We are reader supported. When you purchase through links on our site, we may earn an affiliate commission. Also, as an Amazon affiliate, we earn from qualifying purchases.
If you are planning a backyard pond or rebuilding an old one, getting the liner size right is the most important and most confusing step. Order a liner too small and it will not climb up the walls, it will stretch, tear, and leak. Order a liner too large and you pay for material you will never use. This guide gives you a simple, beginner-friendly formula, clear measurement methods for different shapes, real examples, and tips to avoid the most common mistakes. By the end, you can measure your pond, calculate the correct liner size, and order with confidence.
Why Liner Size Matters
The pond liner is the waterproof skin that holds all the water in place. A liner that is properly sized will drape smoothly over shelves and curves, reach the edges for secure anchoring, and allow you to add stones without pulling the liner tight. A liner that is too small will pull away from edges, expose folds, and create weak spots. Getting the size right also helps you minimize seams, reduce waste, and control costs.
The Easy Liner Formula (Works for Most Ponds)
Use this formula for most backyard ponds with sloped or stepped sides and gentle curves. Measure the inside pond length, width, and the maximum water depth (the deepest vertical point). Then choose your edge overlap. Finally, calculate.
Liner length = Pond length + 2 × Maximum depth + 2 × Edge overlap
Liner width = Pond width + 2 × Maximum depth + 2 × Edge overlap
Example of overlap: 1 ft per side (minimum), 1.5 ft per side for rock coping, or 2 ft per side for tall edges, uneven ground, or windy sites. That means the “2 × Edge overlap” part is 2 ft, 3 ft, or 4 ft total added to each dimension.
Understanding Each Measurement
Pond length and width
For irregular shapes, measure the longest inside length and the widest inside width at the intended waterline. Do not measure along the outside soil or edging. Stretch a string across the pond, mark the spans, and measure the string with a tape for accuracy.
Maximum depth
Measure the deepest vertical point from the waterline to the pond bottom. Include all shelf drops when calculating this depth. If you have multiple shelves, add their vertical drops together until you reach the deepest floor. The total is your maximum depth.
Edge overlap
This is the extra liner that rises above the waterline onto land so you can anchor, hide, and secure the liner. Choose at least 1 ft per side. If you plan to stack rocks, tuck under pavers, or create a wide edge, choose 1.5 to 2 ft per side. Extra overlap is cheap insurance.
When to Add a Little Extra
The formula above already covers slope and shelves in most backyard ponds. Add 5 to 10 percent extra if your pond has complicated curves, very steep slopes, stone ledges that stick out, or a boulder border that reaches deep into the pond. Round up to the next whole foot when ordering.
Tools That Make Measuring Easy
A 25 to 50 ft tape measure or a flexible cloth tape works for most ponds. Use string, stakes, or chalk to mark the longest length and width. A straight board and small level help you find the waterline on uneven ground. A builder’s level or laser level is helpful for large ponds but not required. A notepad and quick sketches are very useful.
Step-by-Step: Measure and Calculate
1) Rectangular or Oval Ponds
Measure the internal length and width at the waterline. Measure the maximum vertical depth to the deepest point. Choose an edge overlap. Apply the formula. This works well for rectangles, squares, and soft ovals because the longest spans are easy to identify.
2) Circular Ponds
Measure the diameter at the waterline. Use the diameter as both the length and width in the formula. Keep the overlap the same on all sides. Your liner will be a square piece that easily covers the circle.
3) Kidney-Shaped and Irregular Ponds
Stretch a string across the longest part of the pond and measure it. Then stretch another across the widest part and measure that. Use those two numbers as length and width. Measure the maximum depth. Choose overlap and calculate. If the shape has deep coves or tight curves, add 5 to 10 percent extra and round up.
4) Formal Straight-Wall Ponds
Formal ponds with vertical walls and right-angle corners are best served by box-welded liners (factory-made to your exact dimensions). If you are using a flat sheet liner, the basic formula still works, but you must allow extra material for the tight inside corners and for clamping under coping stones. Add at least 12 inches extra per dimension beyond the formula. For a clean result, consider a custom box-welded liner.
5) Ponds with Multiple Shelves
If you have two or three shelves, you can still use the simple formula with the deepest total depth. The vertical drops added together equal your maximum depth. For complex terracing, add 5 to 10 percent extra to help the liner drape smoothly over each step and curve.
Worked Examples You Can Copy
Example A: Kidney Pond with Rock Edge
Inside length = 12 ft. Inside width = 9 ft. Maximum depth = 2.5 ft (30 inches). Overlap choice = 1.5 ft per side (for stone coping).
Liner length = 12 + 2 × 2.5 + 2 × 1.5 = 12 + 5 + 3 = 20 ft.
Liner width = 9 + 2 × 2.5 + 2 × 1.5 = 9 + 5 + 3 = 17 ft.
