What size septic tank do I need in Tennessee?
Under Tennessee’s state rule, bedrooms set the septic tank size: 900 gallons for 3 bedrooms and 1,000 for 4. Homes with 2 or fewer bedrooms get 750 gallons, and each bedroom past 4 adds 250 gallons. Williamson County’s own regulations use the same numbers. Angi’s national chart runs bigger, but the permit here follows the Tennessee tables.
What does the Tennessee septic tank sizes chart say for a 3 or 4 bedroom house?
Two sets of rules size septic tanks in the counties we serve. State rule 0400-48-01 covers the counties that permit through TDEC, the Tennessee Department of Environment and Conservation. Williamson County runs its own septic program under its own regulations.
Both size a home’s septic tank by its bedrooms, and both use the same numbers. A house with 2 or fewer bedrooms gets 750 gallons. Three bedrooms call for 900 gallons, and 4 bedrooms call for 1,000. Each bedroom past 4 adds 250 gallons, so 5 bedrooms come to 1,250 and 6 come to 1,500.
Williamson County’s regulations call Table S14-1 a minimum, so a larger tank meets the rule there. No septic tank in the county may hold less than 750 gallons. Every septic system installation we do starts from these tables. We file the permit in whichever office your address calls for.
This septic tank sizes chart sets the two Tennessee tables beside Angi’s national chart.
| Bedrooms | Tennessee state rule (TDEC counties) | Williamson County minimum | Angi national chart |
|---|---|---|---|
| 2 or fewer | 750 gallons | 750 gallons | 1,000 gallons |
| 3 | 900 gallons | 900 gallons | 1,000 gallons |
| 4 | 1,000 gallons | 1,000 gallons | 1,250 gallons |
| 5 | 1,250 gallons | 1,250 gallons | 1,500 gallons |
| 6 | 1,500 gallons | 1,500 gallons | 1,750 gallons |
How big of a septic tank do I need if national charts say more?
Angi’s national chart shows bigger tanks than the Tennessee tables. Angi’s 2026 size guide lists 1,000 gallons for 1 to 3 bedrooms and 1,250 gallons for 4. The Tennessee tables list 900 gallons for 3 bedrooms and 1,000 for 4.
Angi’s guide also says local rules and regulations dictate required septic tank sizes. In Tennessee, that means the state rule in the TDEC counties and Table S14-1 in Williamson County. Use the national chart as a rule of thumb for comparing tanks.
Going bigger than the table is allowed in Williamson County. Angi’s guide adds a caution: a tank that is far too large for the house may not work properly. We size the tank to the house and the permit before any digging starts.
What counts as a bedroom when you size a septic tank?
Both Tennessee tables count bedrooms, not bathrooms or the people in the house. Williamson County defines a bedroom by the International Building Code and International Residential Code the county has adopted.
The county sizes a new tank on the bedrooms proposed, or the bedrooms the house can be expected to have. A room you already plan to turn into a bedroom is worth counting now.
Williamson County lets a documented existing system keep serving the house through a remodel or an addition. That changes when a listed trigger applies. Adding bedrooms beyond what the system was designed to serve is one of those triggers. Then the system has to meet current regulations.
Our guide to septic permits in Williamson County covers how that county office works. Most counties around it permit through TDEC instead.
What counts toward a 1,000 gallon septic tank’s capacity?
Only the liquid counts. Williamson County rates a tank by the liquid it holds up to the outlet. The air space above that liquid does not count. So a tank counts as 1,000 gallons there only when it holds 1,000 gallons of liquid up to the outlet.
The county also sets a minimum detention time for any size tank: 36 hours, based on projected daily flow. That means the tank has to hold at least a day and a half of the house’s projected flow.
The tank is split in two inside. Under the state rule, a new septic tank must be a two-compartment design. The inlet compartment holds two-thirds to three-fourths of the total capacity. Williamson County also requires two-compartment septic tanks, while pump tanks there are single-compartment.
Do you need a bigger septic tank with a garbage disposal?
Not in Williamson County. The county says its Table S14-1 sizes already allow for a household’s garbage grinder, dishwasher, showers, tubs and washing machine. The state rule’s residential table, used in the TDEC counties, lists bedrooms only.
The county still strongly advises against garbage disposals on a septic system, though they are legal. If you use one, the county says to pump the tank every two years.
