Septic System Design

The design of a septic system is the crucial first step in making sure that the onsite sewage disposal system works efficiently and effectively.

Designing a Drainage Field

The first step in the process is determining the volume of wastewater that will be discharged from the house.  The following is a useful formula for that determination:

150 gal x # of bedrooms = volume of wastewater per day

Size of the Drain Field

The next step is to determine the size of the drain field to handle the calculated amount of wastewater.  This is based on two factors: 1) the ability of the ground to absorb and treat the sewage and 2) the amount of wastewater being sent out from the house.  A percolation table is used to determine the rate at which the ground can absorb the wastewater which is called the “application rate.”  Once this is found out, then the formula below can be used to estimate the size that the drain field needs to be:

Volume / Application Rate = Sq ft of Required Trench


Length of the Drainage Trench

There are 3 considerations in figuring out the length of the drainage trenches.  The width is widely considered the first factor.  The second factor is the depth of the trench.  A general rule of thumb is that the deeper the trench the greater is the absorption area and thus the shorter the trench has to be.  The final consideration is how the trenches are laid out.  The more space between the trenches the better.

The Size of the Septic Tank

Several different methods are used in sizing the septic tank.  They are 1) based on the number of bedrooms  2) based on the size of the house and 3) based on the number of fixtures.

Number of Bedrooms

In using this method the number of bedrooms determine the size of the tank.  For example, for a single family dwelling with one or two bedrooms the size of the tank would be 750 gallons.  For three bedrooms, the required capacity would be 1000 gallons.  For four bedrooms, a 1200 gallon tank would  be required.  A 1500 gallon capacity tank would be needed for a house with five or six bedrooms.

Size of the House

The size of the house is another way of determining the required capacity of the septic tank.  If the square footage of the house is less than 1500 then the minimum tank capacity would be 1000 gallons.  For a house greater than 1500 and less 3500 the required tank would be 1500 gallons.  Finally, a minimum of a 2000 gallon tank would be necessary for a house greater than 3500 square feet.

Number of Fixtures

The third way is takes into account the number of fixtures and the number of bedrooms with each fixture having a different value.  For example,

  • a bathtub has two fixture units
  • washer has 2 units
  • shower has 2 units
  • toilet with 1.6 gallons per flush is 3 units
  • toilet with greater than 1.6 gallons is 4 units
  • dishwasher is 2 units
  • single lavatory is 1 unit

The size of the septic tank is calculated by adding up the total number of units and then referencing a table based on the number of bedrooms.   Just as an example, if the fixture count is more than 28 and the number of bedrooms is 4 then the minimum septic tank size would be 1500 gallons.


Needless to say the design of the septic system takes into account many different factors.  However it is of paramount importance to consider all factors in the proper design in order to ensure the proper treatment of wastewater coming from the house,  To ensure a properly designed system, it is necessary to call a professional.

Leave a Reply

Your email address will not be published. Required fields are marked *