Pond Calculator
Pond volume in gallons and liters for rectangular, round and natural ponds — plus the liner size to buy, worked out with the depth and overlap allowance.
Pond volume and the liner that fits it
Two numbers run a pond project: how much water it holds — the figure pump sizing, treatment doses and fish stocking all start from — and how big a liner to buy, which is bigger than the hole in every direction. Volume is surface area × average depth, with the shape taken honestly: a rectangle counts in full, a round or oval pond at π/4 of its bounding box (0.7854), and a natural irregular outline at about 85%.
Gallons = length × width × shape factor × average depth × 7.48052 · Liner side = pond side + 2 × depth + 2 ft overlap
Example: a rectangular pond 10 × 6 ft, averaging 2 ft deep, holds 120 cu ft of water — 898 gallons (3,398 liters). The liner has to run down one wall, across the floor and up the other: 10 + 2×2 + 2 = a 16 × 12 ft liner.
The liner rule is the one people get wrong: the sheet must travel the full profile of the pond — down, across, up — and still land a foot of spare edge on every side for anchoring under stone or soil. Measure depth at the deepest point for the liner even though volume uses the average. And a pond is rarely as deep as its digging: shelves for plants and a beach edge make the average depth a foot or more less than the deep end — average honestly or the gallon figure flatters the pond.
Frequently asked questions
How do I work out the average depth of a pond with shelves?
Weight it by area: if half the pond is a 1 ft shelf and half is a 3 ft deep zone, the average is 2 ft. A quick field method is to sound the depth in a grid of spots and average the readings — close enough for pumps and treatments.
What liner thickness should I buy?
45 mil EPDM rubber is the long-life standard for garden ponds — flexible, fish-safe and repairable. Thinner PVC liners cost less and suit small or temporary ponds but age faster in sunlight. Whatever the size this calculator gives, buy the next stock size up rather than trimming the allowance away.
Why does the liner need a foot of overlap each side?
The overlap is the anchor: it lies flat on the ground around the rim, pinned by edging stone or buried in a shallow trench, so the sheet cannot slip into the pond as it fills and settles. Skimping the overlap is how liners end up a foot short at the far wall.
How many gallons does a koi pond need per fish?
A common planning figure is 250 gallons per koi at adult size, with 1,000 gallons as a sensible minimum pond for koi at all. Goldfish and smaller pond fish need far less — around 10 gallons per inch of fish is the usual rule of thumb — and filtration matters as much as volume.
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