Pond Volume & Liner Calculator

The Pond Volume & Liner Calculator answers the two questions every pond builder needs before digging: how many gallons will it hold and how big a liner do I need to buy. Enter the length, width (or diameter) and depth of your pond and the tool works out the volume in US gallons, UK gallons and litres, then sizes a flexible liner sheet that drapes down every wall and leaves an overlap flap to anchor around the edge. It also suggests a suitable pump flow rate and a safe fish-stocking level, so you can plan the whole build from one page.

How to Calculate Pond Volume

Pond volume is simply the surface area multiplied by the average depth, converted into gallons or litres. Because pond floors are rarely flat, always use the average depth rather than the deepest point.

All dimensions are in feet. One cubic foot of water is about 7.48 US gallons, 6.23 UK gallons or 28.3 litres. Working in metric instead? Multiply length × width × depth in metres to get cubic metres, then multiply by 1,000 for litres.

Pond Liner Size Formula

A flexible pond liner is a single flat sheet that has to line the whole hole. On each axis it must cover the pond, run down one wall and up the other (that is why depth is counted twice) and leave an overlap flap on both edges to hold it in place:

For example, a pond 8 ft long, 6 ft wide and 2 ft deep with a 1 ft overlap needs a liner of 8 + 4 + 2 = 14 ft long by 6 + 4 + 2 = 12 ft wide. For a circular pond the liner is cut as a square using the diameter in place of both length and width.

Pond Liner Overlap Guide

The overlap is the extra material that folds over the edge of the excavation and is buried or weighed down with rocks, soil or edging stone. It stops the liner slipping into the pond as it fills and settles.

What Size Pump Does a Pond Need?

As a rule of thumb, a pond pump should circulate the entire volume of the pond at least once every hour. A 500-gallon pond therefore needs a pump rated for at least 500 gallons per hour (GPH). Ponds with koi, a waterfall, a long run of tubing, or a lot of vertical lift (head height) should size up to roughly twice the volume per hour to keep the water well oxygenated and the filter fed.

How Many Fish Can a Pond Hold?

Fish stocking is limited by surface area, not volume, because oxygen enters the water at the surface. Common conservative guidelines are about one small goldfish per 3 square feet of surface, or one koi per 10 square feet because koi grow large and produce a lot of waste. Always stock gradually while the biological filter matures, and under-stock rather than over-stock for clear, healthy water.

What Affects Your Pond Build?

How to Use This Calculator

1. Choose shape and units: Pick rectangular, circular or oval, and Imperial (feet) or Metric (metres).
2. Enter the pond size: Type the length and width (or diameter) and the average depth.
3. Set the edge overlap: Enter the flap you want around the edge — 1 ft (about 30 cm) is a good default.
4. Click Calculate: Read off the volume in gallons and litres, the exact liner sheet size, a recommended pump, and safe fish stocking, plus a step-by-step breakdown and diagrams.

Frequently Asked Questions

How do I calculate pond volume in gallons?

Multiply length by width by average depth to get the volume in cubic feet, then multiply by 7.48 to get US gallons (or 6.23 for UK gallons). For a round pond use π times the radius squared times the depth. In metric, length times width times depth in metres times 1,000 gives litres.

What size pond liner do I need?

Add twice the depth and twice the overlap to each side of the pond. Liner length = pond length + (2 × depth) + (2 × overlap), and liner width = pond width + (2 × depth) + (2 × overlap). The depth allowance lets the liner run down one wall and up the other, and the overlap is the flap you bury or weigh down with rocks around the edge.

How much liner overlap should I leave?

A 6 to 12 inch (15 to 30 cm) overlap all the way around is normal for most garden ponds. Larger or informal ponds often use 12 inches or more so there is plenty of material to anchor under rocks, soil or edging and to allow for the liner settling as the pond fills.

What size pump does my pond need?

As a rule of thumb a pond pump should circulate the entire volume of the pond at least once every hour. So a 500 gallon pond needs a pump rated for at least 500 gallons per hour (GPH). Ponds with koi, waterfalls or long tubing runs should aim for roughly twice that flow rate.

How many fish can my pond hold?

Fish stocking is limited by surface area, because oxygen is exchanged at the water surface. A common guideline is about one small goldfish per 3 square feet of surface, or one koi per 10 square feet since koi grow much larger. Stock conservatively and add fish gradually while the filter matures.

Should I measure average or maximum depth?

Use the average depth for the volume calculation, because pond floors are rarely flat. For sizing the liner, using the deepest point is safest so the liner is guaranteed to reach the bottom and still leave enough overlap around the edge.

Additional Resources

• Pond Liner - Wikipedia
• Gallon - Wikipedia