Septic system requirements in Montana
What landplan.io applies when it designs a septic system in Montana: the system types allowed, tank sizes, setbacks and soil loading rates, with what septic products cost there.
Updated
Design a septic system for an address in Montana
Reviewed landplan.io designs by Montana's own rules, which our staff reviewed against the state's regulations. Last published 5 October 2026.
Septic systems allowed in Montana
The system types landplan.io permits in Montana:
- Elevated Sand Mound System
- Subsurface Pressure Trench System
- Subsurface Trench System
- Infiltrator Advanced Treatment Leachfield
- Infiltrator Advanced Enviro-Septic® (AES)
- Discharging System
Septic tank size in Montana
The daily design flow of a home: 150 gal/day per bedroom, at least 240 gal/day. The septic tank is sized from that flow:
| Daily design flow | Minimum tank |
|---|---|
| up to 240 gal/day | 1,000 gallons |
| 240 to 360 gal/day | 1,500 gallons |
| 360 to 600 gal/day | 2,000 gallons |
| 600 to 720 gal/day | 2,500 gallons |
| 720 to 840 gal/day | 2,750 gallons |
| 840 to 920 gal/day | 3,000 gallons |
| 920 to 1,040 gal/day | 3,250 gallons |
Septic setbacks in Montana
| From | To | Distance |
|---|---|---|
| Polygon | Existing Septic Component to be Abandoned | 5 ft |
These are the values the landplan.io design engine applies in Montana. Confirm a design with the Montana health department or your county's before you build.
Parcel data in Montana
landplan.io has parcel boundaries for 56 of Montana's 56 counties: the counties where 100% of the state's people live. Type an address there and the design starts from the parcel.
Septic companies, designers and jobs in Montana
- Septic companies and distributors in Montana (26)
- Septic designers in Montana
- Septic design jobs in Montana
Where the rules come from
The regulations and documents landplan.io's Montana rules are read from:
- Subdivisions/On-Site Subsurface Wastewater Treatment Systems, Subchapter 3 (Subsurface Wastewater Treatment Systems) (Admin. R. Mont. (ARM) 17.36.301 - 17.36.343 (ch. 17.36, subch. 3))
- Circular DEQ-4: Montana Standards for Subsurface and Other Wastewater Treatment Systems (Circular DEQ-4 (2023), incorporated by reference in ARM 17.36.323)
Septic rules by county in Montana
- Beaverhead County
- Big horn County
- Blaine County
- Broadwater County
- Carbon County
- Carter County
- Cascade County
- Chouteau County
- Custer County
- Daniels County
- Dawson County
- Deer lodge County
- Fallon County
- Fergus County
- Flathead County
- Gallatin County
- Garfield County
- Glacier County
- Golden valley County
- Granite County
- Hill County
- Jefferson County
- Judith basin County
- Lake County
- Lewis Clark County
- Liberty County
- Lincoln County
- Madison County
- Mccone County
- Meagher County
- Mineral County
- Missoula County
- Musselshell County
- Park County
- Petroleum County
- Phillips County
- Pondera County
- Powder river County
- Powell County
- Prairie County
- Ravalli County
- Richland County
- Roosevelt County
- Rosebud County
- Sanders County
- Sheridan County
- Silver bow County
- Stillwater County
- Sweet grass County
- Teton County
- Toole County
- Treasure County
- Valley County
- Wheatland County
- Wibaux County
- Yellowstone County
Questions
What septic systems are allowed in Montana?
landplan.io permits these system types in Montana: Elevated Sand Mound System, Subsurface Pressure Trench System, Subsurface Trench System, Infiltrator Advanced Treatment Leachfield, Infiltrator Advanced Enviro-Septic® (AES) and Discharging System.
What size septic tank do I need in Montana?
landplan.io sizes the septic tank in Montana from the daily design flow: up to 240 gal/day: 1,000 gallons; 240 to 360 gal/day: 1,500 gallons; 360 to 600 gal/day: 2,000 gallons; larger flows need larger tanks. A home's design flow is 150 gal/day per bedroom, at least 240 gal/day.
Does landplan.io have parcel data in Montana?
Yes, for 56 of Montana's 56 counties.