How do I work out what size pond liner I need?

Add twice the deepest point and an overlap at each edge to the pond's length, and again to its width. The liner goes down one side, across the floor and up the other, so the depth counts twice, and it then lies flat on the ground at the edge to be held down. Anjon, a US liner maker, adds 2 ft (61 cm) to each dimension, which the page reads as 1 ft (30 cm) of overlap at each edge: its 12 by 7 ft (3.7 by 2.1 m) pond, 3 ft (0.9 m) deep, takes a 20 by 15 ft (6.1 by 4.6 m) sheet. Pontec adds 50 cm (20 in) at each edge. The same rule works for a round or an irregular pond, measured at its longest and widest, and shelves need nothing added: over a hole that steps down to its deepest point and back up, the sheet down every face and over every shelf is never longer than the rule.

How much water is in my pond?

The surface area times the average depth: the page's 3 × 2 m (10 × 6.5 ft) pond at 0.6 m (2 ft) average depth holds 3,600 litres (951 US gallons). A rectangle's area is its length times its width; a round pond's is π times the diameter squared, over four. Use the average depth, not the deepest point: shelves and sloping sides hold much less water than a straight-sided box of the same depth. A cubic metre is 1,000 litres (264 US gallons).

What counts as an irregular pond, and how accurate is it?

Anything without a constant width: a kidney, a bean, a figure of eight, a natural outline. Measure the length along several lines across the pond and the width at several places along it, average each set, and the page multiplies the two averages by the average depth. Penn State Extension, whose method this is, rates it as typically within plus or minus 20%; splitting the outline into trapezoids gets nearer 5 to 10%, and the Irregular Area Calculator works that from offsets measured along a straight line. Filling through a water meter gives the real figure.

Why does the page ask for two depths?

Because the liner and the water need different ones. The liner has to reach the very bottom of the hole, so it is sized on the deepest point. The water fills the whole shape, shelves and slopes included, so it is worked on the average depth. A pond 0.9 m (3 ft) at its deepest with wide planting shelves might average 0.6 m (2 ft) or less, and using the deepest point for the water would overstate it by half.

How do I make sure the liner is big enough before I buy it?

For a hole that steps down to its deepest point and back up, shelves and all, the makers' rule is already enough. A rope laid across such a hole, down every face and over every shelf, is never longer than the length plus twice the depth: each stretch of it is no longer than the ground it crosses plus the height it drops or climbs, the crossings add up to the length, and the drops and climbs to no more than twice the depth. The rope matches the rule where every face is vertical and every shelf level, and comes out shorter where the sides slope. A hole the rule does not describe can need more: a shelf with a raised lip, a wall that leans in under the edge, or edges at different heights. For one of those, dig the hole first, then run a rope or a garden hose across it along the longest line, down every face and over every shelf and up the far side, and out to the overlap at each end; do the same across the widest point. The two lengths of rope are the sheet.

Landscaping & Outdoor

Pond Volume and Liner Size Calculator

How much water a rectangular, round or irregular garden pond holds, in litres or gallons, and the liner sheet it needs by the liner makers' rule.

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Every formula citedNamed published standards
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Imperial · sales tax

Answer

Which figure leads the answer. Both are worked out every time and shown below it.

The water is what the pump's turnover, any treatment and fish stocking are worked from. The liner is what you buy before any of that, and it is a sheet much larger than the pond's surface, because it has to line the sides and lie on the ground around the edge as well.

Pond Shape

The outline of the water seen from above.

Rectangular and round ponds are worked exactly. An irregular pond is anything with no constant width, such as a kidney, a figure of eight or a natural outline. For its water the page uses the average of several lengths and the average of several widths, which Penn State Extension rates as typically within plus or minus 20%; for its liner it uses the longest and the widest, because the sheet has to cover the whole hole.

Pond Length

The longer side of the water, measured at the top of the hole.

