How it works
The calculator adds twice the deepest point and twice the edge overlap to the pond’s longest length and widest width, and multiplies the two results for the liner area. The underlay is the same size. With an average depth it also works out the water volume.
Liner length = Pond length + 2 × Deepest point + 2 × Overlap
Liner width = Pond width + 2 × Deepest point + 2 × Overlap
Liner area = Liner length × Liner width
Water = Length × Width × Average depth
A cubic foot holds 1,728 ÷ 231 ≈ 7.48 US gallons and a cubic meter 1,000 liters. Units are converted internally, so the pond can be measured in feet or meters and the depths in inches or centimeters.
Worked example
Inputs
- Pond length (longest)
- 10 ft
- Pond width (widest)
- 8 ft
- Deepest point
- 24 in
- Edge overlap (each side)
- 12 in
- Average depth
- 18 in
Result
- Liner length
- 16 ft
- Liner width
- 14 ft
- Liner area
- 224 sq ft
- Underlay area
- 224 sq ft
- Water volume
- 897.7 gal (120 cu ft)
Common mistakes
- Using the average depth for the liner. The liner has to reach the deepest point, so its size uses the maximum depth; the average is only for the water volume.
- Measuring a free-form pond along its curves. Measure straight across the longest and widest points – the liner is a rectangle that covers both.
- Forgetting the overlap. Without spare liner at the rim there is nothing to tuck under stones, and the water level cannot reach the top.
- Leaving out the underlay. Firestone’s guide places a geotextile fabric under the liner to protect it from stones and roots.
- Counting shelves twice. Depth is added once on each side for the deepest point; shelves on the way down are already covered by that allowance.
Assumptions and limits
- Liner length = pond length + 2 × deepest point + 2 × overlap, and the same for the width – the method in Firestone’s PondGard guide, with its 2 extra feet split into 12 in per edge.
- The underlay is the same size as the liner.
- The water volume is length × width × average depth, treated as a box. Sloping or curved sides hold less, so the real volume is usually lower.
- Volume is in US gallons (231 cubic inches) or liters.
- The liner size is not rounded; pick the next stock size up.
Frequently asked questions
What size liner do I need for a 10 × 8 ft pond, 2 ft deep?
Add twice the depth and the overlap to each side. With 1 ft of overlap per edge, 10 + 2 × 2 + 2 × 1 = 16 ft long and 8 + 4 + 2 = 14 ft wide, so a 16 × 14 ft liner (224 sq ft) or the next larger stock size.
Why is the depth counted twice?
The liner runs down one side of the pond, across the bottom and up the other side, so each dimension needs the depth once for each side wall.
How much overlap should I leave?
Firestone’s PondGard guide adds 2 extra feet to each dimension, which is 1 ft at each edge. Leave more if your edging needs it, and trim any surplus after the pond is filled.
How many gallons is my pond?
Multiply length × width × average depth in feet and multiply by about 7.48 gallons per cubic foot. A 10 × 8 ft pond with an average depth of 1.5 ft holds about 120 × 7.48 = 898 gallons.
Do I need an underlay?
Firestone’s PondGard guide places a geotextile fabric as underlay before the liner goes in. This calculator sizes it the same as the liner; underlay is often sold in rolls, so compare the area with the roll size.
Sources
- Firestone PondGard EPDM Rubber Pond Liner brochure (Item 1947) — Firestone Building Products. How to calculate quantity of liner: longest, widest and deepest dimensions; total length = length + 2 × depth + 2 extra feet, total width likewise (overlap default 1 ft per edge); a geotextile underlay goes under the liner.
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 5, 2026