What this calculator works out
This calculator works out the sheet size a pond liner needs, adding twice the maximum depth to each dimension plus a metre for the overlap around the edge.
Liner is sold as a rectangular sheet, so the shape of the pond does not affect the calculation — only its longest and widest points and its deepest part.
Enter your details
Before you order
- Use the maximum depth, not the average. The liner has to reach the bottom of the deepest point and back up again on both sides.
- Measure the longest and widest points of the excavation, including any shelves or bog area the liner will cover.
- The calculation allows 0.5 m of overlap on each side. Increase it for a large pond or where the edge detail needs burying under stone.
- Underlay is sized the same way and is worth using on anything but pure sand.
The liner runs down one side, across the bottom and up the other side. A pond 0.6 m deep therefore needs 1.2 m of extra length and 1.2 m of extra width before any overlap. Deep ponds consume liner disproportionately, which is why a deep pond costs far more than its surface area suggests.
How this calculator works
Each dimension extended by twice the depth plus the overlap:
Liner length = pond length + (2 × maximum depth) + 1 m
Liner width = pond width + (2 × maximum depth) + 1 m
Area = liner length × liner widthThe 1 m allowance is 0.5 m of overlap on each side. Rolls come in standard widths, so the practical order is usually the next standard size up rather than the exact figure.
Worked example: a 3 m by 2 m pond
Using the default figures — 3 m by 2 m at 0.6 m maximum depth:
- Liner length: 3 + 1.2 + 1 = 5.2 m
- Liner width: 2 + 1.2 + 1 = 4.2 m
- Sheet area: 21.84 m²
A 6 m² pond needs nearly 22 m² of liner — more than three times its surface area. Deepen it to 1 m and the sheet becomes 6 m by 5 m, or 30 m². That is the practical argument for a shelved pond with one deep section rather than uniform depth: the liner is sized by the deepest point regardless of how much of the pond is that deep.
Common mistakes
- Using average depth. The maximum depth sets the liner size.
- Forgetting the overlap. Liner must extend beyond the water line and be secured.
- Skipping underlay. Stones and roots puncture liner from below.
- Measuring the finished water surface rather than the excavation. The liner covers the whole hole.
- Ordering an exact fit. Trimming excess is easy; adding liner is not.
Frequently asked questions
What liner material should I use?
Butyl and EPDM rubber are the usual choices for a permanent pond — flexible, durable, and typically guaranteed for decades. PVC is cheaper and shorter-lived. Preformed rigid liners avoid the sizing question but limit the shape and are harder to install level than they look.
Do I need underlay?
On anything except pure sand, yes. Sharp stones, roots and settling all puncture liner from below, and a leak under a full pond is extremely difficult to find and repair. Proprietary underlay is inexpensive; old carpet and thick sand are traditional alternatives.
How deep should a wildlife pond be?
A shelf at 10 to 20 cm for marginal plants, a gentle slope or beach at one edge so animals can get out, and a deeper section of 45 to 60 cm that will not freeze solid. Depth beyond about 60 cm adds liner cost without much benefit for wildlife.
How do I finish the edge?
Bury the overlap in a shallow trench and cover it with turf, soil or stone so no liner is visible and ultraviolet light cannot degrade it. At least one edge should slope gently into the water to let hedgehogs and amphibians escape, which is more important than any decorative detail.
Is what I enter stored?
No. Measurements are processed in your browser and never transmitted or retained.
Related tools
References
- Royal Horticultural Society — pond construction, liners and planting guidance
- GOV.UK — water abstraction, wildlife protection and garden pond safety
Sources are checked at publication and can change — how I choose and check references.