Septic tank size by state / Pacific
Oregon septic tank size
In Oregon the minimum septic tank for a three-bedroom house is 1,000 gallons, and four bedrooms need the same 1,000 gallons. The figure comes from OAR 340-071-0220(3)(a)(C). That matches the 1,000-gallon median of the 47 states listed here.
What the Oregon rule says
The rule is OAR 340-071-0220(3)(a)(C), administered by Oregon Department of Environmental Quality (DEQ); many counties are DEQ contract agents. In its own words:
The capacity of a septic tank that serves a single family dwelling must be based on the number of bedrooms in the dwelling. For a dwelling with 4 or fewer bedrooms, the tank capacity must be at least 1,000 gallons. Septic tank capacity must be at least 1,500 gallons for dwellings with more than 4 bedrooms.
Source: OAR 340-071-0220(3)(a)(C), read 2026-10-20.
- Oregon has no per-bedroom step: any home over four bedrooms needs at least 1,500 gallons.
- Many counties act as DEQ's contract agents for permits.
Oregon 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 | 1,000 gal | −200 |
| 5 | 1,500 gal | same |
| 6 | 1,500 gal | — |
The first step up comes at 5 bedrooms, from 1,000 to 1,500 gallons, and a six-bedroom house needs 1,500 gallons. These are minimums: the permit can call for more, and a bigger tank needs pumping less often.
How Oregon compares with nearby states
Alaska requires 1,000 gallons for three bedrooms; California sets no statewide figure; Hawaii requires 1,000 gallons for three bedrooms; Washington requires 1,000 gallons for three bedrooms. 37 of the 47 states listed here share Oregon's three-bedroom figure.
Before you size a tank in Oregon
- 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 Oregon'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.