Measure the finished water surface, inside the edge, not the outside of the edging stones.

Tools needed: Tape measure

Pond Width

The shorter side of the water, at right angles to the length.

For a square pond, the same as the length.

Tools needed: Tape measure

Depth at the Deepest Point

From ground level at the edge down to the lowest point of the hole. The liner is sized on this.

Measure from a straightedge laid across the top of the hole to the deepest part of the floor. The makers' rule adds twice this to each dimension: the sheet goes down one side and up the other. Shelves and sloping sides need nothing added: over a hole that steps down to its deepest point and back up, the sheet never needs more than the rule. A pond deeper than it is wide still follows the rule; its sheet is mostly sides.

Tools needed: Tape measure, Straightedge or spirit level

Average Water Depth

The depth of water averaged over the whole pond, shelves and slopes included. The water is worked from this.

Shelves for marginal plants and sloping sides make the average well less than the deepest point. A straight-sided pond with a flat floor has an average equal to its deepest point. For "a typical bowl-shaped pond", Penn State Extension estimates the average depth as 0.4 times the maximum depth.

Liner Overlap at Each Edge

How far the liner lies flat on the ground beyond the water's edge, at each side.

The overlap is what the edging stones or turf hold down. Pontec's rule allows 50 cm (20 in) at each edge. Anjon adds 2 ft (61 cm) to each dimension, which the page reads as 1 ft (30 cm) of overlap at each edge. A pond edged with heavy stone or a wide beach can take more, and a liner tucked into a trench under turf needs enough to reach the trench.

Pond water or liner sheet

972.5 gal

High confidence

The water is the length times the width times the average depth. The liner is each side of the pond plus twice the deepest point and the overlap at both edges: the sheet the makers' rule gives, enough for any hole that steps down to its deepest point and back up, with the overlap lying on the ground all round.

Water the pond holds: 972.47 gal
Water surface area: 65 ft²
Liner sheet length: 18 ft
Liner sheet width: 14.5 ft
Liner sheet area: 261 ft²

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

Anjon Manufacturing (maker of LifeGuard EPDM and PolyGuard PVC pond liners), Pond Liner: Pond Length ft + (Depth ft x 2) + 2 ft = Liner Length, and Pond Width ft + (Depth ft x 2) + 2 ft = Liner Width; a finished pond 12 ft long, 7 ft wide and 3 ft deep needs a 20 x 15 ft liner; the basic formula works for all pond shapes
Pontec (an OASE brand), Calculating the pond size: length + 2 x depth (at the deepest point of the excavation) + 2 x 50 cm = length of the liner, and width + 2 x depth (at the deepest point of the excavation) + 2 x 50 cm = width of the liner
Penn State Extension, Bryan R. Swistock and Tom McCarty, Pond Measurements: Area, Volume and Residence Time (updated 19 May 2025): for an irregular pond, measure numerous lengths and widths and multiply their averages (typically within ±20%), or divide it into trapezoids (±5–10%), or map its edge by GPS (about 1%); "For a typical bowl-shaped pond the average depth can be estimated as 0.4 times the maximum depth"
Geometry and units: a round pond's water surface is π/4 times its diameter squared, the volume is the plan area times the average depth, and one cubic metre is 1,000 litres

Inputs used

Answer: The water the pond holds
Pond Shape: Rectangular or square
Pond Length: 10 ft
Pond Width: 6.5 ft
Pond Diameter: 8 ft
Longest Distance Across: 13 ft
Widest Distance Across: 10 ft
Average Length (of Several Measured): 11 ft
Average Width (of Several Measured): 7 ft
Depth at the Deepest Point: 3 ft
Average Water Depth: 2 ft
Liner Overlap at Each Edge: 12 in

Intermediate steps

Water the pond holds: 972.47 gal
Water surface area: 65 ft²
Liner sheet length: 18 ft
Liner sheet width: 14.5 ft
Liner sheet area: 261 ft²

Final result972.47 gal

Confidence note: The water is the length times the width times the average depth. The liner is each side of the pond plus twice the deepest point and the overlap at both edges: the sheet the makers' rule gives, enough for any hole that steps down to its deepest point and back up, with the overlap lying on the ground all round.

