Short answer
In Texas, tank size is set mainly by your home's bedroom count. A 1–3 bedroom home generally needs at least a 750–1,000 gallon tank, a 4-bedroom home around 1,000–1,250 gallons, and larger homes scale up from there. The drainfield is sized separately, by your soil.
Sized by bedrooms
State sizing rules tie minimum tank capacity to bedroom count — a proxy for daily wastewater flow. More bedrooms means a higher design flow and a larger required tank.
Tank vs. drainfield
The tank holds and settles solids; the drainfield disperses the treated liquid into the soil. They're sized differently — the tank by bedrooms, the drainfield by your soil's absorption rate.
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