Written by the Pondliner.org team · reviewed
Measuring is the only step you cannot redo later. Get the maths right at the planning stage and the install becomes straightforward; get it wrong and you're either short on liner at the worst possible moment, or paying for square metres of polymer you'll trim into the bin.
The formula
- Liner length = pond length + (2 × max depth) + overlap
- Liner width = pond width + (2 × max depth) + overlap
- Overlap = 30 cm minimum each side (so add 0.6 m total)
The "2 × max depth" accounts for the liner draping down one side of the excavation and back up the other. The 30 cm overlap per side gives you something to anchor under coping stones, turf or a flagstone edge — and a small margin for the inevitable adjustment as the liner settles under water weight.
Worked example
A 3 m × 2 m pond with a max depth of 0.6 m needs a liner of (3 + 1.2 + 0.6) × (2 + 1.2 + 0.6) = 4.8 m × 3.8 m. Round up to the nearest standard roll width.
A bigger example: a 6 m × 4 m wildlife pond at 1.0 m max depth needs (6 + 2.0 + 0.6) × (4 + 2.0 + 0.6) = 8.6 m × 6.6 m. That's about 57 m² of liner, which on RPM 0.7 mm typically ships as a single welded sheet to avoid seams in the deep zone.
Shelves and irregular shapes
Measure from the deepest point — shelves don't add to the depth figure, they're already inside it. For kidney or natural shapes, measure the bounding rectangle and the formula still works. You'll trim more off, but the alternative (trying to template a complex shape) almost always leaves you short.
Don't forget the underlay
Order underlay to exactly the same length and width as the liner. The two products work as a system: any area the underlay doesn't cover is an area where a stone can find your liner. For weight selection and where to step up, see our guide on whether you need pond underlay.
How thickness changes the answer
The formula above sizes the sheet. The thickness you choose then sets the cost per square metre. For most UK gardens that's 0.7 mm reinforced — see how thick should a pond liner be for the full breakdown, or our pond liner comparison if you're still choosing between materials.
Frequently asked questions
Why add twice the depth to the liner length and width?
Because the liner has to drape down one side of the excavation and back up the other. A 0.6 m deep pond needs an extra 1.2 m of liner in each direction just to reach the opposite edge.
How much overlap should I leave around the edge?
A minimum of 30 cm per side, so add 0.6 m to both the length and the width. That gives enough to anchor under coping stones and absorb the small movement as the liner settles.
Do I measure underlay the same way?
Yes — order underlay to the exact same length and width as the liner. Any area the underlay doesn't cover is an area where a stone can reach the liner from below.
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