Texas Septic Flow Rate Calculator (OSSF – Chapter 285)
This calculator estimates residential wastewater design flow and minimum absorptive area for septic (OSSF) systems in Texas using values aligned with Texas Administrative Code, Title 30, Chapter 285.
In Texas, wastewater design flow is determined using standard flow tables based on the number of bedrooms in a dwelling. These tables are defined in Chapter 285 and are used to size on-site sewage facilities (OSSF).
OSSF Design Flow Calculator
Disclaimer: This calculator is for general planning only. Design flow values are based on standard residential flow tables from TCEQ Chapter 285. Final system design must be based on a licensed site evaluation and county approval.
Texas OSSF Design Flow (Chapter 285)
Under 30 TAC Chapter 285, residential wastewater flow is determined using standard design flow tables based on bedroom count. Typical values include:
- 2 bedrooms → 180 gallons/day
- 3 bedrooms → 240 gallons/day
- 4 bedrooms → 300 gallons/day
- 5 bedrooms → 360 gallons/day
- 6 bedrooms → 420 gallons/day
These values are used for sizing septic systems and reflect assumed occupancy and water use defined in the regulations.
Absorptive area calculation
Drainfield area is estimated using:
- Area = Design Flow ÷ Soil Application Rate
Soil classification significantly affects system size. Slower clay soils require larger areas, while sandy soils require less.