Septic Tank Size Calculator

About this calculator

Septic tank sizing determines the minimum capacity needed for a household's wastewater system by multiplying the number of occupants by average daily water use per person, then multiplying that daily wastewater volume by the required retention time — typically 24 to 72 hours — that allows solids to settle and partially decompose before the liquid effluent flows out to the drain field. Most residential codes also specify a minimum tank size regardless of household size (commonly around 1,000 US gallons for up to a certain occupant count) to provide adequate settling capacity even for smaller households.

Homeowners, builders and septic installers need this figure when installing a new system, replacing an undersized tank, or adding bedrooms/bathrooms to a home, because a septic tank that's too small doesn't give solids enough time to settle, leading to solids escaping into the drain field and clogging it — a failure that is expensive and disruptive to fix, since it usually means digging up and replacing the entire leach field.

This calculator takes your household size, daily water use per person and desired retention time and returns the septic tank capacity you need, checked against common minimum sizing standards.