How do I calculate the pond liner size?

Liner length = pond length + 2 × maximum depth + 2 ft of overlap, and liner width = pond width + 2 × maximum depth + 2 ft. For a 10 × 8 ft pond that is 2 ft deep: 10 + 4 + 2 = 16 ft by 8 + 4 + 2 = 14 ft. The depth counts twice because the liner goes down one side and up the other, and the 2 ft is 1 ft at each edge to anchor. Round up to the next size your supplier sells: Liner 20 × 15 ft (6.10 × 4.57 m).

How much overlap should I allow?

Firestone’s PondGard guide adds 2 extra feet in total, which is 1 ft at each edge, and this calculator uses that as its default (12 in at each edge). Makers and shops differ a little: some allow more for a large pond or for liner that is buried under a stone edge. At 18 in at each edge (3 ft in total) the 10 × 8 ft, 2 ft deep pond needs 17 × 15 ft instead of 16 × 14 ft.

Which depth do I use: the deepest point or the average?

The deepest point for the liner, because the liner has to reach the bottom of the deepest part and come back up. The average depth is only for the water volume and the pump estimate, and it is lower for a pond with shelves or a sloping floor. Measure the depth of the excavation from the surrounding ground, not the depth of the water, since the liner goes up to the edge.

What about a round, oval or irregular pond?

Use the longest and widest points, as if drawing a rectangle around the pond. A liner is a rectangle that folds into the curves, so it has to cover that rectangle, whatever the shape. The shape matters only for the water: an oval holds π ÷ 4 (about 78.5%) of its bounding rectangle. A 12 × 8 ft pond with 30 in maximum depth and 20 in average depth needs a 20 × 15 ft liner either way, and holds about 940 gal as an oval and 1,197 gal as a rectangle.

Which stock sizes can I buy?

Pre-cut liners are typically sold in 5 ft steps, so the calculator rounds each side up to the next 5 ft by default: Liner 20 × 15 ft (6.10 × 4.57 m) for the 10 × 8 ft pond. That is a typical pattern, not a list from one maker, and the sizes carried change by maker and shop, so check your supplier. Choose the next 1 ft (16 × 14 ft (4.88 × 4.27 m)) or the exact size (16 × 14 ft (4.88 × 4.27 m)) if you are buying cut from a roll. A liner that is too small cannot be stretched, so round up and trim the excess.

How much underlayment do I need?

The same size as the liner, because it goes under all of it. For the 20 × 15 ft (6.10 × 4.57 m) liner that is 300 sq ft. Underlayment is sold in rolls (15 ft wide is typical) and cut to fit, so you buy the same area or slightly more to allow for the width of the roll and for laying it in strips with the edges butted. Pond-supply shops recommend it under an EPDM liner over rocky or rough ground.

How many gallons does my pond hold?

Length × width × average depth in feet gives cubic feet; multiply by 7.48052 for US gallons. The 10 × 8 ft pond with an 18 in average depth: 10 × 8 × 1.5 = 120 cu ft × 7.48052 = 898 US gallons (3,398 L). A 20 × 15 ft pond with a 36 in maximum depth and a 26 in average holds about 4,862 gallons and needs a 30 × 25 ft (9.14 × 7.62 m) liner. A small 6 × 4 ft pond, 18 in deep, needs 15 × 10 ft (4.57 × 3.05 m).

What size pump do I need?

A common rule of thumb is to move the whole pond volume through the filter about once an hour, so the pump flow in gallons per hour is about the pond volume in gallons: 898 gal/h for the 10 × 8 ft pond. This is an estimate. Pond-supply guides give once an hour as the usual minimum for a koi pond and more for heavy stocking, others allow a slower turnover for a lightly stocked pond, and a pump delivers less than its rating when it lifts water or pushes it through a filter or up a waterfall. Pick a pump rated higher than the figure and check its performance chart at your lift.

Can a pond be too deep for the liner formula?

The formula holds for any depth, but a deep, steep-sided pond with shelves takes more liner than a plain box, because every step adds a fold. If the pond has several shelves or a very uneven shape, allow a little more overlap, or measure the actual liner path by laying a tape along the bottom from edge to edge over the shelves. Check the supplier’s widest stock size too: wide sheets may be cut from a roll.

Pond Liner Calculator: What Size Liner Do I Need?

