Sizing
Pond liner calculator
What size pond liner do you need?
To calculate pond liner size, add twice the maximum depth and twice your edge allowance to both the length and the width of the pond. With a 30cm allowance: liner length = pond length + (2 × depth) + 0.6m, and the same for width. A pond 3m long, 2m wide and 0.6m deep needs a liner 4.8m by 3.8m, which is 18.2 square metres. Order pond liner underlay to match.
How to calculate pond liner size
A flexible pond liner has to run down one wall of the excavation, across the base and back up the opposite wall. That is two depths of material in each direction, which is why the depth of your pond is doubled and then added to both the length and the width. It is the step people miss, and it is why the sheet is always much larger than the surface of the water.
The edge allowance is the material left flat around the rim so you can hold it down with stone, turf or paving. Thirty centimetres each side is standard and is added twice for the same reason the depth is. Allow more if the edging is heavy or you want a bog garden, because you cannot stretch a liner to make up a shortfall and you cannot add to it later.
Take the width and depth measurements from the deepest point of the hole. Marginal shelves sit inside the overall dimensions of your pond and the liner folds to fit them, so they need no separate estimate.
Pond liner size chart
Liner sizes for common garden pond dimensions, with a 30cm edge allowance each side.
| Pond size | Pond depth | Pond liner size | Area |
|---|---|---|---|
| 2m x 1.5m | 0.5m | 3.6m x 3.1m | 11.2m² |
| 3m x 2m | 0.6m | 4.8m x 3.8m | 18.2m² |
| 4m x 3m | 0.75m | 6.1m x 5.1m | 31.1m² |
| 5m x 3m | 0.9m | 7.4m x 5.4m | 40.0m² |
| 6m x 4m | 1.2m | 9.0m x 7.0m | 63.0m² |
A small pond for wildlife and a large pond for koi are calculated exactly the same way. Only the depth changes, and depth is the measurement that moves the answer most.
Best pond liner material: EPDM, butyl and rubber
The calculation gives you a sheet size. It does not tell you which pond liner material to buy, and the size you need does not change with the material you choose.
For a garden pond liner, EPDM rubber is the usual choice, sold at 0.75mm and 1.02mm. Butyl pond liner is typically 0.75mm and is a little more forgiving around complex shapes. PVC is the budget option at 0.5mm and the least resistant to sunlight, which matters because the strip above the water line is the part that degrades first. A thicker grade buys durability, not area.
Treat published guarantees carefully. Firestone publishes a 20 year warranty on its PondGard EPDM rubber pond liner, but liner guarantees generally exclude puncture damage, which is what actually ends most liners.
Pond liner calculator FAQs
How do I calculate how much pond liner I need?
Measure the maximum length, width and depth of your pond. Add twice the depth and twice your edge allowance to both the length and the width. With a 30cm allowance the calculation is: liner length = pond length + (2 x depth) + 0.6m, and the same for the width. A pond 3m by 2m and 0.6m deep needs a liner 4.8m by 3.8m.
What size pond liner do I need for my pond?
It depends on the dimensions of your pond rather than its surface area alone, because depth is counted twice. Enter your measurements in the calculator above for the exact size pond liner you need, or use the pond liner size chart below for common garden pond dimensions.
How much overlap do I need for a pond liner?
Around 30cm each side is the standard allowance, which adds 0.6m to each measurement. Allow more if you are edging with heavy stone, planning a bog garden or building a beach slope for wildlife, because all three consume the flap. You cannot add liner later, so err generous when ordering.
Why is pond depth counted twice?
A flexible liner runs down one wall, across the base and back up the opposite wall. That is two depths of material in each direction, so the maximum depth of your pond is added twice to both the length and the width.
Do I measure marginal shelves separately?
No. Measure the overall length, width and the deepest point of the excavation. Marginal shelves sit inside that envelope and the liner folds to fit them, so they need no separate allowance.
What size pond liner underlay do I need?
The same size as the liner. Underlay sits beneath the liner to protect it from sharp stones and roots, so it should cover the same area. Puncture damage from underneath is the most common cause of failure and is normally excluded from a manufacturer guarantee.
What thickness pond liner should I choose?
EPDM is commonly sold at 0.75mm and 1.02mm, butyl at 0.75mm and PVC at 0.5mm. Thicker grades give more margin against punctures but cost more and are stiffer to fold into tight corners. Thickness does not change the sheet size you need, only the durability of it.