Septic tank size by state / Mountain
Colorado septic tank size
In Colorado the minimum septic tank for a three-bedroom house is 1,000 gallons, rising to 1,250 gallons at four bedrooms. The figure comes from 5 CCR 1002-43 (Regulation 43, On-site Wastewater Treatment System Regulation) §43.9.B.2, Table 9-1. That matches the 1,000-gallon median of the 47 states listed here.
What the Colorado rule says
The rule is 5 CCR 1002-43 (Regulation 43, On-site Wastewater Treatment System Regulation) §43.9.B.2, Table 9-1; design flow §43.6.A.2 / Table 6-1, administered by Colorado Water Quality Control Commission / CDPHE; administered by local public health agencies. In its own words:
Sizing for residential capacity for new installations must be based upon the number of bedrooms according to Table 9-1: ... 2 or 3 | 1,000; 4 | 1,250; Each Additional | 250
Source: 5 CCR 1002-43 (Regulation 43, On-site Wastewater Treatment System Regulation) §43.9.B.2, Table 9-1, read 2026-10-20.
- A new home is designed for at least two bedrooms, so a one-bedroom house gets the same 1,000-gallon tank.
- A grinder pump ahead of the tank adds 500 gallons.
- Local public health agencies adopt Regulation 43 and can be stricter.
Colorado 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 | 1,250 gal | +50 |
| 5 | 1,500 gal | same |
| 6 | 1,750 gal | — |
The first step up comes at 4 bedrooms, from 1,000 to 1,250 gallons, and a six-bedroom house needs 1,750 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 Colorado compares with nearby states
Arizona 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; Wyoming requires 1,000 gallons for three bedrooms. 37 of the 47 states listed here share Colorado's three-bedroom figure.
Before you size a tank in Colorado
- 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 Colorado'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.