About this tool
Pond Calculator is a free, in-browser tool for planning a garden pond. Choose a rectangular or round shape, enter the dimensions and average depth in feet, and it returns the water volume, the liner size to buy, and how many koi or goldfish the pond can safely hold. Nothing is uploaded — the math runs on your device.
For a rectangular pond, volume in gallons is length × width × average depth × 7.4805 (US gallons per cubic foot); for a round pond it is π × (diameter ÷ 2)² × depth × 7.4805. Liters are gallons × 3.78541. The liner must wrap up and over both sides, so its length is the longest dimension + 2 × depth + 2 × overlap and its width is the shortest dimension + 2 × depth + 2 × overlap (a round pond needs a square liner of diameter + 2 × depth + 2 × overlap). Safe stocking uses about 250 gallons per koi and 30 gallons per goldfish, rounded down.
Use the volume for sizing pumps, filters and treatments, and the liner size when ordering flexible EPDM or PVC. Stock conservatively — the fish figures are maximums for mature fish, and a lightly stocked pond stays cleaner and healthier.