Rural Home Calc

Septic Tank Size for a 3-Bedroom House (state code tables)

The minimum septic tank for a 3-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 gal360 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 3 bedrooms

Three bedrooms is the most common house size and the last count that stays on Pennsylvania's floor: 400 gpd and 900 gallons. Washington still asks for 1,000 gallons, because its first step covers up to 4 bedrooms. Minnesota is also at 1,000 gallons, the top of its smallest row. Washington's design flow rises to 360 gpd, which is what sizes the drain field.

Drain field for 3 bedrooms (Washington)

Washington sizes the drain field from design flow (360 gpd for 3 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/day360 sq ft
Type 21.0 gal/sq ft/day360 sq ft
Type 30.8 gal/sq ft/day450 sq ft
Type 40.6 gal/sq ft/day600 sq ft
Type 50.4 gal/sq ft/day900 sq ft
Type 60.2 gal/sq ft/day1,800 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: 2 bedrooms · 4 bedrooms · 5 bedrooms.

Sources