Order a 20 × 18 ft or 20 × 20 ft liner, depending on available roll cuts. Round up one foot for easier installation and trimming.
Example B: Circular Pond
Diameter = 8 ft. Maximum depth = 2 ft. Overlap choice = 1 ft per side.
Liner length = 8 + 2 × 2 + 2 × 1 = 8 + 4 + 2 = 14 ft.
Liner width = 8 + 4 + 2 = 14 ft.
Order a 14 × 14 ft liner. If only even sizes are available, choose 14 × 16 or 16 × 16 and trim.
Example C: Formal 10 × 6 ft Pond, 4 ft Deep, Under Coping
Inside length = 10 ft. Inside width = 6 ft. Maximum depth = 4 ft. Overlap = 1 ft per side.
Base formula: Length = 10 + 2 × 4 + 2 × 1 = 10 + 8 + 2 = 20 ft. Width = 6 + 8 + 2 = 16 ft.
Because of vertical walls and tight corners, add 1 ft extra to each dimension. Final target size: about 21 × 17 ft. Round up to the nearest available. Better solution: order a box-welded liner cut to 10 × 6 × 4 ft with a flange for clamping under coping.
Example D: Stream and Waterfall
Stream length = 12 ft. Average channel width at waterline = 1.5 ft. Side depth from waterline to top edge = 0.5 ft each side. Overlap = 0.75 ft per side.
Liner width = Channel width + 2 × side depth + 2 × overlap = 1.5 + 2 × 0.5 + 2 × 0.75 = 1.5 + 1 + 1.5 = 4 ft.
Liner length = Stream length + 2 × overlap at ends = 12 + 1.5 = 13.5 ft. Round up to 14 ft.
Waterfall drops often need extra material to wrap over lips and under spill stones. Add at least 1 ft extra in length at the top and bottom for secure folds.
Choosing Your Liner Overlap
Minimum overlap is 1 ft per side if you are pinning the liner under turf or covering with gravel. For stone copings or wide boulders, use 1.5 ft per side. If your ground is uneven, you need taller edging, or you expect strong wind that could lift exposed liner, use 2 ft per side. Always plan extra at the lowest edge of the pond to prevent water from finding the liner’s end and escaping.
Imperial and Metric Tips
You can use the formula with feet or meters. Just keep units consistent. Example in metric: Liner length = Pond length (m) + 2 × max depth (m) + 2 × overlap (m). If converting, 1 inch = 2.54 cm, 1 foot = 30.48 cm, and 1 meter ≈ 3.28 feet. Round up to the nearest 0.5 m or 0.5 ft when ordering to allow trimming.
What About Underlayment?
Underlayment protects the liner from rocks, roots, and pressure points. Buy underlayment the same size as your liner, or slightly larger. Overlap seams by 6 to 12 inches. A non-woven geotextile is best. Old carpet can hold moisture and smell; if you use it at all, place geotextile on top of the carpet to protect the liner and avoid moldy fibers against the liner.
Material and Thickness: EPDM, RPE, and PVC
EPDM (45 mil) is very flexible, easy to fold around shelves, and fish safe. It is heavy but forgiving for beginners. RPE (30 to 40 mil) is lightweight, tough, and crease-resistant, great for larger ponds and streams, and often comes in very large panels. PVC is inexpensive and flexible but can be less UV stable; use only fish-safe PVC if you choose this route. For rocky soil, frequent freezing, or deep ponds over 3 ft, choose at least 45 mil EPDM or 30 to 40 mil RPE with good underlayment.
Working With Roll Sizes and Minimizing Seams
Liner rolls come in standard widths such as 10, 15, 20, 25, 30, 40, or 50 ft, and are cut to length. After calculating your target length and width, try to match one dimension to a standard roll width so you can buy a single panel. Orient your pond planning so the longest dimension lines up with an available roll width. Avoid seams whenever possible. If a seam is unavoidable, plan where it will sit (on a shelf, not at the deepest point) and add extra material for a proper overlap and bonding zone.
Seaming Basics (If You Must)
Many beginners should avoid seaming. If you must join two sheets, follow the liner manufacturer’s instructions. EPDM uses primer and seam tape with a minimum 6 inch overlap; many installers prefer 12 inches. Add at least 1 ft extra beyond the formula to give room for rolling, cleaning, priming, and setting the seam. RPE often requires thermal welding or specialized tapes designed for RPE. Practice on scrap first.
Streams, Waterfalls, and Bogs: Size These Separately
Streams
Use the stream version of the formula. Liner width = channel width + 2 × side depth + 2 × overlap. Liner length = stream length + 2 × end overlap. Add 1 ft extra at drop lips to wrap over spill stones. Avoid piecing stream liners with many small sections. Long, continuous pieces leak less and are easier to shape.
