Septic tank size by state / Mountain
Montana septic tank size
In Montana the minimum septic tank for a three-bedroom house is 1,000 gallons, rising to 1,500 gallons at four bedrooms. The figure comes from Circular DEQ-4 (December 2023 edition), Section 5.1.6.2 Residential flows. That matches the 1,000-gallon median of the 47 states listed here.
What the Montana rule says
The rule is Circular DEQ-4 (December 2023 edition), Section 5.1.6.2 Residential flows; adopted by ARM 17.36.321 and related rules, administered by Montana Department of Environmental Quality (Circular DEQ-4); local health boards permit. In its own words:
5.1.6.2.A: "For 1 to 3 bedrooms, the minimum capacity is 1,000 gallons per living unit;" "For 4 to 5 bedrooms, the minimum capacity is 1,500 gallons per living unit;" "For 6 to 7 bedrooms, the minimum capacity is 2,000 gallons per living unit;" "For 8 or more bedrooms, the minimum capacity is 2,000 gallons per living unit plus 250 gallons" [per bedroom over 7; rule's examples: 8 BR 2,250, 9 BR 2,500]
Source: Circular DEQ-4 (December 2023 edition), Section 5.1.6.2 Residential flows, read 2026-10-20.
- The tank steps up in pairs: 1,500 gallons for four or five bedrooms, 2,000 for six or seven.
- Montana's design rules are in Circular DEQ-4; county and local boards issue permits.
Montana minimum tank size by bedrooms
| Bedrooms | Minimum tank | Versus the median state |
|---|---|---|
| 1 | 1,000 gal | same |
| 2 | 1,000 gal | — |
| 3 | 1,000 gal | same |
| 4 | 1,500 gal | +300 |
| 5 | 1,500 gal | same |
| 6 | 2,000 gal | — |
The first step up comes at 4 bedrooms, from 1,000 to 1,500 gallons, and a six-bedroom house needs 2,000 gallons. Beyond the table the rule adds 250 gallons for each extra bedroom. These are minimums: the permit can call for more, and a bigger tank needs pumping less often.
How Montana compares with nearby states
Arizona requires 1,000 gallons for three bedrooms; Colorado requires 1,000 gallons for three bedrooms; Idaho requires 1,000 gallons for three bedrooms; Nevada requires 1,000 gallons for three bedrooms; New Mexico requires 1,000 gallons for three bedrooms; Utah requires 1,000 gallons for three bedrooms; Wyoming requires 1,000 gallons for three bedrooms. 37 of the 47 states listed here share Montana's three-bedroom figure.
Before you size a tank in Montana
- 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 Montana'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.