You need the gallons to buy a pump and a filter that fit the pond, to dose water treatments and to stock fish without crowding them. Garden ponds are rarely neat boxes, so the calculator also handles free-form ponds and ponds where you only know the deepest point.
How to use this calculator
- Choose the shape. For a kidney or free-form pond, choose Irregular and enter the average of several lengths and widths.
- Enter the depth: the average depth if you know it, or only the deepest point for a natural bowl-shaped pond.
- Under More options, add a waterfall and its width to size the pump for it too.
Worked examples
A rectangular water garden
Shape: Rectangle or square · Size: 10 × 8 ft · Depth: 2 ft
- Pond volume
- 1,197 US gallons
- 1,200 US gallons per hour
- Liters
- 4,530 L
- Cubic feet
- 160 cu ft
- Surface area
- 80 sq ft
- Pump flow for clearest water, up to
- 4,787 US gallons per hour
- Adult fish, at most
- 239 inches of fish
A round pond
Shape: Circle · Diameter: 8 ft · Depth: 1.5 ft
- Pond volume
- 564 US gallons
- 600 US gallons per hour
- Liters
- 2,135 L
- Cubic feet
- 75.4 cu ft
- Surface area
- 50.27 sq ft
- Pump flow for clearest water, up to
- 2,256 US gallons per hour
- Adult fish, at most
- 113 inches of fish
A kidney-shaped pond
Shape: Irregular (kidney, free form) · Average: 12 × 7 ft · Depth: 2 ft
- Pond volume
- 1,257 US gallons
- 1,300 US gallons per hour
- Liters
- 4,757 L
- Cubic feet
- 168 cu ft
- Surface area
- 84 sq ft
- Pump flow for clearest water, up to
- 5,027 US gallons per hour
- Adult fish, at most
- 251 inches of fish
A koi pond with a waterfall
Shape: Rectangle or square · Size: 16 × 12 ft · Depth: 3.5 ft · Waterfall: Gentle flow · Waterfall width: 18 in
- Pond volume
- 5,027 US gallons
- 5,050 US gallons per hour
- Liters
- 19,028 L
- Cubic feet
- 672 cu ft
- Surface area
- 192 sq ft
- Pump flow for clearest water, up to
- 20,106 US gallons per hour
- Adult fish, at most
- 1,005 inches of fish
A natural bowl-shaped pond
Shape: Oval · Size: 30 × 20 ft · Depth I know: Only the deepest point (bowl-shaped pond) · Depth: 5 ft
- Pond volume
- 7,050 US gallons
- 7,050 US gallons per hour
- Liters
- 26,686 L
- Cubic feet
- 942 cu ft
- Surface area
- 471 sq ft
- Pump flow for clearest water, up to
- 28,199 US gallons per hour
- Adult fish, at most
- 1,410 inches of fish
How pond volume is worked out
Multiply the surface area by the average depth, in feet, to get cubic feet, and each cubic foot holds 7.48 US gallons. A rectangle's area is length times width, a circle's is a little over three-quarters of its diameter squared, and an oval's the same for its length times its width.
For a free-form pond, Penn State Extension suggests averaging several lengths and several widths. The result usually lands within about 20 percent of the true area, close enough for a pump or a treatment dose.
Depth matters as much as area. A lined pond with flat shelves is easy: average the depths. For a natural bowl-shaped pond, Penn State puts the average depth at about 0.4 times the deepest point.
What size pump the pond needs
K-State's goal is to move the pond's whole volume 1 to 4 times an hour, and a mechanical filter only works if all the water passes through it at least once an hour. The calculator gives the smallest pump that does that, and the flow for the clearest water.
A waterfall can need more. K-State multiplies its width in inches by 50 to 75 gallons per hour per inch of width for a trickle, 100 to 150 gallons per hour per inch of width for a gentle flow and 200 to 300 gallons per hour per inch of width for a heavy one. Check the pump's flow at the height it has to lift the water, not its maximum rating.
How many fish a pond can keep
Texas A&M's rule is at most one inch of fish for every 3 to 5 gallons of water, counting the fish at their adult size: goldfish grow to 10 to 12 inches and koi to 2 to 3 ft. By weight, that is about 1 lb of adult fish per hundred gallons without a filter, and 2 lb with a biofilter, a waterfall and aeration.
UF/IFAS suggests at least 500 gallons of water for a fish pond, so the fish have room to swim and the water stays stable.
Common questions
How do I convert pond size to gallons?
Multiply the length, width and average depth in feet, then multiply by 7.48 US gallons per cubic foot. For a round pond, use a little over three-quarters of the diameter squared as the area.
How do I calculate the volume of a pond with sloping sides?
Use the average depth, not the deepest point: measure the depth in several places and average them. For a natural bowl-shaped pond, the average is about 0.4 times the deepest point.
What size pump do I need for my pond?
One that moves the pond's whole volume at least once an hour at the height it lifts the water, and up to 4 times an hour for the clearest water. A waterfall can need more; the calculator checks both.
How many fish can I put in my pond?
At most one inch of adult fish per 3 to 5 gallons of water. Count each fish at its adult size, and stock fewer if the pond has no filter.
How deep should a fish pond be?
Water gardens are often 18 to 24 inches deep, with a section 3 to 4 ft deep for fish to winter in; koi need at least 3 ft.