A 10 × 8 ft garden pond that is 2 ft deep at its deepest needs a liner of 16 × 14 ft, and the result is Liner 20 × 15 ft (6.10 × 4.57 m) after rounding up to the next 5 ft. The sum is the pond length plus twice the depth plus 2 ft of overlap (10 + 4 + 2 = 16 ft), and the same for the width (8 + 4 + 2 = 14 ft). Cut the underlayment to the same size, and the pond holds about 898 US gallons at an average depth of 18 in. Enter the size, the deepest point and the average depth of your pond; the stock sizes are typical, so check what your supplier lists.

Figures based on the sources listed belowUpdated 2026-10-16How we calculate

Pond length

Pond width

Pond shapeThe liner is a rectangle either way: for a curved pond enter the longest and widest points, and the liner is sized to that bounding box because it folds into the curves. The shape only changes the water volume (π ÷ 4 of the box for an oval).

Maximum depth (in)

The deepest point, measured from the surrounding ground to the bottom of the excavation (2 ft is 24 in). The liner follows the deepest point down and back up, so use the maximum, not the average.

Average depth (for the water volume) (in)

Used only for the gallons and the pump estimate. A pond with shelves or a sloping floor holds less than length × width × maximum depth: enter the average depth of the water, which is below the maximum and below the ground level by the freeboard. Leave it at the maximum depth for an upper limit.

Overlap at each edge (in)

The liner left lying on the ground around the pond to anchor it under rocks or edging. 12 in at each edge is 2 ft in total, the usual allowance; the calculator adds it twice (once per side).

Round the liner up toPre-cut liners are mostly sold in 5 ft steps, and a liner that is too small cannot be stretched, so round up. Stock sizes vary by maker: check the sizes your supplier lists, or have a roll cut to the exact size.

Liner 20 × 15 ft (6.10 × 4.57 m)

Liner to order (typical stock sizes): 20 × 15 ft (6.10 × 4.57 m)
Liner needed before rounding up: 16 × 14 ft (4.88 × 4.27 m)
Underlayment (same size as the liner): 300 sq ft
Water volume: 898 US gal (3,398 L)
Pump flow for one turnover per hour (estimate): about 898 gal/h (3,398 L/h), then add margin for lift and hose

Shopping list

EPDM pond liner (45 mil)
Pond underlayment (geotextile)
Pond pump
Pond skimmer or filter
Pond liner seam tape
Measuring tape (100 ft)
Marking paint

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Liner size by pond size and maximum depth

Feet, worked by this calculator with 12 in of overlap at each edge (2 ft in total). Each cell is the liner needed, then in brackets the size after rounding up to the next 5 ft on each side. Depth is the maximum depth.
Pond size	18 in deep	24 in deep	36 in deep
6 × 4 ft	11 × 9 (15 × 10)	12 × 10 (15 × 10)	14 × 12 (15 × 15)
8 × 6 ft	13 × 11 (15 × 15)	14 × 12 (15 × 15)	16 × 14 (20 × 15)
10 × 8 ft	15 × 13 (15 × 15)	16 × 14 (20 × 15)	18 × 16 (20 × 20)
12 × 10 ft	17 × 15 (20 × 15)	18 × 16 (20 × 20)	20 × 18 (20 × 20)
15 × 10 ft	20 × 15 (20 × 15)	21 × 16 (25 × 20)	23 × 18 (25 × 20)
20 × 15 ft	25 × 20 (25 × 20)	26 × 21 (30 × 25)	28 × 23 (30 × 25)

Gallons in a rectangular pond by average depth

US gallons, worked by this calculator: length × width × average depth × 7.48052. The average depth is the average depth of the water, not the deepest point. An oval pond holds about 78.5% of these figures.
Pond size	12 in average	18 in average	24 in average
6 × 4 ft	180	269	359
8 × 6 ft	359	539	718
10 × 8 ft	598	898	1,197
12 × 10 ft	898	1,346	1,795
15 × 10 ft	1,122	1,683	2,244
20 × 15 ft	2,244	3,366	4,488

