Pond Mosaic

Pond liner, volume & wildlife pond tools

Metres and feet, side by side

Pond liner calculator

How much pond liner do you need? Length + 2 × deepest depth + 2 × overlap, and the same for the width. Enter your pond below for the liner and underlay size, the nearest stock sizes and a to-scale drawing.

Units
Pond shape

The maximum depth, even if most of the pond is shallower or has shelves.

Stock sizes to compare

These are example sizes: paste your supplier's list, such as 4x3, 5x4.

Result

Liner size

4.80 m × 3.80 m

15 ft 9 in × 12 ft 6 in · 18.24 m² (196.3 sq ft)

Length
3.00 + 2 × 0.60 + 2 × 0.30 = 4.80 m
Width
2.00 + 2 × 0.60 + 2 × 0.30 = 3.80 m
Underlay
The same size: 4.80 m × 3.80 m
Pond surface
6 m² (64.6 sq ft)

Example stock sizes examples, not a supplier’s list

Smallest that fits: 5 × 4 m · 20 m², 1.76 m² to spare
Next size up: 5.5 × 4 m · 22 m², 3.76 m² to spare

From a PondGard EPDM roll

15 ft (4.57 m) wide, cut to 16 ft (4.88 m) (standard widths 10 to 50 ft; lengths rounded up to whole feet)

Liner laid flat, to scale
4.80 m3.80 mpond floor
Cross-section, to scale
2.00 m wide0.60 moverlapshelf

More pond tools

Autumn and winter at the pond

Net ponds against falling leaves in autumn, stop feeding fish once cold weather sets in, and never smash ice: melt a hole with a pan of hot water instead. Winter is also a good time to dig a wildlife pond, so autumn and winter rain can fill it ready for spring. Winter pond care · Plan a wildlife pond

How much pond liner do I need?

A flexible liner has to run down both sides of the hole, across the floor and over the edge on both sides. So for each direction:

Liner length = longest length + 2 × deepest depth + 2 × overlap

The width works the same way with the widest width. For a 3 × 2 m pond, 0.6 m deep, with 30 cm of overlap: 3 + 1.2 + 0.6 = 4.8 m by 2 + 1.2 + 0.6 = 3.8 m.

Sources

  1. Firestone Building Products, PondGard EPDM pond liner brochure (item 1947, 2014): “How to calculate quantity of liner”, width plus 2 × depth plus 2 extra feet, and the standard roll widths of 10, 15, 20, 25, 30, 40 and 50 ft.
  2. Freshwater Habitats Trust, Amphibian and Reptile Conservation and World of Water, Creating Garden Ponds for Wildlife (2023): (length + 2 depths + 30 cm) by (width + 2 depths + 30 cm).
  3. RHS, How to make a new pond: the liner should overlap the edge by about 15 cm (6 in) all round.
  4. Bradshaws, How much pond liner do I need? (21 November 2019): length + 2 × depth + 1 m, an overlap of 50 cm on each side; underlay = the liner's length × width.
  5. Webb's Water Gardens, How to size a pond liner (20 February 2024): length + 2 × depth + 2 ft, a foot each side; the same square footage of underlay as liner.
  6. Exact conversions: 1 ft = 0.3048 m (NIST Handbook 44, Appendix C; Weights and Measures Act 1985, Schedule 1).

The stock sizes in the calculator are examples to show how the comparison works, not any supplier's range: replace them with your supplier's sizes.

Pond liner questions

How do I work out pond liner size?

Add twice the deepest depth and twice the overlap to the pond's longest length, then do the same with its widest width. A 3 × 2 m pond, 0.6 m deep, with 30 cm overlap needs a 4.8 × 3.8 m liner.

How much overlap should a pond liner have?

The RHS suggests about 15 cm (6 in) all round. Liner makers and sellers often allow more: Firestone adds a foot on each side, and Bradshaws 50 cm, which gives more to tuck under edging.

Do marginal shelves need extra liner?

No. The liner follows the steps down, but the total is never more than a straight drop to the deepest point, so sizing from the maximum depth already covers shelves.

What size underlay do I need?

The same size as the liner. Underlay goes under the whole liner, up the sides and under the overlap, so buy the same length and width, or the same area.