First, you locate all four corners of the tank. Measuring all sides, how deep it is, etc. The below are a couple different methods to calculate the volume. There are several methods to calculate the volume of the septic system tanks depending upon whether they are cylindrical, square or rectangular.
Cylindrical Tanks
Method A
3.14 x radius squared (feet) x depth (feet) equals cubic capacity.
7.5 x cubic capacity equals capacity in gallons.
Example: 60" dia. tank, 48" liquid depth
3.14 x (2.5 x 2.5) x 4 = 78.5 cu. capacity
78.5 x 7.5 = 588.75 gal.
Method B
Diameter squared (inches) divided by 292.5 equals gallons per inch of liquid depth in tank. Depth (inches) x gallons per inch.
Example: 60" dia. tank, 48" liquid depth
60 x 60 # 292.5 = 12.31 gal. per inch
12.31 gal. per inch x 48 = 590.88 gal.
Method C
Diameter squared (in inches) x depth (in inches) times .0034 equals total gallons.
Example: 60" dia. tank, 48" liquid depth
(60" x 60") x 48" x.0034 = 587.52 gal.
Square or Rectangular Tanks
Method A
Length (feet) x depth (feet) x width (feet) equals cubic ft.
Cubic ft. x 7.5 equals gallons.
Example: 96" = length, 48" = width, 60" = depth
8 x 5 x 4 =160 cu. ft.
160 x 7.5 = 1200 gal.
Method B
Length (inches) x width (inches) divided by 231 equals gallons in each inch of liquid depth. Tank depth (inches) x gallons per inch equals total gallons.
Example: 96" = length, 48" = width, 60" = depth
96" x 48" = 4608/231 = 19.95 gal. per inch
19.95 x 60" = 1,197 gal.