Pond Volume Calculator

Calculate how many gallons a garden pond holds, the liner size to buy including depth and overlap, the pump size in GPH and how many goldfish it can support.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To find pond volume, multiply the surface area by the average depth for cubic feet, then multiply by 7.48 for gallons; the liner is each dimension plus twice the depth plus twice the overlap. A 12 x 8 ft rectangular pond 2 ft deep holds 192 cubic feet, or 1,436 gallons, and needs a 20 x 16 ft liner.

Your measurements

A rectangular volume: length times width times depth.

Pond shapeLengthftLongest dimension of the pond outline.WidthftWidest dimension at right angles to the length.Average depthftAverage the shallow shelf and the deep zone, weighted by how much of the pond each covers. The liner row uses this same depth, so run it again with the deepest point before buying liner. Fish need 2 ft, 3 ft where the pond freezes.Liner overlap per sideft1 to 2 ft of liner past the edge on every side to anchor under rocks or turf. Use 2 ft; the rim is never as level as the plan, and a short liner cannot be stretched.

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Pond volume1,436 galAll results

Results

Pond volume: 1,436 gal; At 7.48 gallons per cubic foot
Water volume: 192 cu ft; 96 sq ft of surface x 2 ft average depth
Liner size: 20 ft x 16 ft; 320 sq ft of liner: each dimension plus twice the depth entered plus twice the overlap; 45 mil EPDM comes 10 ft to 50 ft wide cut to length, so buy 20 ft x 20 ft in one piece, plus the same area of underlayment
Pump size: 718 GPH; Turns the whole pond over every 2 hours at zero head; read the pump curve at the waterfall height, and 359 GPH is enough for a still pond without fish
Goldfish capacity: 144 in; Total fish length at 1 in of goldfish per 10 gal; koi need far more, about 250 gal per fish

Pond Volume formula

Area = Length x Width (rectangle);  3.1416 / 4 x Length x Width (oval);  0.8 x Length x Width (irregular)
Gallons = Area x Average depth x 7.48
Liner size = (Length + 2 x Depth + 2 x Overlap) x (Width + 2 x Depth + 2 x Overlap)
Pump GPH = Gallons / 2

Worked example

An oval pond 15 ft long and 10 ft wide with a 2.5 ft average depth and 2 ft of liner overlap: area = 0.7854 x 15 x 10 = 117.8 sq ft. 117.8 x 2.5 = 294.5 cubic feet. 294.5 x 7.48 = 2,203 gallons.

Liner: (15 + 5 + 4) x (10 + 5 + 4) = 24 x 19 ft, or 456 sq ft of liner. A pump rated about 1,100 GPH turns the pond over every 2 hours, and at 1 in of goldfish per 10 gallons the pond can carry about 220 in of fish, say 30 goldfish of 6 to 8 in.

Quick reference

Rectangular pond volume and liner size (2 ft average depth, 2 ft overlap)
Pond size	Cubic feet	Gallons	Liner size
6 x 4 ft	48	359	14 x 12 ft
8 x 6 ft	96	718	16 x 14 ft
10 x 8 ft	160	1,197	18 x 16 ft
12 x 8 ft	192	1,436	20 x 16 ft
15 x 10 ft	300	2,244	23 x 18 ft
20 x 12 ft	480	3,590	28 x 20 ft

Measuring an existing or planned pond

Measure the longest length and the widest width of the outline. For an oval, the area is about 78 percent of the enclosing rectangle; for a free-form shape, 80 percent is a fair guess. Depth is the hard part. Most ponds have a plant shelf around the rim, 8 to 12 in deep and 12 in wide, and a deep zone of at least 24 in. Make the deep zone 36 in where the pond freezes. Estimate what share of the pond each covers and weight the average accordingly.

If the pond is already full, time how long a hose takes to fill a 5 gallon bucket. Then time the fill of the pond from empty and scale up. The true volume matters when dosing treatments and sizing the filter.

