Pond Liner Worksheet

The arithmetic

Liner length

pond length + 2 x depth + 2 x overlap

Liner width

pond width + 2 x depth + 2 x overlap

⚠ Depth counts TWICE

down one side and up the other

Use MAX depth for the liner

it must reach the bottom

Use AVERAGE depth for volume

that is what holds water

Volume (upper bound)

L x W x avg depth x 7.4805

1 cubic foot

7.4805 gal (1728 ÷ 231, exact)

Water weight

about 8.345 lb per gallon

Underlay

same area as the liner

Your pond

Pond

Date

Length

Width

Maximum depth

Average depth

Overlap per edge

LINER LENGTH

LINER WIDTH

LINER AREA

Roll width available

Offcut

VOLUME (gal)

WEIGHT (lb)

Before you dig and before you fill

• Depth added TWICE in each direction
• Liner sized on MAXIMUM depth, not average
• Overlap generous - it cannot be added later
• Underlay ordered to the same area as the liner
• Roll width checked against the liner WIDTH, not the length
• Pond turned against the roll to see if it saves a size
• Ground beneath cleared of stones and roots
• Weight of the filled pond checked if it is raised or on a structure
• Shelves for planting dug before the liner goes in
• Liner laid loose and allowed to settle before trimming

A 10 ft pond needs a 16 ft liner

At 2 ft deep with a foot of overlap each side: 10 + 2 + 2 + 1 + 1 = 16 ft long, and 6 + 2 + 2 + 1 + 1 = 12 ft wide. The depth is in there twice because the sheet goes down one side and up the other. It is the most common surprise in a first build and the reason people come home with a liner too small.

Trim last, not first

Lay the liner loose, fill it slowly, and let the water press it into the shape of the hole before you cut anything. It will move as it settles, and any overlap you trimmed off beforehand is gone. Cutting is a minute's work at the end; guessing at the start is how a liner ends up an inch short at one corner.