Septic tank size by state / Pacific
Washington septic tank size
In Washington 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 WAC 246-272A-0232(2)(a), Table VII. That matches the 1,000-gallon median of the 47 states listed here.
What the Washington rule says
The rule is WAC 246-272A-0232(2)(a), Table VII, administered by Washington State Department of Health; local health jurisdictions. In its own words:
(2) Have the following minimum liquid volumes: (a) For a single-family residence use Table VII: Table VII Required Minimum Liquid Volumes of Septic Tanks — ≤4: 1,000; Each additional bedroom: 250. (b) For OSS treating sewage from a residential source, other than one single-family residence, 250 gallons per bedroom with a minimum of 1,000 gallons;
Source: WAC 246-272A-0232(2)(a), Table VII, read 2026-10-20.
- Above four bedrooms the minimum rises 250 gallons per bedroom.
- Local health jurisdictions issue permits under the state rule.
Washington 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 | 1,250 gal | −250 |
| 6 | 1,500 gal | — |
The first step up comes at 5 bedrooms, from 1,000 to 1,250 gallons, and a six-bedroom house needs 1,500 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 Washington 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; Oregon requires 1,000 gallons for three bedrooms. 37 of the 47 states listed here share Washington's three-bedroom figure.
Before you size a tank in Washington
- 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 Washington'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.