Tank sizing in both Tennessee and New Mexico follows a simple rule: bedrooms determine the tank size, not the number of people who currently live there. The reason is forward-looking — a house can be sold to a family with more people, but the bedroom count caps how many bedrooms there are. Sizing by bedrooms ensures the system handles maximum reasonable occupancy.
Standard sizing chart
- 1-2 bedrooms: 750 gallons (rare; most installations skip to 1,000 minimum)
- 3 bedrooms: 1,000 gallons
- 4 bedrooms: 1,250 gallons
- 5 bedrooms: 1,500 gallons
- 6 bedrooms: 1,750 gallons
- 7+ bedrooms: engineered design, typically 2,000+ gallons
Why bedrooms (not people)?
Sizing by bedroom count protects future buyers. If a 3-bedroom house's tank were sized for the current single owner (1,000 gallons would be excessive), and that owner sold to a family of five, the system would be undersized. Sizing for the maximum reasonable occupancy of the structure prevents that mismatch.
When you might need an upsize
Adding a bedroom
Converting a den or office into a bedroom triggers a permit review in both states. If the existing tank is undersized for the new bedroom count, an upsize is required before the addition can be permitted. This is a common gotcha on additions — owners discover the septic upgrade only after they've planned the rest of the project.
Heavy water use lifestyle
Households that significantly exceed normal water use — large families with multiple teenagers, home offices with day-occupancy, hobbies that produce wastewater — sometimes upsize voluntarily for system longevity rather than regulatory requirement.
Vacation rental conversions
Converting a residential home to a short-term rental in places like Sevierville, Gatlinburg, or Santa Fe can effectively increase occupancy load above the design assumption. Some rentals require commercial-equivalent sizing.
Cost difference between sizes
Going from a 1,000-gallon to a 1,500-gallon tank typically adds $400-$1,000 to a new installation — a small premium over the lifetime of the system. Upsizing later, when an addition forces it, costs $4,000-$8,000 because the old tank has to be abandoned and a new one set.