How to Build a Garden Pond with a Flexible Liner

Where to put your pond

Choose a level spot that gets sun for at least half the day. Some shade helps keep algae down, but a fully shaded pond stays cold and dark. Avoid overhanging trees, which drop leaves and have roots that can damage the liner. Check for buried pipes and cables before digging a large or deep pond. If young children use the garden, think carefully about safety – even shallow water can be a drowning risk.

How deep should a pond be?

Wildlife pond: 25–40cm at the deepest point is plenty. What wildlife really needs is a gently sloping shallow “beach” on at least one side so frogs, hedgehogs and birds can get in and out.

Planting shelves: about 20–30cm deep and 30cm wide for marginal plants in baskets.

Fish pond: needs to be deeper so fish can escape summer heat and winter ice. At least 60cm is commonly advised for goldfish, and much deeper for koi.

How big a liner do you need?

Use this simple formula:

Liner length = pond length + (2 × maximum depth) + overlap

Liner width = pond width + (2 × maximum depth) + overlap

Allow around 30cm of overlap on each edge (60cm in total) to anchor the liner. For example, a pond 3m long, 2m wide and 60cm deep needs a liner of 3 + 1.2 + 0.6 = 4.8m by 2 + 1.2 + 0.6 = 3.8m. Buy an underlay the same size. The Pond Liner Calculator also works out the water volume and a suitable pump size.

Which liner?

EPDM rubber or butyl: flexible, UV-resistant and long-lasting – the best choice for most ponds.

PVC: cheaper but stiffer, with a shorter lifespan.

Pre-formed rigid moulds: easy for small formal ponds, but you're stuck with their shape and depth.

Buy the best liner you can afford – a leak means draining the pond and starting again.

Step-by-step

Mark out the shape with a hosepipe or rope. Curves are easier to line than sharp corners.

Dig in stages. Dig out the whole area to shelf depth first, then dig the deeper central section. Slope the sides gently rather than making them vertical.

Level the edges. Use pegs and a spirit level all the way round. An uneven edge leaves liner showing on one side.

Remove sharp objects. Pick out stones and roots, then spread about 50mm of soft sand over the base and shelves.

Lay the underlay over the sand, pushing it into the contours.

Lay the liner. Drape it loosely into the hole – EPDM is heavy, so get help. Weigh the edges down with smooth stones. Don't cut it yet.

Fill slowly. The weight of the water pulls the liner into shape. Smooth out creases as it fills. Rainwater is best for wildlife ponds; tap water is higher in nutrients and can encourage algae.

Trim and finish the edges. Once full, trim the excess leaving about 30cm, and bury it under turf, soil, stones or paving.

Pumps and filters

Wildlife ponds generally don't need a pump. Fish ponds need a pump and filter. A common guide is a pump that moves the pond's full volume every two hours. So a 3,600-litre pond needs a pump rated at around 1,800 litres per hour – more if it also feeds a waterfall or fountain, because height reduces a pump's flow.