How design loading rate translates into drainfield square footage or spray-area size.

For subsurface absorption, required absorptive area A = Q ÷ Ra, where Q is the design wastewater usage rate from Table III and Ra is the soil application rate for the most restrictive horizon along the media or within two feet below the excavation; the absorptive area counts the excavation bottom (L × W) plus one foot of sidewall around the full perimeter, i.e., 2(L+W) × 1 ft (30 TAC §285.33; Bandera County packet). Q comes from Table III — 225 gpd for a 1–2 bedroom home under 1,500 sq ft, 300 gpd for 3 bedrooms under 2,500 sq ft, 375 gpd for 4 bedrooms under 3,500 sq ft, 450 for 5, 525 for 6, plus 75 gpd per additional 1,500 sq ft (reduced to 180/240/300/360/420/60 gpd with water-saving devices), and the system must be designed to the higher figure if actual water-use records exceed the table (TCEQ 30 TAC Chapter 285 rule text, Table III). For spray disposal the minimum area = Q ÷ Ri, where Ri is the maximum surface application rate from Figure §285.90(1) (TCEQ RG-472); that map rate varies with evapotranspiration minus precipitation, reaching 0.115 gal/ft²/day in West Texas, and TCEQ's review illustrates a wetter-region rate of 0.041 gal/ft²/day, i.e., 300 gpd ÷ 0.041 = 7,317 ft² of spray field (TCEQ, Re-evaluating Surface Application Rates). Worked subsurface example: 300 gpd ÷ 0.20 (Class III) = 1,500 ft² of absorptive area.

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