Pond liner calculator liner, underlay and litres

How big a liner do you need?

Liner length is the pond's longest measurement plus twice its deepest point plus the overlap. Liner width is the widest measurement plus twice the depth plus the overlap. The depth counts twice because the liner goes down one side, across the bottom and up the other.

The overlap here is the total for both edges, so 0.6m means 30cm tucked under the edging on each side. The calculator then rounds each side up to the next half metre as a buying size, and shows the underlay to match. Separately, it works out the water volume from the average depth, because a pond with shelves holds less than a box of the same size.

Worked example: a garden pond 3.0m long and 2.0m wide, 0.75m at its deepest, about 0.5m deep on average, with the default 0.6m overlap.

• Liner length: 3.0 + (2 × 0.75) + 0.6 = 5.1m.
• Liner width: 2.0 + (2 × 0.75) + 0.6 = 4.1m.
• To buy: rounded up to 5.5m × 4.5m. The exact sheet is 20.91m².
• Underlay: match the liner, so about 21m².
• Water: 3.0 × 2.0 × 0.5 = 3.0m³, which is 3,000 litres. At its deepest everywhere it would hold 4,500.
• Pump flow: 1,500 litres an hour to turn the water over every two hours, or 3,000 for once an hour in a pond with plenty of fish.

How much overlap to allow

Allow 30cm on each edge, 0.6m in total, unless the edging needs more. Sellers' calculators disagree, which is why the overlap is a setting you can change rather than a fixed number.

World of Water adds 30cm to the total sheet size to allow for overlapping the edge. Urban Water also gives 30cm as the usual overlap. Pondkeeper adds a full metre to each dimension. The RSPB's wildlife pond guide adds nothing in its formula, then tells you to cut the liner leaving about 30cm of overlap.

The RSPB can do that because sloping sides are shorter than the two full depths the formula allows, so the spare ends up at the edge. A pond with vertical sides and a slab or stone edge needs the full overlap, so the edging has enough liner under it to hold it down.

Measure the hole, not the plan. Stretch the tape straight across the dug pond at its longest and widest points, then measure down at the deepest. If the pond is not dug yet, mark it out and measure the shape you want. An irregular or kidney shape still takes a rectangular sheet: measure the rectangle around it.

Underlay under the liner

Buy the same area of underlay as liner. Pondkeeper's underlay guide says matching the liner size works in most cases, so the calculator shows the liner's area as the underlay figure.

Lay it in pieces if you need to. Pondkeeper overlaps the sheets by 7 to 10cm so no ground shows between them, and says rough patches may need the underlay two, three, four or more pieces deep. Pondkeeper also suggests a layer of sand where the base is uneven.

Before any of it goes down, pick out stones and cut back roots. The underlay is a cushion, not armour, and a sharp stone left in the base can still work its way through.

Litres, pumps and filters

Litres are length × width × average depth × 1,000. For a round pond, pick Round and enter the diameter: the calculator multiplies by 0.785, because a circle fills about 78.5% of the square around it. An oval or kidney pond goes in as the rectangle around it, so its litres come out on the high side. Use the average depth here, not the deepest point, or you will size the pump for water that is not there.

To get the average, take depth readings in a few places across the pond, including the shelves, and add them up. Divide by the number of readings.

Then size the pump from the volume. All Pond Solutions says heavily stocked koi ponds should turn over at least once an hour, and lighter decorative ponds ideally at least once every two hours. It also says the pump's flow should stay within the filter's maximum, and that the filter should be able to process 150% of the pond's volume.

A pump moves less water the higher it has to push it, up to a filter or a waterfall. Read the maker's flow chart at your height, not the headline figure on the box.

Depth, and buying the sheet

Decide the depth before you dig, because it adds twice over to the liner. World of Water suggests at least 90cm for larger fish, and that a more natural pond with smaller fish could be 50cm deep. On the 3m by 2m pond, going from 0.75m to 0.9m deep adds 30cm to both sides of the liner.

Liners are sold in set sizes, so you will nearly always buy a little more than the formula. Pond Liners Online prices its PVC liner rounded up to the nearest metre on the width and the nearest half metre on the length. Rubber liners such as EPDM come in their own set sizes. Check the seller's list and take the next size up on both sides.

Spare liner is not wasted. Folds take up the slack on curves, and an extra strip at the edge gives you room to adjust the water level after the pond settles.

What the calculator does not cover

It sizes one flexible sheet for a dug pond. It does not design a koi pond, a raised pond with walls, or a pond with a bottom drain, all of which need their own planning.

Landscaping round the new pond? The turf calculator works out the rolls for the lawn around it. To price the job for a customer, put the liner, underlay, pump, edging and labour on the quote template.

• No stocking advice. How many fish a pond can carry depends on the filter and the fish. Ask the pump and filter maker.
• No spoil. A 3,000 litre pond means three cubic metres of soil to dig out and move.
• No edging, pipework or electrics. Outdoor pump circuits are electrical work. Use a qualified electrician.

Common questions