Charts and Formulas
Charts and formulas
To calculate the exact internal volume in gallons of a square or rectangular tank.

Formula #1 Multiply width inches, times length inches, times depth inches, divided by 231 = gallons in tank
Example      48” X 96” X 48” / 231 = 957 gallons in tank

Formula #2 Multiply width feet, times length feet, times depth feet = Volume in cubic feet
                   Volume in cubic feet, times 7.48 gal/cu ft = gallons in tank
Example     4’ X 8’ X 4’ = 128 cubic feet
                  128 X 7.48 = 957 gallons in tank

To calculate the exact internal volume in gallons of a round tank.

Formula #1 Internal diameter in inches squared X depth in inches divided by 231 = gallons in tank
Example      53” X 53” X .7854 X 53” / 231 = 506 gallons in tank

Formula #2 3.14 times the radius times the radius times the depth times 7.48 = gallons in tank
                   ( the radius is 1/2 the inside diameter of the tank to find the radius take the diameter in inches
                    and divide by 12 example 53” / 12 = 4.416 then divide by 2 / = 2.208 that’s 1/2 the diameter)
Example     3.14 X 2.208 X 2.208 X 4.416 X 7.48 = 506 gallons in tank

Note: a 500 gallon tank can have more than 500 gallons in it if the tank is full (above the 2 pipes coming in and going out), the lines coming in and going out are also full, and the ground around the leach lines are saturated with water. The water in those lines will come back into the tank while the tank is being pumped, this is called backwash or back flow. It is possible to pump more than 1,000 gallons from a 500 gallon tank under those conditions. Also remember not all tanks are the same. There are many different sizes of tanks.

How often do I need my septic system pumped out? Many variables go into that question. Here are a few guidelines to help. A house hold of 4 with a 500 gallon septic tank needs to get their tank pumped about every 3 - 4 years. A house hole of 4 with 2 X 500 gall tanks needs the primary tank pumped every 3 - 4 years and the secondary tank pumped every 6 –8 years. The more people in the house, and the more non-biodegradable items that go into the tank the more often it needs to be pumped.

How many portable toilets will I need for my jobsite? One portable toilet will accommodate up to 10 workers per 40 hour workweek when serviced one time per week. This rate of 1 to 10 is the ANSI standard for our industry.

How many portable toilets will I need for my special event? We recommend that one portable toilet will accommodate approximately 125 to 150 uses and that the average uses per person is one time every 3 to 4 hours. If food and alcohol is being served an additional 15 to 20 % more units are needed.

Why would I need a hand wash station at my special event? If food is being served it is necessary to have a place to wash hands after you use the toilet. Hand wash stations are great for crawfish boils.


Charts and Formulas