Pond Liner Calculator – What Size Liner Do I Need?
Enter the pond size and we'll work out the liner and underlay sheet you need, the water volume in litres and a sensible pump flow rate.
Pond Liner Calculator
You need
4.8 × 3.8m liner
3,600 litres of water — about 18.2 m² of liner and the same of underlay
- Water volume
- 3,600 L
- Pump flow
- 1,800 L/h
- Liner area
- 18.2 m²
- Underlay
- 4.8 × 3.8 m
How we worked this out
Your shopping list
19 × m² of EPDM rubber pond liner (4.8m × 3.8m)
Most suppliers cut to size from a roll — order the whole sheet, never joined pieces.
£228
19 × m² of protective underlay fleece (4.8m × 3.8m)
Same size as the liner, laid over a raked, stone-free excavation.
£57.00
1 × Pond pump rated about 1,800 L/h
Check the flow at your actual head height, not the headline figure.
£130
Common pond sizes, liners and volumes
| Pond size | Depth | Liner (300mm overlap) | Volume | Fish pond pump |
|---|---|---|---|---|
| 1.5 × 1 m | 0.45 m | 3.0 × 2.5 m | 675 L | 340 L/h |
| 2 × 1.5 m | 0.6 m | 3.8 × 3.3 m | 1,800 L | 900 L/h |
| 3 × 2 m | 0.6 m | 4.8 × 3.8 m | 3,600 L | 1,800 L/h |
| 4 × 3 m | 0.9 m | 6.4 × 5.4 m | 10,800 L | 5,400 L/h |
| 5 × 3 m | 1.2 m | 8.0 × 6.0 m | 18,000 L | 9,000 L/h |
How this calculator works
A pond liner has to cover the bottom, climb both sides and still have enough spare to tuck under the edging. So the sum is length + (2 × depth) + (2 × overlap) for one dimension, and the same for the other. The 300mm overlap default is generous enough to hold the liner under slabs or turf without leaving it short when the liner settles into the corners.
Volume in litres is simply the space in cubic metres × 1,000. Rectangles and circles are straightforward; for an informal shape we take 80% of the rectangle the pond sits inside, which is close enough for stocking and pump sizing.
Pump flow follows the turnover rule: the whole pond every three hours for wildlife, every two hours for fish, and every hour for koi. That gives a target litres-per-hour figure to shop against.
Worked example: a 3m × 2m fish pond
At 600mm deep with a 300mm overlap, the liner needs to be 3 + 1.2 + 0.6 = 4.8m long and 2 + 1.2 + 0.6 = 3.8m wide — about 18.2m² of sheet. The pond holds 3 × 2 × 0.6 × 1,000 = 3,600 litres, so a fish pond wants a pump moving around 1,800 litres per hour.
In EPDM at about £12 per m², the liner is roughly £220, with another £55 of underlay and £130 for a decent pump. The PVC equivalent halves the liner cost but you will likely be relining in twelve years' time.
Digging and lining without the classic mistakes
Dig the deepest zone first, then cut in the marginal shelf around 250mm below the water line for baskets of iris and marsh marigold. Slope one side gently to a beach so hedgehogs, frogs and birds can get in and out — a pond with vertical sides all round is a wildlife trap.
Check the rim is level all the way round with a straight edge and a spirit level before the liner goes in. A pond 20mm out of level shows a permanent band of liner along one side and no amount of stone edging hides it convincingly.
Rake out every stone and root, lay the underlay, then drape the liner loosely and let the weight of the water pull it into shape. Fold the corners into neat pleats rather than stretching them. Fill with rainwater if you can, or leave tap water to stand for a few days before adding plants and fish.
Tips from the trade counter
- Buy one sheet, never joined pieces. Seams are the usual leak point in a DIY pond.
- Order the liner after you dig. Ponds always end up a little different to the plan — measure the hole, then order.
- Keep it out of full shade and away from trees. Falling leaves rot down and foul the water each autumn.
- Aim for a third of the surface covered in planting. It shades the water, starves the algae and gives wildlife cover.
- Size the pump on real head height. Every metre of lift and every bend in the hose cuts the actual flow.
Ponds and young children
A child can drown in very shallow water. If young children use the garden, fit a rigid steel mesh grid just under the surface, fence the pond securely, or postpone the project until they are older — a pond cover net is not enough on its own.
Electrical supplies to pumps and lights must be RCD protected and suitable for outdoor use. New outdoor circuits are notifiable work under Part P and should be installed by a registered electrician.
Frequently asked questions
What size pond liner do I need?
Liner length = pond length + twice the depth + twice the overlap. Liner width works the same way. For a 3m × 2m pond 0.6m deep with a 300mm overlap, that is 4.8m × 3.8m.
How many litres is my pond?
Length × width × average depth in metres × 1,000. A 3 × 2 × 0.6m rectangular pond holds about 3,600 litres. Informal shapes hold roughly 80% of the box they sit in.
What size pump do I need for a pond?
As a rule of thumb, circulate the whole volume every 3 hours for a wildlife pond, every 2 hours for a fish pond and every hour for koi. A 3,600 litre fish pond therefore wants around 1,800 litres per hour.
EPDM or PVC liner?
EPDM rubber is more flexible, easier to fold into corners, lasts 30+ years and is fish safe — it is what most people should buy. PVC is cheaper but stiffer and typically lasts 10–15 years.
Do I need underlay under a pond liner?
Yes. Protective fleece underlay, the same size as the liner, stops stones and roots puncturing it from below. Old carpet is a poor substitute — it rots and holds sharp grit.
How deep should a pond be?
600mm for a wildlife pond, 900mm for fish, and 1.2m or more for koi so the water stays stable in summer and does not freeze solid in winter. Include a shallow shelf and a sloping beach for wildlife to get out.
Does head height affect my pump?
Yes, significantly. A pump rated 3,000 L/h might only deliver 2,000 L/h once it is lifting water 1.5m to a filter or waterfall. Check the pump's flow curve at your actual head height.