Septic tank size by state / Mountain
Arizona septic tank size
In Arizona the minimum septic tank for a three-bedroom house is 1,000 gallons, rising to 1,250 gallons at four bedrooms. The figure comes from Ariz. Admin. Code R18-9-A314(4)(a). That matches the 1,000-gallon median of the 47 states listed here.
What the Arizona rule says
The rule is Ariz. Admin. Code R18-9-A314(4)(a), administered by Arizona Department of Environmental Quality (ADEQ). In its own words:
The design liquid capacity of the septic tank and the septic tank's daily design flow are determined based on the number of bedrooms and fixture count ... 3 | 21 or less | 1000 | 450; More than 21 | 1250 | 600; 4 | 28 or less | 1250 | 600; More than 28 | 1500 | 750
Source: Ariz. Admin. Code R18-9-A314(4)(a), read 2026-10-20.
- Arizona sizes by bedrooms and by fixture count together. The table here is the lower fixture count (seven fixture units per bedroom or fewer).
- With more fixtures the tank moves up one row: a three-bedroom house then needs 1,250 gallons.
Arizona minimum tank size by bedrooms
| Bedrooms | Minimum tank | Versus the median state |
|---|---|---|
| 1 | 1,000 gal | same |
| 2 | 1,000 gal | — |
| 3 | 1,000 gal | same |
| 4 | 1,250 gal | +50 |
| 5 | 1,500 gal | same |
| 6 | 2,000 gal | — |
The first step up comes at 4 bedrooms, from 1,000 to 1,250 gallons, and a six-bedroom house needs 2,000 gallons. These are minimums: the permit can call for more, and a bigger tank needs pumping less often.
How Arizona compares with nearby states
Colorado requires 1,000 gallons for three bedrooms; Idaho requires 1,000 gallons for three bedrooms; Montana requires 1,000 gallons for three bedrooms; Nevada requires 1,000 gallons for three bedrooms; New Mexico requires 1,000 gallons for three bedrooms; Utah requires 1,000 gallons for three bedrooms; Wyoming requires 1,000 gallons for three bedrooms. 37 of the 47 states listed here share Arizona's three-bedroom figure.
Before you size a tank in Arizona
- Bedrooms, not people, set the size in most rules. Rooms that could be used as bedrooms are usually counted, so check how your permitting office counts a den or a finished basement.
- Garbage disposals, grinder or ejector pumps and large tubs change the answer in many states; the points above list Arizona's adjustments.
- The tank is only half the system. The drainfield is sized separately from soil tests, and it is usually the bigger cost.
- A private well on the same lot has its own setbacks from the tank and field; the well pump and pressure tank calculators size the water side.
Figures are the state's minimum as written on 2026-10-20. Local health departments can require more; the permit issued for your property governs. See the methodology for how each state was read.