Pond Calculator
Planning a garden pond? Enter length, width and average depth to get water volume in gallons and the liner size to buy.
Calculate your pond volume and liner
Use average depth, not the deepest point â pond bottoms slope.
Why pond volume matters
Nearly every pond decision flows from volume: pump size (you want to turn over the full volume every 1â2 hours), filter capacity, fish stocking (roughly 1 inch of fish per square foot of surface, or 1 goldfish per 50 gallons as a starting point), dechlorinator dosing, and even how much beneficial bacteria starter to add. Guessing the volume is how ponds end up with undersized pumps and green water.
The math: length à width à average depth gives cubic feet; one cubic foot of water is 7.48 gallons. A 10Ã8 ft pond averaging 2 ft deep holds about 1,197 gallons â a very typical backyard size.
Sizing the liner correctly
The liner has to go down one side, across the bottom, and up the other side â that's why depth counts twice â plus extra to fold over the edge and anchor under rocks or soil:
Liner length = pond length + 2 Ã depth + 2 ft
Liner width = pond width + 2 Ã depth + 2 ft
The extra 2 feet is edge overlap: about a foot on each side tucked under the edging. Skimp here and you'll be fighting a liner that pulls back into the pond every time it rains. When in doubt, round up to the next standard liner size â excess liner hides under rocks, but a short liner can't be stretched.
How deep should it be?
- 2 ft minimum for most climates â shallower ponds freeze solid in winter and overheat in summer.
- 3 ft+ if fish will overwinter â they need a stable, unfrozen zone below the ice, plus a floating de-icer or aerator to keep a gas-exchange hole open.
- 6â12 inch marginal shelves around the edge for bog plants like iris, cattail and pickerelweed â dig them as ledges before laying the liner.
Liner material and underlayment
45-mil EPDM rubber is the gold standard: it stays flexible in freezing weather, is fish-safe, and routinely lasts 20+ years. PVC liners cost less but go stiff and brittle with UV exposure. Whatever you choose, lay protective underlayment first â purpose-made geotextile, or old carpet in a pinch â because one sharp stone under 1,200 gallons of water will find the liner.
Irregular shapes
For kidney or oval ponds, measure the longest length and widest width, then multiply by 0.8 to approximate the shape (0.785 for a true circle, using diameter for both). Multiply by average depth and 7.48 for gallons. The estimate runs slightly high, which is the safe direction for sizing pumps and filters.
Frequently asked questions
How do I calculate pond volume in gallons?
Multiply length à width à average depth (in feet) by 7.48. A 10Ã8 ft pond averaging 2 ft deep holds about 1,197 gallons.
How do I calculate what size pond liner I need?
Liner length = pond length + (2 Ã depth) + 2 ft; liner width = pond width + (2 Ã depth) + 2 ft. The doubled depth covers both sides, the extra 2 ft is edge overlap for anchoring.
How deep should a garden pond be?
At least 2 ft deep for most climates; 3 ft or deeper if fish overwinter. Add 6â12 inch marginal shelves around the edge for bog plants.
What is the best pond liner material?
45-mil EPDM rubber: flexible in cold, fish-safe, 20+ year lifespan. Always use underlayment beneath it to prevent punctures.
How do I estimate pond volume for an irregular shape?
Measure longest length à widest width à 0.8 (0.785 for circles), then à average depth à 7.48. Slightly high estimates are safe for pump and filter sizing.