Rural Home Calc

Septic Tank Size for a 2-Bedroom House (state code tables)

The minimum septic tank for a 2-bedroom house is 900 to 1,000 gallons in the three states below, which publish fixed bedroom tables. Your county can require more.

Minimum tank by state

State and ruleMinimum tankNotes
Washington (WAC 246-272A-0232)1,000 gal240 gpd design flow; two compartments, first holds ½ to ⅔
Pennsylvania (25 Pa. Code § 73.31)900 gal400 gpd design flow; 900 gal minimum for any home
Minnesota (Minn. R. 7080.1930)1,000 gal1,500 gal with a garbage disposal or sewage pump (+50%)

What changes at 2 bedrooms

Two bedrooms is below every state's floor, so all three states require their minimum tank. Pennsylvania's 900 gallons is the smallest; Washington and Minnesota both start at 1,000. Pennsylvania also assumes 400 gallons a day for any home up to 3 bedrooms, so a small house is sized as if it had three. Washington's design flow is 240 gpd, which is the rule's minimum and equals 2 × 120.

Drain field for 2 bedrooms (Washington)

Washington sizes the drain field from design flow (240 gpd for 2 bedrooms) divided by the soil's loading rate in WAC 246-272A-0234 Table VIII. These are primary areas for ordinary septic tank effluent; a reserve area of the same size is also required. Try your own numbers in the drain field calculator.

Washington soil typeLoading rateDrain field area
Type 11.0 gal/sq ft/day240 sq ft
Type 21.0 gal/sq ft/day240 sq ft
Type 30.8 gal/sq ft/day300 sq ft
Type 40.6 gal/sq ft/day400 sq ft
Type 50.4 gal/sq ft/day600 sq ft
Type 60.2 gal/sq ft/day1,200 sq ft

Before you buy a tank

Local health officers can require more than the state minimum; Washington's design-flow rule says so directly. Get the number from your local permit office or a licensed designer. Run other bedroom counts in the septic tank size calculator, or compare: 3 bedrooms · 4 bedrooms · 5 bedrooms.

Sources