Septic tank size by state / Mountain
Wyoming septic tank size
In Wyoming the minimum septic tank for a three-bedroom house is 1,000 gallons, and four bedrooms need the same 1,000 gallons. The figure comes from WDEQ Water Quality Rules Ch. 25, § 25-10(a)(iii)(A) (020-25 Wyo. Code R. § 25-10). That matches the 1,000-gallon median of the 47 states listed here.
What the Wyoming rule says
The rule is WDEQ Water Quality Rules Ch. 25, § 25-10(a)(iii)(A) (020-25 Wyo. Code R. § 25-10), administered by Wyoming Department of Environmental Quality (DEQ), Water Quality Division; delegated county/city programs. In its own words:
The minimum liquid volume of a septic tank shall be 1000 gallons for residences up to a four (4) bedroom capacity. Additional capacity of 150 gallons per bedroom shall be provided for each bedroom over four (4).
Source: WDEQ Water Quality Rules Ch. 25, § 25-10(a)(iii)(A) (020-25 Wyo. Code R. § 25-10), read 2026-10-20.
- Wyoming adds 150 gallons per bedroom above four, not the usual 250.
Wyoming 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 | 1,000 gal | −200 |
| 5 | 1,150 gal | −350 |
| 6 | 1,300 gal | — |
The first step up comes at 5 bedrooms, from 1,000 to 1,150 gallons, and a six-bedroom house needs 1,300 gallons. Beyond the table the rule adds 150 gallons for each extra bedroom. These are minimums: the permit can call for more, and a bigger tank needs pumping less often.
How Wyoming 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; Montana 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. 37 of the 47 states listed here share Wyoming's three-bedroom figure.
Before you size a tank in Wyoming
- 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 Wyoming'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.