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.

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 plus twice the overlap
Pump size: 718 GPH; Turns the whole pond over every 2 hours; 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 kidney or teardrop shape, 80 percent is a fair guess, and you can refine it by sketching the outline on grid paper. Depth is the hard part, because most ponds have a shallow plant shelf around the rim and a deeper zone in the middle. Estimate what share of the pond each covers and weight the average accordingly.

If the pond is already full and you want the exact volume, time how long a hose takes to fill a 5 gallon bucket, then time how long it takes to fill the pond from empty and scale up. Knowing the true volume matters when dosing treatments and sizing the filter.

Liner, pump and filter sizing

The liner has to go down one wall, across the bottom, up the other wall and out past the edge, so add twice the maximum depth and twice the overlap to both dimensions. Use the deepest point, not the average, when buying the liner. Underlayment goes down first to protect the rubber from stones and roots.

Size the pump to turn the volume over every 2 hours, or every hour for a heavily stocked koi pond, and add the head loss from the lift to a waterfall or filter. Biological filters are rated by pond volume and fish load; a filter sized for a pond with no fish is usually too small once fish go in.

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.

Frequently asked questions

How many gallons is my pond?

Multiply length by width by average depth in feet, then by 7.48. A 12 x 8 ft pond 2 ft deep holds 192 cubic feet, or 1,436 gallons.

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

The usual order for a landscaping & outdoor project. See all landscaping & outdoor calculators.

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