What size septic system do I need?

Septic tanks are generally sized by the number of bedrooms,¹ so septic system size and house size are related through bedroom count. If you are asking what size septic system you need, start with the bedroom count of the house you plan to build. The lot’s soil may limit how large a drainfield, the soil area that receives the tank’s liquid, fits.

How are septic tanks sized?

Washington, for example, requires a minimum of 1,000 gallons for up to four bedrooms plus 250 gallons for each additional bedroom (WAC 246-272A-0232).² For how the tank itself is sized and pumped, see septic tank size.

What does that mean for the drainfield and the lot?

Drainfield size is generally based on the home’s design flow, the estimated daily wastewater it produces.³ Two practical effects follow. A lot with limited good soil may not have room for the larger drainfield that more bedrooms call for. Some states count rooms used for sleeping, not only the rooms labeled as bedrooms.⁴ Decide the bedroom count before the site evaluation so the design matches.

Citations

1. Penn State Extension, Septic System Basics; Washington State Board of Health, WAC 246-272A-0232. ↩

2. Washington State Board of Health, WAC 246-272A-0232, official text published by the Washington State Legislature; Klickitat County Health Department, Gravity Septic Permit Installation Application. ↩

3. Klickitat County Health Department, Gravity Septic Permit Installation Application; Florida Department of Environmental Protection, Rule 62-6.008, F.A.C. (formerly 64E-6.008), official text from the Florida Department of State. ↩

4. Nebraska Department of Water, Energy, and Environment, Guidance Document 25-023, “Bedroom” Definition; Florida Department of Environmental Protection, Form DEP 4015 instructions (Florida Department of Health in Hardee County packet). ↩