you size from the maximum depth: the liner steps down each shelf, but the total drop is still the maximum depth and the shelf widths are already inside the pond length and width. Our planner also adjusts the pond volume for shelves and sloping sides.
Choosing the Right Liner for Your Pond Size
The material you choose should reflect both your pond size and how it will be used. For small ornamental ponds under 15m², PVC 0.3mm or LDPE offer excellent value and flexibility. For medium ponds between 15-50m², HDPE 0.35mm or PVC 0.5mm provide superior durability without excessive cost. For large ponds over 50m², HDPE 40yr or EPDM offer the best long-term value — their extended guarantees make them cost-effective over time despite higher upfront cost. For koi ponds specifically, fish-safe EPDM or Butyl are the professional's choice.
Why Underlay is Non-Negotiable
Every UK pond liner survey shows that punctures from sharp stones are the leading cause of liner failure, ahead of UV degradation and root damage. The cost of a 200gsm geotextile underlay is negligible compared to the cost of replacing a liner — or worse, losing a pond full of fish and established plants. Always lay underlay before your liner, cut it to the same dimensions as your liner, and ensure complete coverage of the excavation including the shelves.
Metric vs Imperial — A Note for UK Builders
All pond liners in the UK are sold in metric dimensions (metres), so always calculate in metres. If you measured in feet and inches, switch the planner to Imperial and it converts for you (1 foot ≈ 0.305 metres). Liners are typically available in increments of 0.5m or 1m, so you'll usually be rounding up by a small margin regardless.