How the pond calculator works
Before you dig, a pond needs three numbers: the water it holds, the liner sheet that lines the hole in one piece, and the pump flow through the filter. The deepest point sizes the liner; the average depth sizes the water.
Liner = (length + 2 × depth + 2 × overlap) × (width + 2 × depth + 2 × overlap)The liner has to run down one side wall, across the bottom and back up the other side, which is why the depth is counted twice in each direction, using the maximum depth. The overlap is the flap that lies flat around the rim and gets buried under edging stones or turf; 30 cm (12 in) per side stops it slipping into the water. Volume is length × width × average depth multiplied by a shape factor: 1 for a box, 0.785 (π ÷ 4) for a round or oval pond, and about 0.8 as a rough estimate for kidney shapes. The pump figure is the volume divided by the turnover time you choose.
Worked example
A rectangular goldfish pond is 3 m long and 2 m wide, 0.8 m at its deepest, with planting shelves that bring the average depth to 0.6 m. It holds 3 × 2 × 0.6 = 3.6 m³, or 3,600 litres, which also weighs 3.6 tonnes. The liner follows the deepest point: 3 + 1.6 + 0.6 = 5.2 m long and 2 + 1.6 + 0.6 = 4.2 m wide, a sheet of 21.84 m². Circulating the whole volume every two hours needs a pump that really delivers 1,800 L/h at the height it lifts water to the filter. The 0.8 m maximum would overstate the water by a third. In feet, a 10 × 6.5 ft pond 2.5 ft deep, averaging 1.9 ft, holds about 924 US gallons and needs a 17 × 13.5 ft liner.
Liner sizes for common ponds
| Pond (L × W) | Max / average depth | Water | Liner with 30 cm / 1 ft overlap |
|---|---|---|---|
| 2 × 1.5 m | 0.6 / 0.45 m | 1,350 L | 3.8 × 3.3 m |
| 3 × 2 m | 0.8 / 0.6 m | 3,600 L | 5.2 × 4.2 m |
| 4 × 3 m | 1.0 / 0.75 m | 9,000 L | 6.6 × 5.6 m |
| 5 × 4 m (koi) | 1.2 / 1.0 m | 20,000 L | 8.0 × 7.0 m |
| 8 × 6 ft | 2 / 1.5 ft | 539 gal | 14 × 12 ft |
| 12 × 8 ft | 3 / 2.25 ft | 1,616 gal | 20 × 16 ft |
Shelves and sloping walls typically bring the average depth to 60–80% of the maximum; the liner does not shrink, because the deepest point sets it.
Choosing the pump
Koi produce a lot of waste, so their water should pass through the filter about once an hour; goldfish ponds manage with once every two hours, and a wildlife pond with plants and no fish needs only a gentle flow. Pump boxes quote the maximum flow at zero lift; every metre (3 ft) of lift and every long narrow hose cuts it, so read the flow curve at your real head height. A waterfall adds its own demand: allow roughly 1,500 L/h for every 10 cm of spill width (about 100 gal/h per inch).
Mistakes to avoid
- Measuring the surface outline only. A pond with steep 1 m walls needs a liner 2 m longer and 2 m wider than its surface, before any overlap.
- Skipping the underlay. Lay protective fleece under the liner after removing sharp stones and roots; one puncture means draining the pond.
- Digging too shallow for fish. In cold climates keep at least one zone 75–90 cm (30–36 in) deep so fish can overwinter below the ice.
- Ignoring buried cables. Check for services before digging and have outdoor pump wiring installed on a protected circuit by an electrician.
Frequently asked questions
What size pond liner do I need for a 3 x 2 m pond?
For a 3 × 2 m pond 0.8 m deep with 30 cm of overlap each side, order a liner of about 5.2 × 4.2 m. Deeper ponds need 2 × the extra depth added to both dimensions.
How many gallons is my pond?
Multiply length × width × average depth in feet, then by 7.48. A 10 × 6.5 ft pond 2.5 ft deep holds about 1,216 US gallons if the sides are straight.
What size pump do I need for a 1,000 gallon pond?
For goldfish, aim for about 500 gallons per hour of real flow at your head height; for koi, about 1,000 gal/h. Choose the pump from its flow curve, not the maximum figure on the box.
How deep should a garden pond be?
A wildlife pond works at 45–60 cm (18–24 in) with shallow edges, a goldfish pond at 60–90 cm (2–3 ft), and koi ponds usually at 1.2 m (4 ft) or more.
How much overlap should a pond liner have?
Leave 30 cm (12 in) of liner flat around the whole edge. It is trimmed after the pond is filled and has settled, then hidden under stones or turf.
Last reviewed: 2026-09-27 · How we calculate