Septic tank size by state / East South Central
Mississippi septic tank size
In Mississippi the minimum septic tank for a three-bedroom house is 900 gallons, rising to 1,200 gallons at four bedrooms. The figure comes from MSDH On-site Wastewater Regulations, Miss. Admin. Code Title 15, Subpart 77, Chapter 5 (Individual Onsite Wastewater Disposal Design Standard), Rule 5.1.9 Septic Tank Sizing. That is 100 gallons less than the 1,000-gallon median of the 47 states listed here.
What the Mississippi rule says
The rule is MSDH On-site Wastewater Regulations, Miss. Admin. Code Title 15, Subpart 77, Chapter 5 (Individual Onsite Wastewater Disposal Design Standard), Rule 5.1.9 Septic Tank Sizing, administered by Mississippi State Department of Health (MSDH), Onsite Wastewater. In its own words:
Rule 5.1.9: "The effective liquid capacity of septic tanks for dwellings shall be based on the number of bedrooms proposed or anticipated as shown in the table below". Rows (bedrooms/occupants/without filter/with filter): 2 or less/4 or less/750/750; 3/6/900/900; 4/8/1200/1000; 5/10/1500/1250; 6/12/1800/1500. "For each additional bedroom add 260 gallons"; "For each additional occupant over 2 per bedroom add 130 gallons"
Source: MSDH On-site Wastewater Regulations, Miss. Admin. Code Title 15, Subpart 77, Chapter 5 (Individual Onsite Wastewater Disposal Design Standard), Rule 5.1.9 Septic Tank Sizing, read 2026-10-20.
- Mississippi's three-bedroom minimum is 900 gallons, below the common 1,000.
- For four or more bedrooms the minimum drops when the tank has a baffle or effluent filter (four bedrooms: 1,000 instead of 1,200).
- Add 130 gallons for each occupant over two per bedroom.
Mississippi minimum tank size by bedrooms
| Bedrooms | Minimum tank | Versus the median state |
|---|---|---|
| 1 | 750 gal | −250 |
| 2 | 750 gal | — |
| 3 | 900 gal | −100 |
| 4 | 1,200 gal | same |
| 5 | 1,500 gal | same |
| 6 | 1,800 gal | — |
The first step up comes at 3 bedrooms, from 750 to 900 gallons, and a six-bedroom house needs 1,800 gallons. Beyond the table the rule adds 260 gallons for each extra bedroom. These are minimums: the permit can call for more, and a bigger tank needs pumping less often.
How Mississippi compares with nearby states
Alabama requires 1,000 gallons for three bedrooms; Kentucky requires 1,000 gallons for three bedrooms; Tennessee requires 900 gallons for three bedrooms. 4 of the 47 states listed here share Mississippi's three-bedroom figure.
Before you size a tank in Mississippi
- Bedrooms, not people, set the size in most rules. Rooms that could be used as bedrooms are usually counted, so check how your permitting office counts a den or a finished basement.
- Garbage disposals, grinder or ejector pumps and large tubs change the answer in many states; the points above list Mississippi's adjustments.
- The tank is only half the system. The drainfield is sized separately from soil tests, and it is usually the bigger cost.
- A private well on the same lot has its own setbacks from the tank and field; the well pump and pressure tank calculators size the water side.
Figures are the state's minimum as written on 2026-10-20. Local health departments can require more; the permit issued for your property governs. See the methodology for how each state was read.