Determining the volume of your pond or tank accurately can be critical for many reasons, including sizing of pumps, piping, filters, etc., when calculating stocking rates and for correct chemical dosages.
Volume is calculated by multiplying surface area times depth. Some geometric shapes are simple to calculate, e.g.,squares, rectangles and circles. Unfortunately, many ponds are free form in shape and/or have variable depths. Their volume can be estimated by dividing the pond surface into simple geometric shapes, calculating the volume of each section based on depth and adding all sections. Be sure to include the water in filters, piping, waterfalls, etc.
The following tables present equations that are used to calculate the area and volume of various geometric shapes.

Shape      Area Calculation
Circle           πr2
Ellipse         πr1r2
Rectangle   lw
Right Triangle         1/2 ab
General Triangle    1/2 hc
Regular Polygon    1/2 ns2 cot (180/n)

Shape      Volume Calculation
Cube            abc
Pyramid       1/3 abh
Cone            1/3 πr2h
Cylinder       πr2h
Elliptical Depression     4/9 πabh
Sphere Segment            1/3 πh2 (3r-h)
Volume in Cubic Feet x 7.48 = Gallons

Alternative methods of determining the volume include
1) checking with the contractor who built the pond for records of pond area and volume, 2) obtaining aerial photographs from which the pond area can be estimated, 3) surveying the ponds using a transit to determine area (get depths from topography), 4) drawing the pond on graph paper divided into squares of known size to calculate surface area, 5) determining the area using CAD software, 6) filling the pond using a totalizing water meter or 7) estimating the volume by adding a known measure of salt, then determining the volume by salinity.