Determining the correct size for your septic tank is one of the most important parts of designing or replacing a septic system. In both Maricopa and Pinal Counties, septic tank sizing is primarily based on the number of bedrooms in the home. In some cases, the number of plumbing fixtures may be considered as an alternative method.

Gallons Per Day (GPD) – What Does It Mean?

GPD stands for gallons per day, and it refers to how much wastewater your household is estimated to generate. The standard design assumption is 150 gallons per day per bedroom. This number is based on the idea that two people may occupy each bedroom, and each person contributes 75 gallons of wastewater daily.

Here’s an example:

4-bedroom home
 4 bedrooms x 150 gallons = 600 GPD
600 GPD x 2 = 1,200 gallon septic tank

Your septic tank should be sized at twice your daily flow to provide the right amount of treatment and retention time.

Minimum Tank Size Rules in Arizona

Maricopa and Pinal Counties both require that all septic tanks be a minimum of 1,000 gallons, regardless of the number of bedrooms.

Common Septic Tank Sizes by Bedroom Count

2 Bedrooms
 2 x 150 = 300 GPD
300 x 2 = 600 gallon tank
But 1,000 gallons is still the minimum requirement.

3 Bedrooms
 3 x 150 = 450 GPD
450 x 2 = 900 gallon tank
Again, the minimum is 1,000 gallons.

4 Bedrooms
 4 x 150 = 600 GPD
600 x 2 = 1,200 gallon tank

5 Bedrooms
 5 x 150 = 750 GPD
750 x 2 = 1,500 gallon tank

For homes with more than five bedrooms, it’s best to consult with a septic professional to determine the right sizing based on your actual household usage.

What Counts as a “Bedroom”?

When it comes to septic design, a bedroom isn’t just any room with a bed. According to code, a bedroom is any room that:

• Has at least 70 square feet (excluding closets)
• Has a ceiling height of at least 7 feet
• Has electrical service and ventilation
• Has a closet or a space for one
• Has an egress window
• Offers privacy with an entry door

This can include rooms commonly labeled as an office, den, exercise room, playroom, or even an enclosed Arizona room with ventilation.

When Fixture Counts Are Used

Sometimes, especially in homes with more bathrooms than bedrooms, septic design is based on the total number of plumbing fixtures instead. Every sink, tub, toilet, or other fixture gets a value, and those values are added up to determine how much water the system needs to handle.

What We Recommend

• Always check local requirements. Rules can vary slightly by county.
• Plan for the future. If you’re thinking about adding a bedroom or bathroom later, size the tank accordingly now.
• Work with a licensed pro. HomeField Onsite Environmental will help you get it right the first time.

Getting the septic tank size right from the start means fewer problems down the road, better performance, and a system built to last. HomeField Onsite Environmental is here to help make sure your system meets all the right requirements—without the guesswork.