Determining what size of septic tank your home needs can be tricky. There is a lot of misinformation out there explaining how this is correctly done. This blog post is written according to the sizing requirements from Maricopa and Pinal Counties in Arizona. If you are reading this outside of this area, you may want to check with your local county requirements.
Determining the size of the septic tank in our installation market is done in one of two ways. First, and most commonly used, is the metric for measuring the septic tank based on the bedroom count inside the home.
Let’s talk GPD and Tank Sizing
GPD is an acronym that stands for Gallons Per Day. This acronym is used a lot when talking about septic systems. The GPD determines the amount of water that is going into the septic system on a daily basis. When sizing a septic tank by bedroom count, the calculation is 150 GPD per bedroom in the home. This number is calculated based on the double occupancy of each bedroom and the average water usage per person in the home at 75 gallons per day. When calculated this way, we are factoring in what should be the maximum load of water into the septic system per day.
EXAMPLE: If Jen has a 4-bedroom home, her GPD per day would be 600.
4 bedrooms X 150 GPD = 600 GPD
Now you understand what GPD is and how that is calculated for the number of bedrooms in the home, now let’s tie that together with how the tank size is calculated
The septic tank size must be 2 times larger (in gallons) than the daily GPD.
EXAMPLE: If Jen has 4 bedrooms in her home, her GPD will be 600.
600 X 2 = 1200
If Frank has a 5-bedroom home, his GPD will be 750.
750 x 2 = 1500
Let’s talk tank size by bedroom count
The minimum required septic tank size in both Maricopa and Pinal county is 1000 gallons. That 1000-gallon septic tank is enough capacity for up to three bedrooms in a home.
EXAMPLE: If Ayzli has a 3 bedroom home, her GPD will be 450
450 X 2 = 900
Bedrooms are calculated at 150 gallons per day (GPD). I know it sounds weird to consider bedrooms instead of bathrooms but we are really accounting for occupancy of the home.
“Bedroom” means, for the purpose of determining design flow for a septic system for a dwelling, any room that has:
- Floor space of at least 70 sq ft, excluding closets;
- Ceiling height of at least 7 feet;
- Electrical service and ventilation;
- A closet or an area where a closet could be constructed;
- At least one window is capable of being opened and used for emergency egress; and
- A method of entry and exit into the room which allows it to be considered distinct from other rooms in the dwelling to afford a level of privacy customarily expected for such a room.
This could include, but is not limited to: offices, dens, craft rooms, exercise rooms, playroom, library, ventilated Arizona rooms, etc.
A great standardization of tank size is illustrated in the infographic below. When you go beyond 8 bedrooms, larger tank sizes can be considered or even splitting the home between two separate systems.
As I stated at the beginning of this post there are two different ways to determine the septic tank size required for the home. Counting bedrooms is the most commonly used method since in most homes, the bedrooms typically outnumber the bathrooms. In homes where the bathrooms outnumber the bedrooms, we will want to calculate the size needed by plumbing fixtures as well. The greater of the two calculations will become the required size.