This barndominium septic system size estimator turns your bedroom count and living area into the design wastewater flow that Texas rules use to size an on-site sewage system. It is built for rural builds around Tyler in Smith County, where most lots need a septic system.
This is an estimate; a licensed site evaluator/designer and the Smith County permitting authority set the final design.
In Spanish: Calculadora del tamaño de la fosa séptica de un barndominium en Texas
| Basis | Gal/day |
|---|
The table behind the number
Texas rules list the usage rate for a single-family home by bedrooms and living area. Each row has a rate without water-saving devices and one with them. This is the full single-family part of the table.
| Single-family home | Standard | Water-saving |
|---|---|---|
| 1 or 2 bedrooms, under 1,500 sq ft | 225 | 180 |
| 3 bedrooms, under 2,500 sq ft | 300 | 240 |
| 4 bedrooms, under 3,500 sq ft | 375 | 300 |
| 5 bedrooms, under 4,500 sq ft | 450 | 360 |
| 6 bedrooms, under 5,500 sq ft | 525 | 420 |
| Over 5,500 sq ft: each extra 1,500 sq ft or part of it | +75 | +60 |
Gallons per day. Source: 30 TAC §285.91, Table III, Wastewater Usage Rate.
Worked example: a 4-bedroom barndominium with 3,000 sq ft of living space. By bedrooms the table gives 375 gallons a day. By area, 3,000 sq ft is over the 2,500 sq ft cap of the 3-bedroom row and under the 3,500 sq ft cap of the 4-bedroom row, so 375 again. With water-saving devices the same house is 300.
What this number is used for, and what it is not
- It is a flow, not a tank. The designer uses the flow, plus the soil evaluation, to size the tank and the drainage or treatment system. Table III has no tank size.
- Your plan can move it. Adding a bedroom, a loft that could be a bedroom, or square footage can move the home to the next row of the table.
- A shop is a separate question. A shop bathroom or floor drain is not covered by this table. Ask the designer how it is handled; our septic and well guide explains why shop drains need an answer before the permit is filed.
Not a design: this is an estimate; a licensed site evaluator/designer and the Smith County permitting authority set the final design. Rules change, so confirm with them before you buy land or lock a floor plan.
How to use the result
- Settle bedrooms and living area
Decide the bedroom count and living area before the septic application is filed. Changing the plan afterward can mean redoing the design.
- Run the estimator both ways
Try standard and water-saving fixtures to see how much the table lowers the flow.
- Give the numbers to your designer
Ask how they classify the home, the shop and any loft, and what flow they will design for.
- Ask what the bid includes
Make sure each builder bid says who pays for the septic design, permit and installation. Use the form below to request quotes from independent local builders.
Read the barndominium septic and well guide for Smith County for soils, system types and sequencing, and the Smith County permits guide for who issues what. We are a referral service that connects you with independent local builders; we do not design or install septic systems.
Source
- Texas Administrative Code, Title 30, §285.91(3), Table III, Wastewater Usage Rate (Texas Commission on Environmental Quality, 30 TAC Chapter 285). Rule text and table (in English).
Frequently asked questions
How is the design wastewater flow for a Texas house septic system set?
Table III of 30 TAC §285.91 lists a usage rate in gallons a day by the number of bedrooms and the square feet of living area. A one- or two-bedroom home under 1,500 sq ft is 225 gallons a day, a three-bedroom under 2,500 sq ft is 300, four under 3,500 is 375, five under 4,500 is 450 and six under 5,500 is 525. Above 5,500 sq ft, each additional 1,500 sq ft or part of it adds 75.
Do water-saving fixtures lower the flow?
Yes. The same table has a second column for homes with water-saving devices: 180, 240, 300, 360 and 420 gallons a day for the same five size steps, and 60 per extra 1,500 sq ft above 5,500 sq ft. Your designer decides which fixtures qualify and documents them.
Why does the estimator take the higher of the bedroom and square-foot results?
Each row of the table pairs a bedroom count with a maximum living area. A 3-bedroom barndominium with 3,000 sq ft of living space does not fit the 3-bedroom row, so it falls to the next row. We read the table that way. Your designer and the permitting authority decide how a specific plan is classified.
Does this give me a septic tank size?
No. Table III gives wastewater flow only. Tank size and drainage area are worked out by the designer from that flow and from the soil evaluation, under other parts of Chapter 285. This page does not guess a tank size.
Which square footage do I enter?
Enter the living area of the house. Ask your designer how a shop, loft, porch or open bay with plumbing is counted, because that is a classification call and we cannot make it for you. Also tell the designer about every room that could be used as a bedroom.
What if my water use is higher than the table?
The table notes say that if actual water use records are greater than the table rates, the system shall be designed for the higher flow. Designers may also use actual water data if the TCEQ or its authorized agent finds it acceptable.