What this calculation does not cover

The liner is the sheet the makers' rule gives: the pond's length and width, twice the deepest point and an overlap at each edge. It is enough for any hole that steps down to its deepest point and back up, shelves included: a rope laid across such a hole, down every face and over every shelf, is never longer than the length plus twice the depth, matching it where every face is vertical and every shelf level, and shorter where the sides slope. A hole the rule does not describe can need more: a shelf with a raised lip, a wall that leans in under the edge, or edges at different heights. For one of those, lay a rope or a hose across the dug hole along its longest and widest lines, out to the overlap at each end, and measure the rope.
The sheet is the size the hole needs, not a size anybody sells. Liners come in set widths and lengths that differ from maker to maker, so round each dimension up to the next size your supplier stocks, and keep the longer side of the sheet along the longer side of the pond.
For an irregular pond the water is worked from the average length and the average width, the quick method Penn State Extension gives for irregular ponds and rates as typically within plus or minus 20%. Measure more lines across the pond and the average improves; for a closer figure, take the plan area from the Irregular Area Calculator's offsets method, or fill the pond through a water meter and read the volume off it.
The average depth is yours, and it is the figure most likely to be off. Shelves for marginal plants, sloping sides and a deep zone in the middle all pull it below the deepest point. For "a typical bowl-shaped pond", Penn State Extension estimates the average depth as 0.4 times the maximum depth; a garden pond with steep walls and narrow shelves sits nearer its deepest point than that.
The figure is the pond itself. A pump chamber, a filter box, a waterfall, a stream and the pipework between them hold more water that circulates with it, and pump turnover and any dose of treatment are worked on the whole system rather than the pond alone.
Nothing here sizes the excavation, the underlay or the edging. The hole is dug a little larger than the finished water to take the underlay and any sand or soft blinding, the spoil bulks up once it is out of the ground, and the overlap is held down by the edging stones or turf the liner is tucked under.

Estimated cost — your price

Price per gal ($)

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Schematic, drawn to the proportions you entered — not to scale on screen.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-10-05 · v1.0.0

Regulatory standards & verification citations4

Anjon Manufacturing (maker of LifeGuard EPDM and PolyGuard PVC pond liners), Pond Liner: Pond Length ft + (Depth ft x 2) + 2 ft = Liner Length, and Pond Width ft + (Depth ft x 2) + 2 ft = Liner Width; a finished pond 12 ft long, 7 ft wide and 3 ft deep needs a 20 x 15 ft liner; the basic formula works for all pond shapes
Pontec (an OASE brand), Calculating the pond size: length + 2 x depth (at the deepest point of the excavation) + 2 x 50 cm = length of the liner, and width + 2 x depth (at the deepest point of the excavation) + 2 x 50 cm = width of the liner
Penn State Extension, Bryan R. Swistock and Tom McCarty, Pond Measurements: Area, Volume and Residence Time (updated 19 May 2025): for an irregular pond, measure numerous lengths and widths and multiply their averages (typically within ±20%), or divide it into trapezoids (±5–10%), or map its edge by GPS (about 1%); "For a typical bowl-shaped pond the average depth can be estimated as 0.4 times the maximum depth"
Geometry and units: a round pond's water surface is π/4 times its diameter squared, the volume is the plan area times the average depth, and one cubic metre is 1,000 litres

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Sizing a Water Feature's Pump

