How do I calculate the size of pond liner I need?

The formula for pond liner size is: Liner length = Pond length + (2 x Maximum depth) + 600mm overlap. Apply the same formula to the width. The overlap allows 300mm on each side for securing the liner at the edge. For a pond that is 3m long, 2m wide and 0.6m deep, you need a liner at least 4.8m x 3.8m.

Do I need pond underlay?

Yes, pond underlay (also called protective fleece) is strongly recommended under every pond liner. It protects the liner from punctures caused by stones, roots, and sharp objects in the soil. Underlay should be the same size as the liner. The cost of underlay is minimal compared to the cost of repairing or replacing a damaged liner.

What is the best pond liner material?

Butyl rubber (EPDM) is the best pond liner material for UK garden ponds. It is flexible, UV-resistant, and typically lasts 25-50 years. Standard thickness is 0.75mm for garden ponds, or 1.0mm for large or koi ponds. PVC liners are cheaper but have a shorter lifespan of 10-15 years and become brittle in cold weather.

How much overlap should I leave around the pond edge?

Allow at least 300mm (12 inches) of overlap on all sides of the pond. This excess liner is used to secure the edge using rocks, paving slabs, or turf. Our calculator adds a total of 600mm (300mm per side) to both the length and width dimensions. For wildlife ponds with gently sloping edges, you may want to add extra.

What size liner do I need for a circular pond?

For a circular pond, calculate the liner as a square: diameter + (2 x depth) + 600mm overlap. For example, a circular pond 2m in diameter and 0.5m deep needs a liner at least 3.6m x 3.6m. You cannot use a circular liner — you need a square piece that is large enough to cover the entire circle with overlap.

TradeCalcs

Pond Liner Calculator — What Size Pond Liner Do I Need?

Calculate the exact pond liner size with depth and overlap allowance for rectangular, circular and kidney-shaped garden ponds.

Pond Length

Longest dimension

Pond Width

Widest dimension

Maximum Depth

Deepest point of the pond

Pond Shape

Include pond underlay

Protective fleece beneath the liner — strongly recommended

Liner Price Per m² (optional)

Enter your supplier price for a cost estimate

How We Calculate This

Our pond liner calculator uses the standard pond liner formula to determine the minimum liner size you need, accounting for the pond depth and edge overlap.

The formula

Liner length = Pond length + (2 × Max depth) + 0.6m

Liner width = Pond width + (2 × Max depth) + 0.6m

Liner area = Liner length × Liner width

By pond shape

Rectangular: Standard formula as above
Circular: Use diameter for both length and width — liner is always square
Kidney/irregular: Use maximum length and width measurements — same formula

Key notes

Overlap: 300mm (12") per side — 600mm total added to each dimension
Underlay: Same dimensions as the liner
Liner sizes are rounded up to the nearest 0.5m for purchasing

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.