Methodology & assumptions

The liner formula is the one pond-liner makers publish: liner length = pond length + 2 × maximum depth + overlap, and liner width = pond width + 2 × maximum depth + overlap. The depth is counted twice because the liner goes down one side to the deepest point and up the other. Firestone, the maker of PondGard liner, writes the overlap as 2 extra feet in total, so 1 ft at each edge to anchor the liner under rocks or edging. This calculator asks for the overlap at each edge (default 12 in) and adds it twice, so the default is the same 2 ft; some makers and retailers allow more or less, so change the field if your supplier advises it. Use the deepest point of the excavation, measured from the surrounding ground, and the longest and widest points of the pond. Shapes: a liner is a rectangle, so a round, oval or freeform pond is sized by its bounding rectangle (its longest and widest points). The liner folds into the curves, and the folds use the extra material at the corners, so a liner cut to a smaller shape would not reach the edge. Shelves, steps and a sloping floor also take up more liner than a plain box, so a pond with a lot of them can need a little more than the sum gives; the 2 ft overlap and the rounding up to the next stock size cover some of this, and a liner that is too small cannot be stretched. Stock sizes: the result is rounded up to the next 5 ft on each side by default, because pre-cut liners are typically sold in 5 ft steps (for example 10 × 15, 15 × 20, 20 × 25 and 20 × 30 ft). That is an estimate of what is typical, not a list from any one maker, and the sizes sold vary by maker and by shop, so check the list of the supplier you buy from. Larger liners are also sold cut from a roll to any length, with widths up to 50 ft on the Firestone brochure, in which case choose "exact size". The underlayment is cut to the same size as the liner, because it goes under all of it; it is sold in rolls (15 ft wide is typical) and is trimmed to fit, so buy the same area or a little more. Water: a rectangular pond holds length × width × average depth. An oval is π ÷ 4 × length × width × average depth, because an ellipse fills π ÷ 4 (about 78.5%) of its bounding rectangle; a freeform pond is somewhere near that, so treat the number as an estimate. The average depth is the average depth of the water, which is less than the maximum depth when there are shelves or a sloping floor, and less again when the water stands below the ground level. Gallons: 1 cu ft = 1,728 ÷ 231 = 7.48052 US gallons, and 1 US gallon = 3.785 L. Pump: a common rule of thumb for a pond pump is to move the whole volume through the filter about once an hour, so a pond of 900 gallons needs about 900 gallons per hour. This is an estimate, not a rule. Pond-supply guides give once an hour as the usual minimum for a koi pond and more for a heavily stocked one, other guides allow one turnover in two hours for a lightly stocked pond, and a pump delivers less than its rating once water is lifted or pushed through a filter or up a waterfall, so pick a pump rated higher than the figure shown and check its performance chart. Metric toggle: lengths in m, depths in cm, area in m². The size in the headline is shown in both feet and metres. This tool sizes the liner and the water only; it does not cover the excavation (see the excavation calculator), edging, or the pond's fish load.

How to calculate the size of a pond liner

Mark out the pond. Mark the outline with a hose or marking paint, and measure the longest and the widest points. For a curved pond, measure as if drawing a rectangle around it.
Find the deepest point. Measure the maximum depth of the excavation from the surrounding ground, including any deeper hole for fish or a pump.
Add depth and overlap. Liner length = pond length + 2 × maximum depth + 2 ft of overlap. Liner width = pond width + 2 × maximum depth + 2 ft.
Round up to a stock size. Choose the next liner size up that your supplier sells. A liner that is too small cannot be stretched.
Order the underlayment and work out the water. Underlayment is cut to the same size as the liner. Water volume = length × width × average depth × 7.48052 gallons (× π ÷ 4 for an oval).

Worked examples

Project	Result
10 × 8 ft pond, 24 in at its deepest, 18 in average, 12 in overlap at each edge	Liner 20 × 15 ft (6.10 × 4.57 m), needed 16 × 14 ft (4.88 × 4.27 m), about 898 US gal
6 × 4 ft small pond, 18 in deep, 14 in average	Liner 15 × 10 ft (4.57 × 3.05 m), needed 11 × 9 ft (3.35 × 2.74 m), about 209 US gal
12 × 8 ft oval pond, 30 in at its deepest, 20 in average (sized by its bounding rectangle)	Liner 20 × 15 ft (6.10 × 4.57 m), needed 19 × 15 ft (5.79 × 4.57 m), about 940 US gal
20 × 15 ft large pond, 36 in at its deepest, 26 in average	Liner 30 × 25 ft (9.14 × 7.62 m), needed 28 × 23 ft (8.53 × 7.01 m), about 4,862 US gal
The 10 × 8 ft pond with 18 in of overlap at each edge (3 ft in total)	Liner 20 × 15 ft (6.10 × 4.57 m), needed 17 × 15 ft (5.18 × 4.57 m)

