Pond Calculator

Pond Calculator: Volume and Liner Size for Garden Ponds

Every pond build starts with two numbers. The first is volume: how much water the pond holds, which decides the pump, the filter, the treatment doses and how many fish the pond can support. The second is the liner size: the sheet you actually have to buy, which is always bigger than the hole looks, because it has to travel down one side, across the bottom and up the other. This pond calculator works out both numbers for rectangular and round garden ponds, in US gallons and litres, from the length, width or diameter and the average depth.

Measure the pond at the waterline, at its longest and widest points. For depth, ponds are rarely one depth all over, so enter an average: if half the pond is a 1 ft plant shelf and half is a 3 ft deep middle, an average of about 2 ft is a fair entry. The sections below explain the liner maths, the average-depth question, pump sizing and fish stocking in more detail.

The liner formula, explained

A liner does not lie flat on the surface of the water; it lines the hole. Travelling from one bank it goes down the side, across the bottom and up the far side, so every dimension grows by twice the depth before you add anything else. On top of that you need an anchoring edge, about 1 ft (0.5 m in metric) on each side, to bury under soil or weigh down with stones so the edge cannot slip into the pond. Put together: liner length = pond length + twice the depth + the anchoring edge, and the same again for the width.

So an 8 × 6 ft pond, 2 ft deep, needs a 14 × 12 ft liner: 8 + 4 + 2 across the length, 6 + 4 + 2 across the width. For a round pond the same maths applies to the diameter, and the liner is a square sheet of that size; the corners fold or pleat into the curve as you fill, which is normal with flexible liner. Buy the sheet before you finish digging if you can, lay it loosely over the hole, and let the weight of the water settle it into shape before trimming the edge. If your size lands between two stocked sizes, order the next size up — a liner that is a foot too big trims in minutes, while one that is a foot too small does not work at all.

Average depth: shelves and the deep middle

Most garden ponds are built in steps: a shallow shelf around the edge, 9 to 12 inches deep, for marginal plants, and a deeper middle where the fish live. Neither number on its own is the depth to enter. For volume, use the average of the shelf depth and the deepest depth, weighted roughly by how much of the pond each covers; a simple halfway figure is a reasonable start, and the pump and filter tolerances cover the difference. For the liner, what matters is the deepest point the liner has to reach, so if your shelves are wide, enter the deepest depth for the liner maths.

To keep things simple and honest: this calculator uses the single depth you enter for both jobs. That is exactly right for a pond of even depth, and close enough for most shelved ponds if you enter the average for volume. If your pond has wide shelves and a small deep area, run it twice — once with the average depth for the volume, once with the deepest depth for the liner — and use each result for its own job.

Pump sizing: turn the water over

The working rule for pond pumps is turnover: the pump should move the pond's full volume every one to two hours. For the 718-gallon example pond above, that means a pump rated around 400 to 700 GPH (gallons per hour). Check the rating at the height the pump actually has to lift the water, not the headline figure, which is measured with no lift at all; a pump feeding a waterfall loses flow with every foot of height. Wildlife ponds without fish can run gentler, while heavily stocked fish ponds and koi ponds want the full two-hour turnover or better.

Filters are rated the same way, by the pond volume and flow they are built for. Match the filter to at least your pond's volume at the flow your pump delivers, and size up if you plan to add fish later. Getting the volume right here is the whole game: a filter that is fine on paper for 500 gallons will struggle all summer on a pond that actually holds 718.

Stocking: fish need volume and depth

Fish need water that stays stable. A pond at least 2 ft deep gives them a layer that does not freeze solid in winter and does not overheat in summer, which is why depth matters as much as surface area. As starting rules only, allow roughly 50 gallons per goldfish, and far more for koi — 250 gallons each or more, because koi grow past 2 ft long and produce waste to match. These are rough guides, not guarantees: strong filtration, shade and surface area all change what a pond can carry, and it is always kinder to stock under the limit than over it.

Add fish slowly, a few at a time over a season, and let the filter bacteria build up to meet them. And remember that the volume you calculated is the pond when full to the brim; the working volume with plants, shelves and a sensible freeboard is a little less, which is another reason to keep a margin.

What to calculate next

A pond usually arrives with other jobs attached. These tools cover the common ones:

• Pool Volume Calculator — for swimming pools, not ponds: volume for chemicals and heating.
• Cubic Yard Calculator — work out the soil you dig out of the hole, in cubic yards, for barrowing or a skip.
• Retaining Wall Calculator — blocks for stone edging and raised pond walls.

Frequently asked questions

Estimates only — flexible liners forgive small errors; order the next liner size up if you are between sizes.