Project guide
Ponds & pools: start with the water volume
Almost every decision about a pond or pool depends on how much water it holds: liner size, pump and filter capacity, treatment doses, heating, and how long it takes to fill. These two calculators give you that number for the shapes people actually build.
Which calculator to use when
What the project involves
Ponds and pools look like simple boxes but rarely are. Garden ponds have sloped sides, shelves for marginal plants, and irregular outlines, so their true volume is noticeably less than length times width times depth. The Pond Volume calculator applies an editable shape factor for that, and also sizes a flexible liner from the dimensions plus depth and edge overlap. Pools usually have a shallow end and a deep end, so the Pool Volume calculator works from the average of the two depths.
Once you have the volume, the rest follows. Pumps and filters are sized by how often they turn over the full volume; water treatments and pool chemicals are dosed per litre or gallon; and the fill time tells you whether a garden hose is realistic or whether a water delivery makes more sense. Getting the volume right first prevents under-sized pumps and over- or under-dosed water.
Order of work
- Measure the longest length, the widest width, and the depth at the deepest point, or the shallow and deep ends for a pool.
- Run the Pond Volume or Pool Volume calculator. For ponds, adjust the shape factor if your sides are steeper or gentler than the default.
- For a new pond, order the liner using the liner length and width from the calculator, which include depth and edge overlap.
- Choose a pump and filter whose flow matches the suggested turnover for your volume, and check the head height on the product chart.
- Use the volume to dose treatments exactly as the product label instructs, and plan the fill using the estimated fill time.
Common mistakes
- Calculating a sloped pond as a full box, which can overstate volume and lead to overdosing treatments.
- Sizing a liner from the surface dimensions only, without adding twice the depth and an edge overlap.
- Choosing a pump by its maximum flow and ignoring the flow loss at the height it has to lift water.
- Using the deep-end depth for the whole pool instead of the average depth.
- Assuming the hose flow rate: time a bucket fill to get your real litres per minute before trusting a fill-time estimate.
Common questions
FAQ
Why is my pond volume lower than length × width × depth?
Most ponds have sloped sides and planting shelves, so they hold less water than a box of the same outer size. The shape factor accounts for this; raise it towards 1 for steep, box-like sides and lower it for shallow, bowl-shaped ponds.
How big a pump does my pond need?
A common approach is to circulate the full volume every one to two hours, with fish ponds often at the faster end. The calculator turns your volume and chosen turnover into a target flow; check that the pump delivers that flow at your head height, not just at zero lift.
How do I find my real fill rate?
Time how long a hose takes to fill a bucket of known size and convert it to litres per minute. Enter that figure in the Pool Volume calculator; household supply varies a lot, so a measured rate is far more reliable than a default.
Can I use the pool calculator for a round or oval pool?
It is designed for rectangular pools. For other shapes, use the manufacturer’s stated capacity where available, or treat the result as a rough upper bound and verify it before dosing chemicals.