What size septic tank do I need?
Missouri's minimum is 1,000 gallons for a house with one to three bedrooms, 1,250 gallons for four bedrooms, and 1,500 gallons for five. The numbers come from the state's construction standards for onsite systems.
| Bedrooms | Minimum tank size | Design flow |
|---|---|---|
| 1 to 3 | 1,000 gallons | 240 to 360 gallons a day |
| 4 | 1,250 gallons | 480 gallons a day |
| 5 | 1,500 gallons | 600 gallons a day |
These are state minimums. A county can require more, and the approved design for your property is what controls.
Why does the state count bedrooms?
Bedrooms set how many people a house can hold, so the system is designed for the house, whoever lives in it. Missouri's standards use 120 gallons a day for each bedroom, with a minimum of 240 gallons a day for any house. A three-bedroom house is designed for 360 gallons a day.
Finishing a basement room as a fourth bedroom raises the design flow, and the permitting office may ask whether the existing tank and field can carry it. Ask before the remodel.
Does tank size set the drain field?
No. The tank settles the solids; the drain field is where the soil treats the liquid, and the soil decides its size. A qualified soil evaluator examines the soil in test pits and reports how it absorbs water and how deep it is to rock or a seasonal water table. Slow soil needs more trench for the same house than soil that drains well. The septic systems page covers how that report leads to a system type.
How far must a tank be from a well?
The tank must be at least 50 feet from a private well, and the drain field at least 100 feet. The state's other common setbacks are in the table.
| Measured from | Septic tank | Drain field |
|---|---|---|
| Private well | 50 feet | 100 feet |
| Stream, lake, or pond | 50 feet | 50 feet |
| Property line | 10 feet | 10 feet |
| House foundation | 5 feet | 15 feet |
| Basement foundation | 15 feet | 25 feet |
On a small or steep lot these distances often decide where the system can go, and sometimes which type of system fits at all.
How often should the tank be pumped?
The EPA says household septic tanks are typically pumped every three to five years, and that the system should be inspected at least every three years. A system with a pump, float switches, or other mechanical parts should be inspected about once a year.
The right interval depends on how many people use the house and how much goes down the drain. At an inspection, a pumper measures the sludge and scum in the tank and can say whether it is due.
What to have ready for a quote
Send with your request: the property address; whether it is a new home, a replacement, or a repair; the number of bedrooms; the soil report and system plan if you have them; and what the yard is like, including hills, rock, and wet spots.
J1S is licensed by the State of Missouri as an advanced onsite wastewater installer and builds from the approved design. We quote from that design and a look at the site.
Sources
- Missouri Code of State Regulations, 19 CSR 20-3: minimum construction standards for onsite sewage systems, including tank sizes, design flow, and setbacks.
- EPA, How to Care for Your Septic System: inspection and pumping intervals.
- Missouri Department of Health and Senior Services, onsite wastewater program.