How to use the pond liner size calculator
- Choose feet or meters and the shape, rectangle or oval.
- Enter the length and width at the water line and the maximum depth.
- Enter the side slope, how far each side leans out per unit of depth (0 for vertical walls), and the overlap you want on each edge.
- Read the liner length and width, the bottom size, the length of each sloped side and the volume of water.
Formula
length of a sloped side = depth x sqrt(1 + slope^2)
bottom length = top length - 2 x slope x depth (the same for width)
liner length = bottom length + 2 x side length + 2 x overlap
rectangular volume = depth / 6 x (top area + bottom area + (L + l) x (W + w))
oval volume = pi x (a b D - s (a + b) D^2 / 2 + s^2 D^3 / 3)
L and W are the top length and width, l and w the bottom length and width, a and b are half the top length and width, D is the depth and s the slope. Liner length and width describe a flat rectangle big enough to cover the pond.
A 10 by 6 foot pond, 2 feet deep
With sides that lean out half a foot per foot of depth, each sloped side is 2 x sqrt(1 + 0.25) = 2.236 feet and the bottom is 8 by 4 feet. With a 1 foot overlap the liner is 8 + 4.472 + 2 = 14.47 feet long and 4 + 4.472 + 2 = 10.47 feet wide, which is 151.6 square feet. The pond holds 90.7 cubic feet, about 678 US gallons. An oval pond of the same size holds 71.2 cubic feet, about 533 gallons.
With vertical walls the liner for the same pond is 10 + 4 + 2 = 16 feet long and 12 feet wide, and it holds 120 cubic feet, about 898 gallons. The rectangular volume was checked by adding up thin slices from top to bottom, which gives the same 90.667 cubic feet.
Why you add more than the pond size
The liner has to run down one side, across the bottom, up the other side and then out over the edge so that it can be held in place. Deep ponds use much more liner than their surface suggests, since each side of depth adds a length to both ends. The tool shows each step so you can see where the liner goes.
Folds, shelves and rolls
A flat sheet does not fit a curved hole without folds, and shelves for plants or steps use extra length. Many pond builders buy a little more than the minimum. Liners are sold in set sizes, so round up to the next one. To plan the stone for the edge, use the Mulch and Gravel Calculator, and for comparing water volumes the Pool Volume Calculator.
More tools for the workshop and home
For related projects around the house and workshop, try the Firewood Cord Calculator, Fabric Shrinkage Calculator and Cabinet Door Size Calculator.
Assumptions and limits
- The pond is a rectangle or an oval with straight sloping sides and a flat bottom.
- The depth is the same across the bottom.
- The liner lies flat against the sides, with no extra for folds, shelves or corners.
- The slope is the same on all sides.
- The size, depth, slope and overlap in the form are examples to replace with your own.
Frequently asked questions
What slope should I use?
It depends on the soil and the design. A steeper side holds more water for the same top size, and a gentler side is easier to climb out of. Enter the slope you plan to dig.
How is the oval volume found?
The tool adds up the area of an ellipse at every depth, which has a closed form for straight sloping sides. The result is smaller than for a rectangle of the same top size.
Should I measure the pond at the top or the bottom?
Enter the length and width at the water line. The tool works out the bottom from the depth and the slope.
Is the volume the amount of water I need to buy?
It is the volume of the hole up to the top, so it is more than you add once a margin is left below the edge. Fill it to the level you plan and use a smaller depth for the volume.