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. The second way is by fixture unit counts.
Let's talk GPD and Tank Sizing
GPD is an acronym that stands for Gallons Per Day. 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 that you know how GRP is calculated from the number of bedrooms, let’s see how it determines the tank size. The GRP helps us size the tank to handle the wastewater from the bedrooms efficiently.
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
BEDROOM COUNT
Bedrooms use 150 gallons per day (GPD) for calculations. 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 a space where you could build a closet
- At least one window must open and serve as an emergency exit
- A method of entry and exit that makes the room distinct from others, providing the level of privacy typically expected for such a space
This includes offices, dens, craft rooms, exercise rooms, playrooms, libraries, ventilated Arizona rooms, and more.
The infographic below shows a standard tank size. For homes with more than 8 bedrooms, consider larger tanks or splitting the home into 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 common method because, in most homes, bedrooms usually outnumber 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.
