Common Calculations to Know for Building and Maintaining Your Water Garden

Common Calculations to Know for Building and Maintaining Your Water Garden

Posted on Mar 25th 2021
Author: Bill Hoffman

Knowing your pond inside and out will help you to have a successful and pleasant ponding experience for years to come. From planning your build to maintaining the water quality, these calculations below will prove useful at some point in your ponding experience.


Figuring Out Pond Liner Size:


Once you have finished digging, you will want to go to the extremes when you measure. Record the widest point (W), the longest point (L), and the deepest point (D). Be sure to include the height of your waterfall, as well as the length and width of your stream. It is always best to use one solid liner, when possible, to reduce the possibility of leakage.

To determine what size liner you will need, use the following formula: L + 2D; W + 2D or Length plus twice the depth; and Width plus twice the depth.

For example: If your longest measurement is 10ft, the widest point is 8ft, and your deepest is 2ft you would have:

10 + 2(2); 8+ 2(2) or 10 + 4; 8 + 4.

The minimum liner size you would need is 14 x 12.

Since liner is sold in widths of 5-foot increments, it is best to round up. (i.e., 15 x 12)


Figuring Out Underlayment Size:


While using the liner size as a guide, determine how much underlayment you will need starting with multiplying the length and width of the liner.

For example: If your liner is 15 x 12 = 180 sq ft. then you will need approximately 180 sq. ft. of underlayment.

Since underlayment typically come in 12’ widths, divide your sq. ft. by 12 to determine the length you will need.

In this instance, it works out to be perfectly 15-feet.

**Underlayment does not need to be one piece. It may be cut and overlapped where needed to get full coverage.


The Importance of Knowing the Surface Area of your Pond:


Surface area is determined by multiplying the length by the width (L x W) of the pond water’s surface.

Your 8 x 10 pond has a surface area of 80 sq. ft: 8 x 10 = 80.

Surface area comes into play when determining how many plants your pond needs and how many fish it will sustain.

  • Underwater Plants (Submersibles/seaweed): 1 weighted bundle per 2 sq. ft. of surface area.
    • i.e., 80 / 2 = 40 weighted bundles
  • Floating Pond Plants: 40 – 60%of the surface are covered
  • Fish are determined by their size in inches as far as how many your pond can handle. Do not populate your pond more than 1” of fish per sq. ft. of surface area – especially in the first season.
    • For example: 80 sq. ft. of surface area can sustain 80” of fish. Take care to only introduce a few fish at a time so as not to overwhelm the biological filtration and eco-system.

The Importance of Knowing your Pond Volume:


The volume of your pond is an estimate of how many gallons of water it holds.

For a more accurate assessment, we use average measurements instead of extremes like we did for figuring the liner size. Your example of an 8 x 10 pond may be rounded or misshapen with shallower areas than others. (Let’s say the example pond averages at 7’ W x 8’ L x 1.5’ D.) Record average measurements of the length (L), width (W), and depth (D). Now multiply them all together to get cubic ft. (7 x 8 x 1.5 = 84 cu. Ft.)

Multiply the cubic feet by 7.48; which is how many gallons there are in 1 cu. Ft. So, 84 x 7.48 = 628.32 gallons. Since we are taking averages, it is okay to round. (i.e., 630 gallons)

Knowing a close estimate of how many gallons of water your pond holds will be useful in determining how to size your pump and in measuring water additives and treatments for a healthy eco-system and happy pond fish.