For this article, we will be discussing underground septic tanks, and size refers to the septic tank’s total volume handling capacity. See our article on above ground septic tanks for more information on these tanks and systems.
The requirements for minimum septic tank capacity depend on several factors. State, county and/or city guidelines can mandate approved sizes as well as tank material and placement. The soil conditions and regional geography have a large role in system effectiveness, which can impact drain field size and septic tank size. Whether the septic tank is intended for residential or commercial use will affect septic tank size per regulations; here we discuss home use. The specific septic tank system in place or that will be installed can also change the needed tank size.
In short, the size of septic tank needed will depend on: (1) the specific septic system type; (2) local government regulations; (3) suitability of the ground geology, and (4) the expected volume of wastewater based on size of the home.
Contrary to some belief, polyethylene (i.e. plastic) septic tanks make excellent choices for modern septic tank systems. Plastic septic tanks are much more resistant to cracking versus concrete septic tanks. Also, plastic septic tanks will not rust, are weatherproof, watertight, cheaper, lightweight, are easier to install and, if installed correctly, will not float.
1) The Specific Septic System Type
There are 7 types of septic tank systems and the tank size needed may vary by the specific system you plan to use. An overview of each system type and their size requirements are beyond the scope of this article. Here, when referring to system type, we mean conventional, gravity-fed anaerobic septic systems. The anaerobic septic system is the most common and the one that most visualize when they think of a septic tank.
The 7 common types of septic systems are listed below, and new polyethylene septic tanks can be used in most, if not all, of these systems that require a tank:
- Conventional, Gravity-Fed, Anaerobic System
- Above Ground Septic System
- Pressure System
- Aerobic System
- Mound System
- Recirculating Sand or Gravel Filter System
- Bottomless Sand Filter System
If your septic tank system is anything but a conventional, anaerobic system, the guidelines in this article may not fully apply.
2) Local Government Regulations
Local government regulations for septic tanks vary widely across the United States. This is due to the vastly different soil geographies and water features that change by state and can even sometimes change by a few miles. This is why checking with local government regulations is such a key factor when finding the right septic tank size and the right location on the property for installation.
For an example and an informational overview of codes, rules, regulations often put forth by governing departments, as well as common terminology and definitions, review the Wastewater Treatment Standards – Residential Onsite Systems document from the New York State Department of Health
3) Suitability of the Ground Geology
The underground soil type has a key role in system effectiveness and therefore septic tank size. This point is heavily related to local government regulations. It is tied into the requirements and recommendations of a designated department that oversees septic tank installations, often the department of health. To find how suitable the ground is for a septic tank system, a certified professional must come out to the proposed installation area to perform certain tests. The most common test is a soil percolation evaluation, often referred to as the perc test. A perc test will determine how viable the underground soil is to properly handle and filter septic tank effluent.
Scheduling for these local ground geology tests can be the responsibility of the contractor installing the septic tank or the landowner. If you are installing a septic tank yourself, it will be your responsibility to contact your local oversight department to come perform perc tests and/or ground area evaluation. The results of the analysis and tests will decide if the desired location is acceptable and whether a specific septic tank system or size will be necessary.
4) The Expected Volume of Wastewater
The most important point to determine what size septic tank is needed is the average volume of wastewater that will be produced for the septic tank to handle. In a septic system only residence, all wastewater ends up at the septic tank unless a separate system for handling greywater is in place. Much research has gone into calculating and approximating these values for residential homes, commercial structures, and facilities. Most governmental recommendations for the smallest septic tank capacity in home use is based on the number of bedrooms in the house.
The recommendation for home use is a 1000 gallon septic tank as a starting point. The 1000 gallon size tank is a minimum and *can be suitable for a 2 bedroom, 3 bedroom house. Some recommendations say to add an extra 250 gallons of septic tank capacity for each bedroom over 3 bedrooms. Additional fixtures above the norm can also increase the tank and/or drain field size required; examples include large volume bathtubs, laundry sinks, bidets, double toilets, etc. This is often when considered collectively for the whole household rather than individually.
*As indicated throughout this article, septic tank size recommendations are highly variable based on where you live, local government standards, underground soil type, house size and how much wastewater your specific home is expected to produce.
Minimum Septic Tank Capacity Table
See the following table for details on Minimum Septic Tank Capacity based on the number of residential bedrooms:
|Number of Bedrooms||Minimum Septic Tank Size||Minimum Liquid Surface Area||Drainfield Size|
|2 or less||1000 – 1500 Gallons||27 Sq. Ft.||800 – 2500 Sq. Ft.|
|3||1000 – 2000 Gallons||27 Sq. Ft.||1000 – 2880 Sq. Ft.|
|4||1250 – 2500 Gallons||34 Sq. Ft.||1200 – 3200 Sq. Ft.|
|5||1500 – 3000 Gallons||40 Sq. Ft.||1600 – 3400 Sq. Ft.|
|6||1750 – 3500 Gallons||47 Sq. Ft.||2000 – 3800 Sq. Ft.|
Note the following regarding the table above:
- Minimum Liquid Surface Area, from the State of New York, refers to the surface area provided for the liquid by the tank’s width by length dimensions.
- Drainfield Size range is dependent on soil type. The drainfield recommendations given above are provided by the State of Michigan and can vary significantly based on local guidelines and area geography.
Additional Thought: Can a Septic Tank Be Too Big?
Without considering cost, a good question may be: “Can a septic tank be too big?”. The answer is no. As long as the septic tank is installed correctly, a septic tank cannot be too big, it can only be too small. By most recommendations, installing a larger size septic tank is often the safer, more preferred option. The reasons for this are:
- A larger septic tank can accommodate for home usage fluctuations, such as with parties or long-term guests.
- A larger septic tank can keep you from having to install a new tank if your family size increases or plan to make additions to your home such as extra bedrooms, bathrooms, plumbing fixtures, etc.
Takeaways | What Size Septic Tank Do I Need
The septic tank size recommendations presented here are recommendations only. They are based on compiled information from government and academic research. The exact septic tank size you need will vary on the variables outlined in this article. There is not a “one-size-fits-all” answer to the septic tank size for your specific home. There is a lot of variance based on where you live. Using the provided Minimum Septic Tank Capacity Table can give general insight into the septic tank and system size that may be best suited to your application and help estimate costs.
To find what size septic tank you need, know the size of your home and water-using features installed, then inquire with local government. Always check and verify with your state, city or local county’s department assigned to handle septic tanks, soil tests, and approvals prior to starting any septic tank installation job. This is often your local department of health.
If looking for a chart of tank sizes, check out our post on available septic tank volumes and size dimensions.
National Tank Outlet offers a wide selection of underground polyethylene septic tanks in both single chamber and double chamber in various styles, configurations and volumes.
For more information on conventional septic systems, consider this article from Texas A&M University for an overview of onsite wastewater treatment systems.