How is system size determined — bedrooms, square footage, or occupancy?

Design flow (Q) comes from Table III of §285.91, which sets a single-family rate on the *combination* of bedrooms and living area: 225 gal/day for 1–2 bedrooms under 1,500 sq ft, 300 for 3 bedrooms under 2,500 sq ft, 375 for 4 bedrooms under 3,500 sq ft, 450 for 5 bedrooms under 4,500 sq ft, 525 for 6 bedrooms under 5,500 sq ft (and 180/240/300/360/420 respectively with water-saving devices), plus 75 gal/day for each additional 1,500 sq ft above 5,500 (30 TAC Chapter 285, Table III). Q then drives tank volume (750 gal for Q ≤ 250 gpd, 1,000 gal for 251–350, 1,250 gal for 351–500, 2.5Q for 501–1,000) and absorptive area (Area = Q ÷ Ra, where the long-term application rate Ra ranges from >0.50 gal/sq ft/day in Class Ia soil down to 0.1 in Class IV clay) (30 TAC Chapter 285, §285.91(1)–(2)). Occupant count is not used — AgriLife notes the flow rate is based on whichever is larger, the home's square footage or the bedroom count — although actual metered water-use records, if higher than Table III, must be used instead (Texas A&M AgriLife, Living with an Aerobic Treatment Unit and Spray Field; 30 TAC Chapter 285, Table III).

Reviewed August 2026. Links go to the sources this answer was checked against.