On a septic inspection, one of the many things we do is to calculate the septic tank volume. This is important because you want to know if you have a 500 gallon tank, a 1,000 gallon tank, or larger. The size of the tank is important in comparison to the size of the home and how many occupants there are.

That being said, how do you calculate the tank volume? It will seem complicated once you see the math, but it is really quite simple and doesn't take long at all.

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

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.

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.

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

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.

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.

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.