Advanced options
Liner formula: (length + 2 x depth + 0.6m overlap) by (width + 2 x depth + 0.6m). Buy the next stock size up, never down. Butyl and EPDM liners last decades; PVC is cheaper and shorter-lived. Always use underlay (or sand plus old carpet) over every stone the dig reveals.
What The Pond Liner Calculator Works Out
- Liner size needed: length plus twice the maximum depth plus 0.6 m of overlap, by the same for the width. Depth counts twice because the liner travels down and back up, and the overlap becomes the buried anchor at the edges. A 3 m x 2 m pond dug to 0.6 m needs a 4.8 m x 3.8 m liner, over 18 m2 for a modest hole.
- Underlay: the same area in protective fleece (or builders' sand topped with old carpet), the insurance against the one flint the dig missed.
- Approx water volume: length x width x depth, scaled to 70% because real ponds with shelves and sloping sides hold far less than the box. Volume sizes the pump, the treatments and the plants.
- Edge overlap (advanced): edit the 30 cm each side. Paved edges hidden under slabs can manage 20 cm; a planted bog edge or a turf edge that will be buried wants 40 cm or more.
- Pond shape (advanced): sets the volume factor: 70% for an irregular pond with marginal shelves, 80% for an oval or kidney, 100% for a straight-sided formal rectangle.
- Stock size to buy (advanced): liners sell in stock widths in half-metre steps up to 5 or 6 m and then whole metres; each dimension is rounded up to the next stock size, never down, and the surplus is trimmed after filling.
- Liner material and cost (advanced): the stock area at a price per m2 for your chosen material. Typical UK prices are about GBP 5 per m2 for 0.5mm PVC (10 to 15 years), GBP 9 for EPDM rubber (25 years plus) and GBP 12 for butyl, which stretches into contours and outlives its owner. The table compares all three for your size.
- Underlay cost (advanced): fleece underlay at about GBP 1.50 per m2 for the same stock area, with a combined liner plus underlay figure.
- Pump flow needed (advanced): a wildlife pond needs no pump and outperforms fountains for biodiversity per pound. A goldfish pond wants a pump that turns over half the volume every hour, a koi pond the full volume, with the filter sized to match.
- Time to fill (advanced): the volume at a typical hose flow of 15 litres a minute, plus the metered water cost at about GBP 2.50 per m3. Fill slowly onto the liner so it settles with few wrinkles, and let tap water stand a few days before planting.
Further reading: RHS: garden ponds.