Frequently asked questions

How do I calculate the pond liner size?: Liner length = pond length + 2 × maximum depth + 2 ft of overlap, and liner width = pond width + 2 × maximum depth + 2 ft. For a 10 × 8 ft pond that is 2 ft deep: 10 + 4 + 2 = 16 ft by 8 + 4 + 2 = 14 ft. The depth counts twice because the liner goes down one side and up the other, and the 2 ft is 1 ft at each edge to anchor. Round up to the next size your supplier sells: Liner 20 × 15 ft (6.10 × 4.57 m).
How much overlap should I allow?: Firestone’s PondGard guide adds 2 extra feet in total, which is 1 ft at each edge, and this calculator uses that as its default (12 in at each edge). Makers and shops differ a little: some allow more for a large pond or for liner that is buried under a stone edge. At 18 in at each edge (3 ft in total) the 10 × 8 ft, 2 ft deep pond needs 17 × 15 ft instead of 16 × 14 ft.
Which depth do I use: the deepest point or the average?: The deepest point for the liner, because the liner has to reach the bottom of the deepest part and come back up. The average depth is only for the water volume and the pump estimate, and it is lower for a pond with shelves or a sloping floor. Measure the depth of the excavation from the surrounding ground, not the depth of the water, since the liner goes up to the edge.
What about a round, oval or irregular pond?: Use the longest and widest points, as if drawing a rectangle around the pond. A liner is a rectangle that folds into the curves, so it has to cover that rectangle, whatever the shape. The shape matters only for the water: an oval holds π ÷ 4 (about 78.5%) of its bounding rectangle. A 12 × 8 ft pond with 30 in maximum depth and 20 in average depth needs a 20 × 15 ft liner either way, and holds about 940 gal as an oval and 1,197 gal as a rectangle.
Which stock sizes can I buy?: Pre-cut liners are typically sold in 5 ft steps, so the calculator rounds each side up to the next 5 ft by default: Liner 20 × 15 ft (6.10 × 4.57 m) for the 10 × 8 ft pond. That is a typical pattern, not a list from one maker, and the sizes carried change by maker and shop, so check your supplier. Choose the next 1 ft (16 × 14 ft (4.88 × 4.27 m)) or the exact size (16 × 14 ft (4.88 × 4.27 m)) if you are buying cut from a roll. A liner that is too small cannot be stretched, so round up and trim the excess.
How much underlayment do I need?: The same size as the liner, because it goes under all of it. For the 20 × 15 ft (6.10 × 4.57 m) liner that is 300 sq ft. Underlayment is sold in rolls (15 ft wide is typical) and cut to fit, so you buy the same area or slightly more to allow for the width of the roll and for laying it in strips with the edges butted. Pond-supply shops recommend it under an EPDM liner over rocky or rough ground.
How many gallons does my pond hold?: Length × width × average depth in feet gives cubic feet; multiply by 7.48052 for US gallons. The 10 × 8 ft pond with an 18 in average depth: 10 × 8 × 1.5 = 120 cu ft × 7.48052 = 898 US gallons (3,398 L). A 20 × 15 ft pond with a 36 in maximum depth and a 26 in average holds about 4,862 gallons and needs a 30 × 25 ft (9.14 × 7.62 m) liner. A small 6 × 4 ft pond, 18 in deep, needs 15 × 10 ft (4.57 × 3.05 m).
What size pump do I need?: A common rule of thumb is to move the whole pond volume through the filter about once an hour, so the pump flow in gallons per hour is about the pond volume in gallons: 898 gal/h for the 10 × 8 ft pond. This is an estimate. Pond-supply guides give once an hour as the usual minimum for a koi pond and more for heavy stocking, others allow a slower turnover for a lightly stocked pond, and a pump delivers less than its rating when it lifts water or pushes it through a filter or up a waterfall. Pick a pump rated higher than the figure and check its performance chart at your lift.
Can a pond be too deep for the liner formula?: The formula holds for any depth, but a deep, steep-sided pond with shelves takes more liner than a plain box, because every step adds a fold. If the pond has several shelves or a very uneven shape, allow a little more overlap, or measure the actual liner path by laying a tape along the bottom from edge to edge over the shelves. Check the supplier’s widest stock size too: wide sheets may be cut from a roll.

Handy tools

Unit Converter: Convert the liner size or the water volume to another unit, or a measurement from feet and inches to metres.