Pond Liner & Underlay Calculator
Work out how many litres your aquarium or pond holds, plus the equipment you'll need – heater, filter, substrate and salt for tanks; liner, pump, filter and UV for ponds.
Results
- Actual water once set up
- –About 80–90% of the glass volume, after substrate, rocks and the gap at the top.
- Weight of water
- –Plus glass, cabinet, substrate and rockwork – make sure your floor and stand can take it.
- Heater
- –
- Filter flow
- –
- Substrate
- –Approximate, based on sand or gravel at about 1.5 kg per litre.
- Marine salt to fill
- –At around 35 g per litre (salinity about 1.025), mixed with RO water.
- Surface area
- –
- Average depth
- –
- Liner size
-
Minimum –
Length and width plus twice the deepest point, plus 30 cm overlap on each edge.
Standard sizes that fitEPDM rubber–PVC–Underlay–The smallest standard size that covers your pond. Lay underlay the same size as the liner.
- Pump flow
- –
- Filter
- –
- UV clarifier
- –
- Koi stocking guide
- –A rough guide of about 1,000 litres per adult koi – good filtration matters more than the number.
- Weekly water change
- –10–20% a week suits most tanks.
All figures are estimates. Not sure what you need? Call us on 01322 664397 or pop into Birchwood Aquatic Centre in Swanley and we'll help you choose the right equipment.
Rubber or PVC liner – which should I choose?
| EPDM rubber | PVC | |
|---|---|---|
| Price | Higher | Lower – the budget option |
| Flexibility | Very flexible and stretchy – moulds into shelves, curves and awkward shapes | Supple but doesn't stretch – best for simpler shapes |
| Cold weather | Stays flexible in frost | Stiffer in the cold – easiest to fit on a warm day |
| Weight | Heavier – may need two people for big liners | Lighter and easier to handle |
| Lifespan | The longest-lasting choice – ideal for permanent and koi ponds | Good for smaller and wildlife ponds |
| Best for | Koi ponds, large or deep ponds, complex shapes | Small to medium garden ponds and tighter budgets |
Whichever you choose, lay a pond underlay first – it protects the liner from stones and roots, and punctures aren't covered by liner guarantees. Ask us in store and we'll help you pick the right liner and underlay for your pond.
Disclaimer: Our pond liner calculator is intended as a guide only, and the sizes it gives are estimates. The liner you need also depends on the finished shape of your excavation, shelves, edging and how the liner folds into the corners, which can't be factored in. Always measure your finished hole before buying, and allow extra if in doubt.