How to calculate pond volume
Measure the length, width and average depth in feet. Multiply them to get cubic feet, and multiply by 7.48 to get gallons. Natural ponds with sloping sides and curves hold less than a box of the same size, so use 6.7 instead.
Circle: gallons = 3.1416 × (diameter ÷ 2)² × depth × 7.48
Irregular: gallons ≈ average length × average width × average depth × 6.7
Worked example
A rectangular pond 10 ft long, 6 ft wide and 2 ft deep: 10 × 6 × 2 = 120 cubic feet × 7.48 = 898 gallons. You need a liner of 10 + 2×2 + 2 = 16 ft by 6 + 2×2 + 2 = 12 ft, and a pump of at least 900 gallons per hour.
Liner size
Liner width = pond width + 2 × max depth + 2 ft overlap
Use the maximum depth here, not the average, so the liner reaches the deepest point.
Pump and fish
- Pump: turn the whole pond over at least once an hour for a fish pond (once every 2 hours for a plant-only pond). Add more for waterfalls: about 100 gallons per hour for each inch of waterfall width.
- Koi: they grow to 2 to 3 ft. Plan at least 1,000 gallons and about 3 ft of depth for the first few, and about 250 gallons per koi.
- Goldfish: about one per 50 gallons, or 1 inch of fish per 10 gallons.
- Depth: in cold climates, at least 2 to 3 ft so fish can overwinter below the ice.
Keeping fish indoors? Use the aquarium calculator.
Frequently asked questions
How do I calculate how many gallons are in my pond?
Multiply length × width × average depth in feet, then multiply by 7.48 for a box-shaped pond or 6.7 for a natural, irregular pond. A 10 × 6 × 2 ft pond holds about 900 gallons.
How big does a koi pond need to be?
At least about 1,000 gallons and 3 ft deep, with about 250 gallons per koi, because koi grow to 2 to 3 ft long.
What size pump do I need for my pond?
A pump that moves the full volume of the pond every hour. A 1,000 gallon pond needs at least 1,000 gallons per hour, plus extra for a waterfall.
How big a pond liner do I need?
Pond length plus twice the maximum depth plus 2 ft, by pond width plus twice the maximum depth plus 2 ft.
How many goldfish can I keep in a pond?
About one goldfish per 50 gallons, or 1 inch of fish per 10 gallons, so a 500 gallon pond suits about 10 goldfish.
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Last updated October 2026.