Estimating surface areas of ponds with different shapes:

Circular Ponds:

The surface area of a circular pond can be estimated by measuring either the radius or the diameter of the pond (Figure 1). The radius is the distance measured from a point in the center of the pond to a point on the bank. The diameter is the distance measured all the way across the pond, from one bank to the opposite bank, passing through the center of the pond as illustrated below.

Figure1. Radius and diameter of a circle

If the radius is measured, the area of the pond is determined by the equation:

area = 3.14 x radius x radius

where 3.14 is actually the number "Pi"
Because the radius is measured in feet, the units of area will be square feet.

Example when using the radius of a pond:

A circular pond has a radius of 50 feet.
The area of the pond = 3.14 x 50 ft x 50 ft = 7,850 square feet

Now convert the area in square feet to acres:
1 acre = 43,560 square feet
For the example above: 7,850 sq. ft ÷ 43,560 sq. ft/acre = .18 acre

If the diameter of the pond is measured, the area of the pond is determined as:

area = (3.14 x diameter x diameter) ÷ 4

where 3.14 is the number "Pi"
Because the diameter of the pond is measured in feet and then this measurement is squared, the unit of area for a circular pond is square feet.

Example when using the diameter of a pond:

A circular pond has a diameter of 90 feet.
The area of the pond = (3.14 x 90 ft x 90 ft) / 4 = 6,358 square feet

Now convert the area in square feet to acres:
1 acre = 43,560 square feet
For the example above: 6,358 sq. ft ÷ 43,560 sq. ft/acre = .146 acre

Square or Rectangular Ponds:

The area of a square or rectangular pond can be determined by measuring the length (in feet) of the four sides of the pond (Figure 2) and using the following equation:

Figure 2. Sides of a rectangle

Area (in square feet) = ((A + C) ÷ 2) x ((B + D) ÷2)

Example for a rectangular pond:

The figure below represents measurements of a pond.

The area of the pond is calculated as:

Area = ((50 + 45) ÷ 2)) x ((500 + 495) ÷ 2))
= (95 / 2) x (995 / 2)
= 47.5 x 497.5
= 23,631 square feet

Now convert the area in square feet to acres:

1 acre = 43,560 ft2

For the example above: 23,631 sq. ft ÷ 43,560 sq. ft / acre = .54 acre

Triangular ponds with one 90% angle:

Use the equation: area = (A x B) ÷ 2

Measure the pond and calculate as follows:

To calculate the pond area for the example above:

Area = (100 ft x 150 ft) ÷ 2
= 15,000 sq. ft ÷ 2
= 7,500 sq. ft

Convert to acres: 7500 sq. ft ÷ 43,560 = .17 acre

Average depth of ponds:
Most existing farm ponds are relatively shallow, reaching eight to ten feet in their deepest sections. On average, a pond which is eight feet at its deepest point will have an average depth of six feet. A pond that is six feet deep at its deepest point will average about four feet in depth. Exceptions to these estimated depths occur when ponds are constructed in hollows with extremely steep banks (greater average depth) or when the pond banks have an extremely shallow slope so that there is a great deal of shallow surface area. If the pond is over 10 years old or if the pondowner has no knowledge of the depth of the pond, multiple measurements of depth should be taken over the surface of the pond and the values averaged for an estimation of the average depth (Figure 5) . Measurements of depth can be made by simply dropping a weight tied to a rope tagged in increments of one foot into the water and noting the feet of rope required for the weight to hit the pond bottom. Please note that many of these measurements should be taken across the entire pond surface to give an accurate estimation of pond depth.

Figure 5. A sampling scheme to determine average depth of a pond.

The last step in determining the potential of the pond for use in aquaculture is to determine the volume of the pond. The pond volume is calculated by multiplying the pond surface acreage by the average depth in feet. The units for this value are acre-feet. For the example of the triangular pond given earlier, the surface acreage of the pond was determined to be 0.17 acres. Assuming that the pond is an average of 6 feet deep, the volume of the pond in acre-feet is calculated as:

0.17 acres x 6 feet = 1.02 acre-feet

The volume of the pond in gallons of water can now calculated with the conversion factor of 1 acre-foot of water = 325,850 gallons of water. For our example,

1.02 acre-feet x 325,850 gallons per acre-foot =
332,367 gallons of water in the pond

This may sound like a great deal of water but if a pond is used as a reservoir to feed fresh water to a separate fish production pond of 1 surface acre in size, the recommended rate of flow that the production pond should receive is 12 gallons per minute. Over a 24 hour period, this production pond requires 17,280 gallons of water. The triangular pond in the example above could supply this production pond for only 19 days if no rainfall is received to recharge the pond!

Due to inherent format limitations of HTML, this document cannot display the standard mathematical equations for pond calculations. Equations used in this document are mathematically correct, but are presented in unconventional formats.