Septic tank size by state / Pacific
Alaska septic tank size
In Alaska 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 18 AAC 72.530(e)(2) (table). That matches the 1,000-gallon median of the 47 states listed here.
What the Alaska rule says
The rule is 18 AAC 72.530(e)(2) (table); design flow 18 AAC 72.530(b)(1), administered by Alaska Department of Environmental Conservation (DEC). In its own words:
the net minimum volume of a septic tank must be in accordance with ... the following table: Number of Bedrooms 0 - 3: 1,000 gallons; 4 - 8: 1,000 plus 250 gallons for each bedroom over three; 9 - 13: 2,500 gallons; 14 - 18: 3,000 gallons; Greater than 18: 1,125 + (0.75* design flow)
Source: 18 AAC 72.530(e)(2) (table), read 2026-10-20.
- A basement sump or lift station ahead of the tank raises the minimum by 250 gallons.
- Tanks may be combined in series or in parallel to reach the required volume.
- Design flow is 150 gallons a day for each bedroom.
Alaska 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 | 1,750 gal | — |
The first step up comes at 4 bedrooms, from 1,000 to 1,250 gallons, and a six-bedroom house needs 1,750 gallons. Beyond the table the rule adds 250 gallons for each extra bedroom. These are minimums: the permit can call for more, and a bigger tank needs pumping less often.
How Alaska compares with nearby states
California sets no statewide figure; Hawaii requires 1,000 gallons for three bedrooms; Oregon requires 1,000 gallons for three bedrooms; Washington requires 1,000 gallons for three bedrooms. 37 of the 47 states listed here share Alaska's three-bedroom figure.
Before you size a tank in Alaska
- 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 Alaska'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.