Septic Tank Size for a 5-Bedroom House (state code tables)

The minimum septic tank for a 5-bedroom house is 1,250 to 1,500 gallons in the three states below, which publish fixed bedroom tables. Your county can require more.

Minimum tank by state

What changes at 5 bedrooms

Five bedrooms pushes every state above its minimum. Washington adds 250 gallons for the bedroom over four, for 1,250. Pennsylvania's flow is 600 gpd, so the tank is 1,250 + 1.5 × 100 = 1,400 gallons. Minnesota stays at 1,500 because 4 and 5 bedrooms share a row. Washington's drain field design flow is 600 gpd, so soil type matters a great deal here.

Drain field for 5 bedrooms (Washington)

Washington sizes the drain field from design flow (600 gpd for 5 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.

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 · 4 bedrooms.