Septic Tank Size Calculator (by bedrooms)
Minimum septic tank size and design flow by bedroom count, using Washington State's on-site sewage rule.
What size septic tank do I need?
Minimum liquid capacity by bedrooms in three states that publish fixed tables. Your county may require more.
| Bedrooms | Washington | Pennsylvania | Minnesota (no disposal) | Minnesota (with disposal) |
|---|---|---|---|---|
| 1 | 1,000 gal | 900 gal | 1,000 gal | 1,500 gal |
| 2 | 1,000 gal | 900 gal | 1,000 gal | 1,500 gal |
| 3 | 1,000 gal | 900 gal | 1,000 gal | 1,500 gal |
| 4 | 1,000 gal | 1,250 gal | 1,500 gal | 2,250 gal |
| 5 | 1,250 gal | 1,400 gal | 1,500 gal | 2,250 gal |
| 6 | 1,500 gal | 1,550 gal | 2,000 gal | 3,000 gal |
| 7 | 1,750 gal | 1,700 gal | 2,000 gal | 3,000 gal |
Worked example
A 3-bedroom single-family home in Washington needs at least 1,000 gallons of liquid volume (4 or fewer bedrooms). A 6-bedroom home needs 1,000 + 2 × 250 = 1,500 gallons. The first compartment has to hold half to two-thirds of the total, so 500 to 667 gallons for a 1,000-gallon tank. Klickitat County's permit worksheet uses 120 gallons per day per bedroom with a 240 gpd minimum, so 3 bedrooms is 360 gpd.
How it works
Septic tank minimums are set by each state, province and county, and they differ. This calculator uses Washington's WAC 246-272A-0232 (in effect since April 1, 2025): 1,000 gallons for up to 4 bedrooms, plus 250 for each extra bedroom, and at least two compartments. Before you buy or design anything, check your local health department's rule. More jurisdictions will be added as their codes are sourced.
Questions
What size septic tank does a 4-bedroom house need in Washington?
1,000 gallons minimum liquid volume under WAC 246-272A-0232 Table VII.
Did the Washington rule change?
Yes. Before April 2025, homes with 3 or fewer bedrooms needed 900 gallons. The current table sets 1,000 gallons for up to 4 bedrooms.
Does a septic tank need two compartments?
In Washington, yes. The first compartment must hold half to two-thirds of the total, either in one tank or two tanks in series.
Sources
Guides
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