Rural Home Calc

Septic Tank Size for a 4-Bedroom House (state code tables)

The minimum septic tank for a 4-bedroom house is 1,000 to 1,500 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 gal480 gpd design flow; two compartments, first holds ½ to ⅔
Pennsylvania (25 Pa. Code § 73.31)1,250 gal500 gpd design flow; 900 gal minimum for any home
Minnesota (Minn. R. 7080.1930)1,500 gal2,250 gal with a garbage disposal or sewage pump (+50%)

What changes at 4 bedrooms

Four bedrooms is where the states split. Washington stays at 1,000 gallons, the last bedroom count in its first row. Pennsylvania's design flow rises to 500 gpd, so the tank jumps to 900 + 3.5 × 100 = 1,250 gallons. Minnesota moves to its 4 to 5 bedroom row at 1,500 gallons, and 2,250 with a garbage disposal.

Drain field for 4 bedrooms (Washington)

Washington sizes the drain field from design flow (480 gpd for 4 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/day480 sq ft
Type 21.0 gal/sq ft/day480 sq ft
Type 30.8 gal/sq ft/day600 sq ft
Type 40.6 gal/sq ft/day800 sq ft
Type 50.4 gal/sq ft/day1,200 sq ft
Type 60.2 gal/sq ft/day2,400 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 · 3 bedrooms · 5 bedrooms.

Sources