Knowing the volume of water your pond holds helps calculate factors such as pump size, chemical dosages and the type of filtration system needed. Ponds may not be an exact rectangle or have a shoreline that produces a perfect circle, but they are often a close approximation or combination of these shapes. To determine the volume of water (in gallons) in your pond, choose the calculation method that best fits the pond's shape.
Things You'll Need
 Tape measure
 Calculator
Rectangular Shaped Ponds

Measure the length and width of the pond. Stretch a tape measure from wet edge to wet edge.

Measure the average depth of the pond. If the pond has a combination of deep and shallow spots, take depth measurements at 5 evenly spaced points along a straight line that transects the length of the pond, and 5 more measurements along a straight line that transects the width of the pond. Add the 10 measurements, and divide by 10 to get the average depth.

Multiply the pond’s length by its width to get the square footage of the pond at its surface. Multiply the square footage by the average depth of the pond to arrive at the volume of water expressed in cubic feet. For example, a 12foot by 8foot pond with an average depth of 3 feet would be12 feet long x 8 feet wide x 3 feet deep for a result of 288 cubic feet of water.

Calculate the number of gallons of water by multiplying the cubic foot calculation by 7.48, which is the number of gallons of water in one cubic foot. In the example, 288 cubic feet x 7.48 equals 2,154.24 gallons of water in this pond.
Circular Ponds

Measure the diameter of the pond, and divide that figure by 2 to get the radius.

Square the radius measurement. For example, if the pond’s diameter was 10 feet, the radius is 5 feet. Squaring the radius equals 25.

Measure the depth at the center of the pond. For purposes of an example, assume it measures 3 feet.

Determine the shape of the pond’s sides. If they drop off quickly in a steep slope, the pond resembles a cylinder. If the sides taper toward the bottom, the pond resembles a cone.

Multiply a steepsided pond’s squared radius number by 3.14. In the example, 25 x 3.14 yields 78.5. For tapered sides, multiply the squared radius by 1.047. In the example, 25 x 1.047 yields 26.175.

Multiply the figure you just calculated by the pond’s depth to find the volume of water in cubic feet. Using the example for a steepsided pond, multiply 78.5 x 3 feet for 235.5 cubic feet of water. For the tapered pond example, multiply 26.175 by 3 feet of depth for 78.525 cubic feet of water.

Convert the cubic foot calculation to gallons of water by multiplying the result by 7.48. For the steepsided example pond, 235.5 cubic feet x 7.48 is 1761.54 gallons of water. For the tapered pond example, 78.525 cubic feet x 7.48 is 587.367 gallons of water.
Tips & Warnings
 For irregularly shaped ponds that do not fit well into circles or rectangles, divide the pond up into sections that more closely fit the models. For example, a kidneyshaped pond would most resemble two circles pushed together. Then make the calculations and add them up to arrive at the total number of gallons of water in the pond.
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