How to calculate pond volume and liner size in 13 steps

AnswerWhich figure leads the answer. Both are worked out every time and shown below it.
Pond ShapeThe outline of the water seen from above.
Pond LengthThe longer side of the water, measured at the top of the hole.
Pond WidthThe shorter side of the water, at right angles to the length.
Pond DiameterStraight across the water through the centre.
Longest Distance AcrossThe longest straight line across the water, end to end. The liner's length is sized on this.
Widest Distance AcrossThe widest point across the water, at right angles to the longest line. The liner's width is sized on this.
Average Length (of Several Measured)The mean of several lengths measured parallel to the longest line, at intervals across the pond.
Average Width (of Several Measured)The mean of several widths measured square to the longest line, at intervals along it.
Depth at the Deepest PointFrom ground level at the edge down to the lowest point of the hole. The liner is sized on this.
Average Water DepthThe depth of water averaged over the whole pond, shelves and slopes included. The water is worked from this.
Liner Overlap at Each EdgeHow far the liner lies flat on the ground beyond the water's edge, at each side.
Pond water or liner sheetThe tool computes the pond water or liner sheet from those figures and shows the formula, its sources, and a confidence rating alongside it.

Frequently asked questions

How do I work out what size pond liner I need?: Add twice the deepest point and an overlap at each edge to the pond's length, and again to its width. The liner goes down one side, across the floor and up the other, so the depth counts twice, and it then lies flat on the ground at the edge to be held down. Anjon, a US liner maker, adds 2 ft (61 cm) to each dimension, which the page reads as 1 ft (30 cm) of overlap at each edge: its 12 by 7 ft (3.7 by 2.1 m) pond, 3 ft (0.9 m) deep, takes a 20 by 15 ft (6.1 by 4.6 m) sheet. Pontec adds 50 cm (20 in) at each edge. The same rule works for a round or an irregular pond, measured at its longest and widest, and shelves need nothing added: over a hole that steps down to its deepest point and back up, the sheet down every face and over every shelf is never longer than the rule.
How much water is in my pond?: The surface area times the average depth: the page's 3 × 2 m (10 × 6.5 ft) pond at 0.6 m (2 ft) average depth holds 3,600 litres (951 US gallons). A rectangle's area is its length times its width; a round pond's is π times the diameter squared, over four. Use the average depth, not the deepest point: shelves and sloping sides hold much less water than a straight-sided box of the same depth. A cubic metre is 1,000 litres (264 US gallons).
What counts as an irregular pond, and how accurate is it?: Anything without a constant width: a kidney, a bean, a figure of eight, a natural outline. Measure the length along several lines across the pond and the width at several places along it, average each set, and the page multiplies the two averages by the average depth. Penn State Extension, whose method this is, rates it as typically within plus or minus 20%; splitting the outline into trapezoids gets nearer 5 to 10%, and the Irregular Area Calculator works that from offsets measured along a straight line. Filling through a water meter gives the real figure.
Why does the page ask for two depths?: Because the liner and the water need different ones. The liner has to reach the very bottom of the hole, so it is sized on the deepest point. The water fills the whole shape, shelves and slopes included, so it is worked on the average depth. A pond 0.9 m (3 ft) at its deepest with wide planting shelves might average 0.6 m (2 ft) or less, and using the deepest point for the water would overstate it by half.
How do I make sure the liner is big enough before I buy it?: For a hole that steps down to its deepest point and back up, shelves and all, the makers' rule is already enough. A rope laid across such a hole, down every face and over every shelf, is never longer than the length plus twice the depth: each stretch of it is no longer than the ground it crosses plus the height it drops or climbs, the crossings add up to the length, and the drops and climbs to no more than twice the depth. The rope matches the rule where every face is vertical and every shelf level, and comes out shorter where the sides slope. A hole the rule does not describe can need more: a shelf with a raised lip, a wall that leans in under the edge, or edges at different heights. For one of those, dig the hole first, then run a rope or a garden hose across it along the longest line, down every face and over every shelf and up the far side, and out to the overlap at each end; do the same across the widest point. The two lengths of rope are the sheet.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.