Pond Liner Size Calculator
Enter your pond dimensions to get the liner and underlayment size you need, including the sides and edge overlap. EPDM is the durable standard, and you should always lay underlayment beneath it.
Use the maximum depth, the deepest point of the pond, so the liner reaches all the way down each wall. Default overlap is 2 feet to anchor the edge.
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liner size (ft)
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total sq ft
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underlayment (ft)
Buy this size or the next size up. Liner comes in fixed widths, so round up rather than cutting it close. Always lay underlayment beneath the liner to protect it from stones and roots, and choose fish-safe EPDM for a pond you plan to keep for years.
Right-sized liner gear for a ft pond liner
Auto-matched to your liner size. Sizes are starting points; round up to the nearest stocked sheet.
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How to Size a Pond Liner
A pond liner has to do more than cover the floor of the hole. It must run down one wall, across the bottom, up the opposite wall, and then leave enough material at the top to anchor the edge. That is why a liner is always quite a bit larger than the surface of the pond. The formula is simple once you see it laid out. Your liner length equals the pond length plus twice the maximum depth plus your overlap, and your liner width equals the pond width plus twice the maximum depth plus the same overlap. The maximum depth is added twice because the liner drops down on both sides, and the overlap, usually about 2 feet, is added once for each edge. The calculator above does this for you the moment you enter your numbers, and it shows the total square footage so you can compare rolls and sheets.
Why Maximum Depth, Not Average
When you size a liner you use the maximum depth of the pond, which is the opposite of the rule for calculating volume. Volume math uses average depth because sloped sides and shelves reduce the real number of gallons. A liner, by contrast, has to physically reach the lowest point of the deepest zone and climb back out, so it is sized to the deepest spot. Measure straight down from the top edge of the excavation to the bottom of the deep zone and enter that figure as your max depth. If your pond has a deep central pocket for overwintering koi, that pocket sets the liner size even though most of the pond is shallower.
Always Use Underlayment
Underlayment is a thick geotextile fabric that sits between the bare soil and the liner. It is cheap insurance. Roots, sharp stones, and shifting soil will eventually work against any liner, and a single puncture in a buried liner is a miserable repair. Buy underlayment in the same size as your liner so the whole excavation is cushioned, and in rocky ground consider doubling it on the floor. Some pond keepers add a layer of sand on the base as well, though a purpose-made underlayment fabric is the reliable standard and easier to work with on shelves and curves.
EPDM Is the Durable Standard
For a pond you intend to keep, fish-safe EPDM rubber is the material to buy. It is a flexible 45 mil membrane that molds itself to curves, shelves, and the deep zone without fighting you, and the fish-safe grade is non-toxic to koi and plants. It resists UV and routinely lasts for decades. PVC liners cost less but are stiffer, harder to fit around contours, and tend to grow brittle and crack as they age in sunlight. One important caution: roofing EPDM is not the same product and may contain additives that harm fish, so always confirm a liner is labeled safe for ponds and fish before it goes in the ground. Round your order up to the nearest stocked sheet and keep any offcuts for patches.
Keep going: size the rest of your pond.
Frequently Asked Questions
What size pond liner do I need?
The rule is liner length equals pond length plus twice the maximum depth plus your overlap, and liner width equals pond width plus twice the maximum depth plus your overlap. The extra depth on each side lets the liner drop down both walls, and the overlap, usually about 2 feet, gives you material to anchor around the edge with rocks or soil. Enter your dimensions above and the calculator returns the exact liner size, total square footage, and the matching underlayment size.
How much overlap should I leave around the edge?
Plan for at least 1 to 2 feet of liner past the waterline on every side. That overlap is what you tuck under your edging stones, fold into a planting shelf, or bury in a trench so the liner stays put and water cannot wick out over the rim. Two feet is a safe default and the value this calculator uses. Larger or steeper ponds, or ones with a stream and waterfall, often want a little more, so round up rather than cutting it close.
Do I really need underlayment under the liner?
Yes. Always install underlayment beneath the liner. It is a thick geotextile fabric that cushions the liner from sharp stones, roots, and settling soil that would otherwise puncture it over the years. Buy underlayment in the same size as your liner so the whole excavation is protected. Some builders add a second layer of underlayment in rocky ground, or use old carpet as a backup cushion, but a purpose-made pond underlayment is the reliable standard.
What is the best pond liner material for koi?
EPDM rubber is the durable standard for koi ponds and water gardens. It is a flexible 45 mil rubber membrane that conforms to curves and shelves, lasts decades, and the fish-safe grade is non-toxic to koi. PVC liners are cheaper but stiffer and shorter lived, and they can crack with age and UV exposure. For a pond you plan to keep, fish-safe EPDM is worth the extra cost. Make sure any liner you buy is labeled safe for fish, not roofing EPDM.
Should I measure average depth or maximum depth for the liner?
Use your maximum depth for liner sizing, not average depth. The liner has to reach all the way down the deepest wall and back up the other side, so it is sized to the deepest point plus the overlap. This is the opposite of volume math, where average depth gives the truer gallon count. Measure from the top edge of the pond straight down to the lowest part of the deepest zone, then enter that figure as your max depth above.
How do I figure the square footage of liner to buy?
Multiply the calculated liner length by the calculated liner width. That total square footage is what you compare against rolls or pre-cut sheets when shopping, since liner is priced by the square foot. Our calculator shows this number for you. Liner comes in fixed widths, so you will usually round up to the next available sheet size, which is fine. A little extra material around the edge is far better than coming up short mid-build.
Can I join two liners to cover a big pond?
It is best to use a single sheet whenever possible, because every seam is a potential leak. EPDM can be joined with manufacturer seam tape and primer for very large ponds, but the splice must be done carefully on a clean, dry surface. For most backyard ponds a single oversized sheet is cheaper in the long run and far less risky. Size the liner with this calculator first, then order the closest single sheet that covers it.