A disposal changes the pumping schedule, and the tank size in the table stays the same. Our crew handles septic tank pumping whenever your tank comes due.
Is the drain field sized the same way as the septic tank?
Not quite. The drain field is sized apart from the tank, by the bedrooms and the soil.
Some sites are sized by perc tests. On those, the state rule calls for at least 370 square feet of trench bottom per bedroom. For a 3 bedroom house, that comes to at least 1,110 square feet.
Williamson County designs a perc-tested conventional system at 75 minutes per inch, even when the test comes back faster. That means at least 405 square feet of trench bottom per bedroom, or 1,215 square feet for 3 bedrooms.
Our guide to types of septic systems in Tennessee covers the field designs the rules allow. If an existing field is failing, drain field repair is a separate job from the tank.
How much does a septic tank cost by size?
Angi’s 2026 cost guide prices the tank alone, before installation, by size:
- 750 gallons: $700 to $1,200
- 1,000 gallons: $900 to $1,500
- 1,200 gallons: $1,200 to $1,600
- 1,500 gallons: $1,500 to $2,500
By material, the same guide puts plastic tanks at $500 to $2,000 and concrete at $700 to $2,000. Fiberglass runs $1,200 to $2,000 in that guide.
These are national prices for the tank by itself, before the rest of an installed system. Our guide to septic system cost in Tennessee covers installed prices. We look at the site and hand you one price on a free on-site estimate.
Frequently asked questions
What size septic tank do I need for a 3 bedroom house?
Under Tennessee’s state rule, a 3 bedroom house calls for a 900 gallon septic tank. Williamson County’s Table S14-1 lists the same 900 gallons. The county calls that a minimum, so a bigger tank is allowed there. Angi’s national chart lists 1,000 gallons for 1 to 3 bedrooms.
What size septic tank do I need for a 4 bedroom house?
A 4 bedroom house calls for a 1,000 gallon septic tank under Tennessee’s state rule. Williamson County’s Table S14-1 lists the same 1,000 gallons. Angi’s national chart lists 1,250 gallons for 4 bedrooms, but that is a rule of thumb. A fifth bedroom adds 250 gallons under both Tennessee tables.
What is the standard size of a septic tank?
There is no single standard size in Tennessee. The state rule goes by bedrooms: 750 gallons for 2 or fewer, 900 for 3 and 1,000 for 4. Each bedroom past 4 adds 250 gallons, and Williamson County uses the same sizes. Angi’s national chart lists 1,000 gallons for 1 to 3 bedrooms and 1,250 for 4.
How many people will a 1,000 gallon septic tank support?
The Tennessee tables count bedrooms, not people. Under the state rule and Williamson County’s Table S14-1, 1,000 gallons is the size for a 4 bedroom house. Angi’s guide says occupants of a typical U.S. home use 50 to 100 gallons of water per day. For low pressure pipe systems, Williamson County projects 150 gallons a day per bedroom.
How much does a 1,000 gallon septic tank cost?
Angi’s 2026 cost guide puts a 1,000 gallon tank at $900 to $1,500 for the tank alone. That is a national range, before the rest of an installed system. Under the Tennessee tables, 1,000 gallons is the size for a 4 bedroom house. We look at the site and hand you one price on a free on-site estimate.
Can I install a bigger septic tank than the minimum?
Yes, in Williamson County. The county’s regulations call Table S14-1 a minimum, so a larger tank meets the rule there. Angi’s size guide adds a caution: a tank far too large for the house may not work properly. In the TDEC counties, the state rule bases the tank on the same bedroom table.
Do I need a bigger septic tank with a garbage disposal?
Not in Williamson County. The county says its Table S14-1 sizes already allow for a household’s garbage grinder and other fixtures. It strongly advises against disposals on a septic system and says to pump every two years with one. The state rule’s residential table lists bedrooms only.
How large does a septic field need to be?
The field is sized apart from the tank, by bedrooms and soil. Where perc tests size it, the state rule calls for at least 370 square feet of trench bottom per bedroom. On perc-tested Williamson County sites, a conventional field gets at least 405 square feet per bedroom. That is at least 1,215 square feet for 3 bedrooms.
Septic Superior is a family septic company based in Thompson’s Station. More than 20 years of septic work in Williamson County sit behind the company. On a new septic system, we handle the permit start to finish and set the tank it calls for. Call (615) 957-9682 or use our contact form for a free on-site estimate, and we hand you one price.