Liner, pump and filter sizing

The liner goes down one wall, across the bottom, up the other wall and out past the edge. That is why twice the depth and twice the overlap are added to both dimensions. This calculator uses the depth you entered for both rows, so run it again with the deepest point before buying liner. Buy 45 mil fish-safe EPDM, not roofing grade. It comes 10 to 50 ft wide in 5 or 10 ft steps, cut to length, so the 20 x 16 default becomes a 20 x 20 ft piece. Put 8 oz non-woven underlayment or 2 in of sand under it and dig 2 in deeper to allow for it. Level the rim with a line level or laser first.

Size the pump to turn the volume over every 2 hours, or every hour for a heavily stocked koi pond. The GPH rating is at zero head. A 3 ft waterfall through 20 ft of 1-1/2 in tubing typically cuts a 1,500 GPH pump to about 1,000, so read the curve at your lift. The pump plugs into a GFCI-protected outdoor receptacle with an in-use cover (NEC 210.8(A), 406.9(B)); a new circuit is an electrician's job. A filter sized for a pond with no fish is usually too small once fish go in.

What this calculator leaves out

The result is water, liner and pump only. Not included: underlayment (same area as the liner), skimmer and filter, tubing and check valve, the GFCI circuit, or edge rock at roughly 1 ton per 10 ft of rim. Call 811 before digging, and ask the city: some treat water over 18 to 24 in deep as a pool and require a barrier.

Mistakes that cost money

Sizing the liner from the average depth: it must reach the bottom of the deep zone. Size from the deepest point plus 2 ft overlap.
No underlayment: a root or stone punctures 45 mil EPDM under 4 tons of water. Lay 8 oz non-woven geotextile first.
Roofing EPDM instead of fish-safe pond liner: roofing grade can carry talc and algaecides that kill fish. Buy 45 mil fish-safe EPDM.
Pump on a plain outdoor outlet: NEC 210.8(A) requires GFCI outdoors, and a pump leak in a pond is a shock hazard.
Rim out of level: water shows a 1 in error as bare liner on the high side. Level the edge with a laser.
Pump sized at zero head: every foot of lift and 10 ft of tubing cuts flow. Pick from the curve at the waterfall height.

Key facts

One cubic foot of water is 7.48 gallons and weighs 62.4 lb; a 1,000 gallon pond holds over 4 tons of water.
A pond pump should circulate the full volume every 1 to 2 hours; a waterfall needs about 100 GPH per inch of spillway width.
45 mil EPDM rubber is the standard flexible pond liner and is sold in widths of 10, 15, 20, 25, 30, 40 and 50 ft.
The old rule for goldfish is 1 in of fish per 10 gallons; koi grow to 2 ft or more and need 250 gallons or more each.
A pond with fish should be at least 2 ft deep, and 3 ft or deeper where it freezes so the fish can winter below the ice.
A submersible pump plugs into a GFCI-protected outdoor receptacle with an in-use cover (NEC 210.8(A), 406.9(B)); its cord is never spliced or extended under water.

Frequently asked questions

Can I use roofing EPDM or a tarp instead of pond liner?

Not for a pond with fish. Roofing EPDM can carry talc and algaecides, and a poly tarp goes brittle in sunlight within a couple of seasons. Buy 45 mil fish-safe EPDM in one piece over underlayment; it carries a 20 year warranty and takes a patch if you ever nick it.

What size pond liner do I need?

Add twice the depth plus twice the overlap to both the length and the width. A 12 x 8 ft pond 2 ft deep with 2 ft of overlap needs a 20 x 16 ft liner.

What size pump does a pond need?

Half the pond volume in gallons per hour, which turns the water over every 2 hours. A 1,436 gallon pond needs about a 718 GPH pump, more if it feeds a waterfall or a long pipe run.

How many fish can my pond hold?

About 1 in of goldfish per 10 gallons, so a 1,436 gallon pond supports around 144 in of goldfish. Koi need far more room, about 250 gallons each, and grow much larger.

Sources and references

Ornamental Ponds and Water Gardens in Texas, EWF-014 (liners, filtration, fish stocking per 100 gallons) (Texas A&M AgriLife Extension)
Create a Garden Pond for Wildlife, EC 1548 (liner, depth, pumps and filters) (Oregon State University Extension)
Creating Water Features in the Landscape (flexible liners, depth, fish) (Mississippi State University Extension)
Handbook 44 Appendix C, General Tables of Units of Measurement (cubic feet to gallons) (NIST)

Next steps for this project

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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.