What septic tank size means

Septic tank size is the liquid volume of the tank, in gallons. The tank holds wastewater long enough for solids to settle and scum to float, so mostly clarified liquid moves on to the drainfield, the soil area that receives the tank’s liquid. A tank that is too small can send solids into the drainfield, which can clog it.

How is septic tank size set?

Septic tanks are generally sized by the number of bedrooms.¹ 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 so adding a bedroom beyond four raises the minimum. Ask the agency that issues septic permits where the lot is what minimum applies before you add a bedroom. For the bedroom side of the question, see what size septic system you need.

Does tank size affect pumping?

Household septic tanks are typically pumped every three to five years, and tank size is one of the factors that affect how often, along with household size and the total wastewater the home generates.³

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. U.S. EPA, How to Care for Your Septic System; Washington State Department of Health, Caring for Your Septic System. ↩