Measuring a pond
A pond’s volume is its area times its average depth. A rectangle is its length times its width, a circle and an oval are π times the two across, divided by four. An irregular pond is taken at its average length and its average width: measure it across in a few places each way and average them. For the depth, measure at several points, shelves included, and average those too.
A cubic foot holds 7.48 US gallons, a US gallon being 231 cubic inches (NIST).
Worked example. A pond of 10 × 6 ft, 2 ft deep.
- Volume: 10 ft × 6 ft × 2 ft = 120 cu ft
- In gallons: 120 × 7.48 = 898 US gallons, or 3,398 L
- Liner: 10 ft + 2 × 2 ft + 2 × 1 ft long, by 6 ft + 2 × 2 ft + 2 × 1 ft wide: 16 × 12 ft
- Pump: 898 gallons an hour, moving the pond once an hour
A round pond 8 ft across and 2 ft deep holds 752 gallons and takes a liner of 14 × 14 ft. An oval of 10 × 6 ft at the same depth holds 705, less than the rectangle round it.
The liner and its underlayment
The liner covers the floor, runs up every wall and lies past the edge. Pond Pro Direct puts it as the pond’s length plus twice its deepest point plus 2 ft, and its width the same way: 1 ft on each side, to anchor the edge, is its least. For an irregular pond, or one with a tall waterfall, it gives 1.5 ft a side, and the calculator takes that when you choose Irregular. Its own example, a pond of 10 × 8 ft and 2 ft deep, comes to 16 × 14 ft here as there. The underlayment under the liner is cut to the same size.
The depth that counts for the liner is the deepest point, where the liner goes down furthest; the calculator uses the average depth for it only while you leave the deepest point empty. If the example pond goes down to 3 ft at its middle, its liner is 18 × 14 ft. An irregular pond is lined across its longest and its widest, not its averages: at 12 ft longest and 8 ft widest, with 1.5 ft a side, the example’s liner is 19 × 15 ft, while its water is still worked out on the averages, 898 gallons.
Liners are sold in set sizes: the same guide says to round up to the next size there is.
The pump
The same supplier’s guide to pumps asks for the whole volume of the pond to pass through the filter at least once an hour in a water garden or a pond with a few goldfish, 1.5 times an hour with many goldfish, and twice an hour for koi, 2.5 to 3 times in summer heat: 898 gallons an hour for the example, 1,347 with many goldfish, 1,796 with koi.
It sizes a pump by what it delivers at its total head, which is less than the open-flow figure on its box. The head is the height the pump lifts the water to, plus 1 ft for every 10 ft of tubing and 1 ft for each elbow; the maker’s chart of flow against head says what the pump delivers there.
Common mistakes
- Sizing the liner on the average depth of a pond whose floor slopes: type the deepest point.
- Lining an irregular pond on its averages. The liner has to reach across the longest and the widest.
- Buying a pump by the figure on its box, or by the lift alone. Add the tubing and the elbows to the lift, and read the flow at that head on the maker’s chart.
Questions about ponds
How many gallons is a pond of 10 × 6 ft and 2 ft deep?
898 US gallons (3,398 L): 120 cu ft at 7.48 gallons to the cubic foot.
How do I work out the size of a pond liner?
The length plus twice the deepest point plus twice the overlap, and the width the same way: 16 × 12 ft for the example pond.
How much overlap does a pond liner need?
At least 1 ft past each edge in Pond Pro Direct’s guide, and 1.5 ft for an irregular pond or a tall waterfall. Twice the overlap is added to the length of the liner, and twice to its width.
What size underlayment do I need?
The same size as the liner.
What size pump does my pond need?
One that moves the pond’s volume at least once an hour, more with fish, at its total head: 898 gallons an hour for the example, 1,796 with koi.
Sources
- Pond Pro Direct, Pond Liner Sizing Guide: How Much Liner Do You Need?. Liner length = pond length + 2 × depth + 2 ft, and liner width = pond width + 2 × depth + 2 ft: the 2 ft is 1 ft of overlap on each side, for anchoring the edge. One foot per side is the minimum; in complex builds with tall waterfalls or irregular shapes, use 1.5 ft per side. The depth is the maximum depth of the pond, never the average. A pond of 10 × 8 ft, 2 ft deep, needs a liner of 16 × 14 ft. Size the underlayment identically to the liner. Always round up to the next size available. Read on .
- Pond Pro Direct, How to Choose a Pond Pump: GPH, Head Height & Sizing Guide (8 April 2026). Move the full pond volume through filtration at least once per hour for water gardens and lightly stocked goldfish ponds, 1.5 times per hour for heavily stocked goldfish ponds, twice per hour for koi ponds, and 2.5 to 3 times per hour for koi in summer heat. Size a pond pump by its flow at the actual head height, never by its open-flow rating. Total head is the static lift plus friction: add 1 ft per 10 ft of tubing and 1 ft per 90° elbow. Read on .
- National Institute of Standards and Technology, Guide to the SI (Special Publication 811), Appendix B.9: conversion factors. A foot is 0.3048 m and an inch 2.54 cm, exactly. A cubic foot is 0.02831685 m³, a cubic yard 0.7645549 m³, a US gallon 3.785412 L, a square foot 0.09290304 m² and an acre 4,046.873 m². A US dry quart is 1.101221 L. Read on .
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