Pond Calculator
Calculate pond accurately for your build. Get material quantities, waste allowances, and project cost breakdowns. Get results you can export or share.
Formula
Volume (cu ft) = L x W x D | Gallons = cu ft x 7.48
For rectangular ponds, multiply length times width times average depth in feet, then multiply by 7.48 to convert cubic feet to US gallons. Circular ponds use pi x radius^2 x depth. Oval ponds use (pi/4) x length x width x depth. Liner size adds twice the depth plus 2 feet overlap to each dimension.
Worked Examples
Example 1: Backyard Koi Pond
Problem: Calculate volume and liner size for a rectangular pond 15 ft x 8 ft x 3 ft deep.
Solution: Volume = 15 x 8 x 3 = 360 cu ft\nGallons = 360 x 7.48 = 2,693 gallons\nLiner = (15 + 6 + 2) x (8 + 6 + 2) = 23 x 16 ft\nPump needed = 1,347-2,693 GPH
Result: 2,693 gallons with a 23 x 16 ft liner
Example 2: Circular Garden Pond
Problem: Calculate the volume of a circular pond 10 ft in diameter and 2.5 ft deep.
Solution: Radius = 10/2 = 5 ft\nVolume = pi x 25 x 2.5 = 196.3 cu ft\nGallons = 196.3 x 7.48 = 1,469 gallons\nLiner = (10 + 5 + 2) x (10 + 5 + 2) = 17 x 17 ft
Result: 1,469 gallons with a 17 x 17 ft liner
Frequently Asked Questions
How do I calculate pond volume in gallons?
For a rectangular pond, multiply length by width by depth (all in feet) to get cubic feet, then multiply by 7.48 to convert to gallons. For a circular pond, use pi times radius squared times depth. For an oval or kidney-shaped pond, multiply length times width times depth times 0.785 (which is pi/4). Natural ponds with irregular shapes can be estimated by dividing them into simpler geometric sections and adding the volumes. Average depth is often used since most ponds slope from shallow edges to a deeper center.
What size pond liner do I need?
To calculate liner size, add twice the maximum depth plus two feet of overlap to both the length and width of the pond. For example, a 20x10 foot pond that is 4 feet deep needs a liner that is at least 30x20 feet: 20 + (2 x 4) + 2 = 30 feet long, and 10 + (2 x 4) + 2 = 20 feet wide. The extra two feet provides material for anchoring the edges under rocks or soil. For irregularly shaped ponds, use the maximum length and width dimensions.
How many fish can my pond hold?
A conservative rule is one inch of fish per two gallons of water for goldfish, or one inch per gallon for lighter stocking. Koi need more space due to their size and waste production, typically 250 to 500 gallons per adult koi. A 1,000-gallon pond could comfortably house 3 to 4 adult koi or about 20 to 30 small goldfish. Overstocking leads to poor water quality, algae blooms, and stressed fish, so adequate filtration must scale with fish load.
What size pump do I need for my pond?
A pond pump should circulate the entire pond volume at least once every two hours for fish ponds, or once per hour for koi ponds with heavy stocking. For a 5,000-gallon pond, you need a minimum 2,500 GPH pump (5,000 GPH for koi). Also factor in head height, which is the vertical distance the pump must push water up to a waterfall or filter. Most pumps lose 10 to 20 percent flow rate per foot of head height, so choose a pump rated above your minimum requirement.
How do I get the most accurate result?
Enter values as precisely as possible using the correct units for each field. Check that you have selected the right unit (e.g. kilograms vs pounds, meters vs feet) before calculating. Rounding inputs early can reduce output precision.
Can I share or bookmark my calculation?
You can bookmark the calculator page in your browser. Many calculators also display a shareable result summary you can copy. The page URL stays the same so returning to it will bring you back to the same tool.