Knowing your pond size is an important step in determining the appropriate amount of product to purchase or aeration system to install. When determining the size of your pond, there are two main measurements to know about your pond: surface area (acre) and volume (acre-feet).
Larger bodies of water typically use “acres” as the unit of measure for surface area. One acre is 43,560 square feet. Determining the acreage of a pond will be necessary to calculate the pond volume. Pond volume is measured in acre-feet of water. Not to get too far ahead of ourselves, but one acre-foot of water is one acre of water one foot deep. Confused? Stay with it and we’ll help break this down.
Determining Surface Area of a Pond.
Satellite imagery of ponds has become readily available with several online resources with the most well known being Google Earth. Determining the surface area of a water body is made much easier using satellite images and corresponding measuring tools that are available with these types of programs. There are a few general formulas that can be used based on the overall shape of the pond whether satellite imagery is available or not.
Square or Rectangular Ponds: Determine the acreage by multiplying average length (in feet) times average width (in feet) and dividing the result by 43,560.
FORMULA: Average Length (ft.) X Average Width (ft.) ÷ 43,560 = Acres
Example: A mostly rectangular ponds measures 350 feet long and 200 ft wide. To calculate acreage: 300 X 200 = 70,000 ÷ 43,560 = 1.6 acres.
Circular Ponds: An alternative to ℼr² for surface area of circular ponds – Determine acreage by squaring the diameter, dividing the answer by 43,560 and multiplying by 0.8.
FORMULA: Diameter (ft.) X Diameter (ft.) ÷ 43,560 X 0.8 = Acres
Example: A circular pond measures 235 feet across. To calculate acreage: 235 X 235 = 55,225 ÷ 43,560 = 1.27 X 0.8 = 1.0 Acres
Elliptical Ponds: Determine acreage by multiplying overall length (ft.) times maximum width (ft.), dividing by 43,560 and multiplying the answer by 0.8.
FORMULA: Length (ft.) x Width (ft.) ÷ 43,560 X 0.8 = Acres
Example: An elliptical pond is 325 feet long across the middle and 135 feet wide. To calculate acreage: 325 X 135 = 43,875 ÷ 43,560 = 1.00 x 0.8 = 0.8 Acres.
If a pond is a combination of these shapes, use these formulas for the various sections of the pond and add them up for an accurate surface area measurement.
Determining Pond Volume
There are some good ways to estimate the volume of water in a pond. A pond’s water volume is typically measured in acre-feet of water. This is a fairly simple calculation: Acreage X Average Depth (ft) = acre-feet
Average Depth: Determining average depth is most often calculated by dividing the maximum depth by two. This works if the pond bottom has a relatively uniform bottom slope.
FORMULA: Maximum Depth (ft.) ÷ 2 = Average Depth (ft.)
Example: A pond with maximum depth of 12′ and uniform slopes. To calculate average depth: 12 ÷ 2 = 6 feet average depth.
Pond Gallons: It is less common to know the actual gallons of water in a large pond. But if the gallons are known the formula for volume in acre-feet is: Gallons ÷ 325,829 = 1 acre-foot. Quick example: A pond with four million gallons
4,000,000 ÷ 325,829 = 12.28 acre-feet
Putting it All Together
With all the formulas out of the way, let’s recap. Knowing your pond’s acreage and volume (even as a solid estimate) is an important step in taking care of your pond. This knowledge helps to size aeration equipment or how much water treatment product to use with your pond. If you have questions on the size of your pond or need help determining which EasyPro product to use, please feel free to use our “Contact Us” form for assistance.
(Calculations and examples from How to Identify and Control Water Weeds and Algae 5th Edition, Applied Biochemists et al, 1998, page 76-77)