Pond Calculator
Pond volume ≈ length × width × average depth × a basin factor (0.9 rectangular, 0.7 oval) for sloping sides. A 10 × 6 ft pond with 18 in average depth holds about 81 ft³ — roughly 606 US gallons. The liner adds twice the maximum depth and twice the edge overlap to each side: 16 × 12 ft here.
How it's calculated: Volume ≈ L × W × average depth × basin factor; liner = (L + 2 × max depth + 2 × overlap) by (W + 2 × max depth + 2 × overlap).
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Volume ≈ Length × Width × Average depth × basin factor (0.9 rectangular, 0.7 oval)
Liner size = (Length + 2×max depth + 2×overlap) by (Width + 2×max depth + 2×overlap)
Run the calculation to see each step with your own numbers.
A rectangular pond 10 ft × 6 ft, 18 in average depth, 24 in max, 12 in overlap.
Volume ≈ 10 × 6 × 1.5 × 0.9 = 81 ft³ ≈ 606 US gallons
Liner ≈ (10 + 4 + 2) × (6 + 4 + 2) = 16 ft × 12 ft
- Average depth means the mean over the whole surface — for a bowl-shaped pond that's roughly 40–60% of the maximum depth.
- Basin factors (0.9 / 0.7) approximate the volume lost to sloping sides and shelves — a straight-walled formal pond is closer to 1.0.
- Liner is a single flat sheet; the calculation assumes it drapes into the deepest point.
- Bog filters, streams and waterfalls are extra.
- Why isn't it just length × width × depth?
- A real pond has sloped sides and planting shelves, so it holds less than a box of the same footprint. The basin factor accounts for that; a straight-sided formal pool is closer to the full box volume.
- Average depth or maximum depth?
- Both, for different things. The average depth (roughly 40–60% of the maximum for a bowl-shaped pond) sets the water volume; the maximum depth sizes the liner, which has to reach the deepest point.
- How much liner overlap do I need?
- About 12 inches past the finished edge on every side, to fold under the edging stone and anchor the liner. The calculator adds this to both dimensions.
- How many gallons is my pond?
- Multiply the surface length by width and average depth in feet, then by the basin factor (0.9 rectangular, 0.7 oval) and by 7.48 for US gallons. A 10 × 6 ft pond with 18 in average depth holds about 606 gallons.
- What size pond liner do I need?
- Add twice the maximum depth and twice the edge overlap to both the length and the width. A 10 × 6 ft pond, 24 in deep at most, with 12 in of overlap needs a 16 × 12 ft liner.
Volume + liner
Standard geometry with the water-garden industry basin-factor and liner-sizing rules of thumb (widely published by pond-supply manufacturers). Unit conversions per NIST SP 811.
About the Pond Calculator
The Pond Calculator estimates a garden pond's water volume and the size of the flat liner sheet to buy, from the surface dimensions and the depth. It applies a basin factor — about 0.9 for a rectangular pond, 0.7 for an oval — to account for the volume lost to sloping sides and planting shelves. Bog filters, streams and waterfalls are extra.
What you enter
- Pond shape
- Length (at the surface)
- Width (at the surface)
- Average depth — The mean depth across the whole pond, not the deepest point.
- Maximum depth — Used only to size the liner.
- Liner overlap each side — Extra liner past the edge to anchor under stone. ~12 in typical.
- Waste allowance — Default 0%. Adjust to your own allowance — the exact figure is always shown too.
What you get back
- Water volume — the exact result of the calculation.
- Liner length — the exact result of the calculation.
- Liner width — the exact result of the calculation.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
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