Pond liner calculator
Work out what size pond liner and underlay to buy. The liner has to cross the pond, run down and back up each side, and leave an overlap on every edge to anchor under paving or turf.
The pond liner formula
liner length = pond length + (2 × maximum depth) + (2 × overlap)liner width = pond width + (2 × maximum depth) + (2 × overlap)
Example: a 10 × 8 ft pond, 2 ft deep, with 1 ft overlap needs 10 + 4 + 2 = 14 by 8 + 4 + 2 = 12 ft — and 3 ft deep needs 16 × 14 ft. In metres, a 3 × 2 m pond 0.8 m deep with 0.3 m overlap needs 5.2 × 4.2 m.
Shelves, curves and slopes
- Marginal shelves don't change the size: the liner still drops a total of the maximum depth on each side, just in steps.
- Curved and irregular ponds: use the longest length and widest width. You'll have folds and some waste, but no shortfall.
- Sloped sides need slightly less than the formula — the pond volume calculator works out the slanted side length for the Sloped sides shape.
Overlap and underlay
1 ft (30 cm) per edge is the usual overlap; 1.5–2 ft (45–60 cm) is safer on sandy soil or where heavy edging stones sit on the liner. Buy underlay the same size as the liner to protect it from stones and roots. Liners come in stock widths, so round up to the next size the supplier sells.
Questions
What size liner do I need for a 10 × 8 ft pond?
At 2 ft deep with 1 ft overlap, 14 × 12 ft. At 3 ft deep, 16 × 14 ft. The pond liner size chart lists other sizes.
How much overlap should a pond liner have?
At least 1 ft (30 cm) on each edge, more on loose soil or under heavy stone. Too little overlap is the most common liner mistake — you can trim extra, but you cannot add it.
Do I need underlay under a pond liner?
Yes. Underlay (geotextile fleece) or a layer of sand stops stones and roots puncturing the liner. Use the same dimensions as the liner.