Waterfalls
For waterfalls, measure the total drop distance and the run. Ensure the liner rises high on the sides above the water flow to prevent leaks around the edges. The liner should extend under any spill plates or stones and continue into the pond liner by at least 12 to 18 inches to create a watertight overlap.
Bog filters or Planting Zones
If your pond has a separate lined bog, calculate it as its own rectangle using the same formula, then connect it to the main pond with a spillway or pipe. Keep the bog liner at least 2 inches higher than the pond waterline to prevent backflow when the pump turns off.
Edge Treatments That Affect Overlap
Stone coping requires more overlap so the liner can tuck under and rise behind stones. Concrete collars need liner to extend under the collar with a protective layer in between. Turf or gravel edges need less overlap but still require enough liner to pin securely under the covering and stay above natural water runoff. Always keep the liner’s top edge above the highest nearby soil grade so rainwater does not siphon dirt into your pond.
Estimating Pond Volume (Bonus)
Knowing your water volume helps size pumps, filters, and UV clarifiers. For rectangles, volume in US gallons ≈ length (ft) × width (ft) × average depth (ft) × 7.48. For irregular shapes, multiply by 0.8 to account for curves, or take several length, width, and depth measurements and average them. Example: Inside dimensions 12 × 9 × 2.5 ft. Volume ≈ 12 × 9 × 2.5 × 7.48 ≈ 2,028 gallons. In liters, multiply cubic meters by 1,000 (1 cubic meter = 1,000 liters).
Ground Prep and Depth Considerations
Remove roots, sharp stones, and debris. Compact the soil on shelves and slopes so they do not settle later and pull the liner tight. Round over the tops of shelves and the transition into the bottom to prevent stress points. If your pond is deeper than 3 ft, consider steeper sides only where safe, and make shelves at stable widths so they support rock without collapsing. Underlayment is essential under and over the liner when placing heavy boulders.
Climate Notes
Freeze-thaw cycles can shift edges. Use extra overlap and a secure anchor trench in cold climates. In hot, sunny climates, choose UV-stable materials and cover exposed liner quickly with stone, gravel, or edging. Wind can lift uncovered liner; minimize exposed sections and anchor early.
Common Mistakes to Avoid
Do not measure outside dimensions or include edging stones in the pond width and length. Do not forget vertical depth when you have multiple shelves; always add all the shelf drops to find the maximum depth. Do not choose the smallest overlap; small edges are the first place leaks occur. Do not cut the liner early; place it, fill partially, let it settle, then trim. Do not place large rocks directly on unprotected liner; always use underlayment or padding. Do not leave seams at the deepest point; position them on shelves where possible.
Quick Checklist Before You Order
Confirm the longest inside length and width at the waterline. Confirm the maximum depth by adding all vertical drops to the bottom. Choose your edge overlap based on edge design. Calculate liner length and width using the simple formula, then add 5 to 10 percent if your shape is complex. Round up to the next foot or half meter. Check roll widths and choose an orientation that avoids seams. Add matching underlayment to your order.
FAQs
Is the simple formula accurate for very steep walls?
Yes, but add extra. For near-vertical walls or formal ponds, add at least 1 ft to each dimension beyond the formula or use a box-welded liner for a cleaner fit.
How much overlap is really enough?
Minimum is 1 ft per side. Most stone-edged ponds are easier with 1.5 ft per side. For uneven terrain or if you plan to tuck under heavy coping, use 2 ft per side.
Can I seam two pieces to save money?
You can, but it is riskier for beginners. If you must, follow the manufacturer’s seaming instructions, add extra material for the bond area, and place seams on shelves, not at the bottom.
Do shelves change the formula?
You still use the maximum depth (sum of shelf drops). For complicated shelves and tight coves, add 5 to 10 percent extra to ensure a relaxed fit.
What thickness should I buy?
45 mil EPDM or 30 to 40 mil RPE are reliable for most ponds. Choose thicker or tougher materials with lots of rockwork, deep water, or heavy foot traffic.
Putting It All Together: A Simple Workflow
Sketch your pond and mark the longest length and width. Measure inside dimensions at the waterline and the deepest vertical depth. Decide your edge style and choose an overlap. Calculate liner length and width using the formula. Adjust for complexity and round up. Check roll sizes to avoid seams and order underlayment the same size. Install underlayment, set the liner, fill slowly, smooth folds toward one area, and trim only after the liner has settled with water in the pond.
Conclusion
Calculating the correct pond liner size does not have to be difficult. Measure the longest length and width inside the pond, find the maximum depth by adding all vertical shelf drops, pick a generous overlap, and apply the simple formula. Add a little extra for complex shapes, round up to available sizes, and choose a quality underlayment. With a well-fitted liner, your pond will hold water reliably, look clean around the edges, and be easier to maintain for years. Use the examples in this guide as a template for your own measurements, and you will order the right liner the first time.