Build a Garden Pond

Of everything on this site, a garden pond is one of the most forgiving builds: no structure, no permits in most places, and small enough that a mistake is measured in a bag of gravel rather than a wall. But it has exactly one critical sum — the liner. Order it short and the whole dig is wasted, because the sheet that has to reach the bottom, climb both sides and still lap over the edge does not forgive optimistic measuring.

The formula is simple once you see it: liner length is pond length plus twice the maximum depth plus twice the edge overlap, and the same for width. A 3 × 2 m pond at 0.8 m deep with a 30 cm overlap needs a 5.2 × 4.2 m sheet — nearly double the plan size, which is why guessing fails. From there the numbers flow: water volume from average depth, pump flow from volume and a turnover time, gravel for the edging, and mulch for the beds around it.

The sequence below runs in build order. One rule sits above all of it, and it's in the safety section: anything electrical near water — pumps, filters, pond lighting — needs outdoor-rated equipment and, in most countries, a qualified installation.

1. Choose the size and depth. Around 60 cm (2 ft) of depth suits most temperate-climate garden ponds — deeper for koi, shallower for wildlife ramps. Depth feeds every calculation that follows, so decide it first.
2. Shape it and plan the shelves. Marginal planting shelves typically sit 20–30 cm below the waterline. Curves are welcome, but measure the shape as a rectangle or circle so the calculators have something to work with.
3. Size the liner. Length plus twice the depth plus twice the overlap — typically 30 cm (12 in) of overlap per edge, more on soft ground. Underlay goes under the liner and is ordered with about 10% extra area.
4. Work out the water volume. Plan area times average depth gives cubic metres and litres — the number the pump, any treatments and the first fill all work from.
5. Pick the pump by turnover. Typical guidance is a 1–2 hour turnover for koi ponds and 2–4 hours for goldfish and wildlife ponds: volume divided by hours gives the flow in litres per hour.
6. Edge it. Rocks, cobbles or turf hide the liner overlap — gravel by volume from the edge area and depth you choose, with heavier stone where the liner turns vertical.
7. Plant and mulch around it. Beds beside a pond get mulched like any other border, with the mulch kept back from the water's edge so it stays out of the pond.