Septic tank size by state / Mountain
New Mexico septic tank size
In New Mexico the minimum septic tank for a three-bedroom house is 1,000 gallons, rising to 1,200 gallons at four bedrooms. The figure comes from 20.7.3.201(Q)(1) NMAC and Table 201.2 (Capacity of septic tanks). That matches the 1,000-gallon median of the 47 states listed here.
What the New Mexico rule says
The rule is 20.7.3.201(Q)(1) NMAC and Table 201.2 (Capacity of septic tanks); design flow 20.7.3.201(P)(1), administered by New Mexico Environment Department (Liquid Waste Program). In its own words:
Q: "The minimum liquid capacity of a septic tank shall be determined as follows:" (1) "for residential units, the liquid capacity shall be based on the number of bedrooms using Table 201.2; and" Table 201.2: 1 BR 750; 2-3 BR 1000; 4 BR 1200; 5-6 BR 1500; 7-9 BR 2000.
Source: 20.7.3.201(Q)(1) NMAC and Table 201.2 (Capacity of septic tanks), read 2026-10-20.
- A one-bedroom home may use 750 gallons; two and three bedrooms 1,000.
- The table stops at nine bedrooms.
New Mexico minimum tank size by bedrooms
| Bedrooms | Minimum tank | Versus the median state |
|---|---|---|
| 1 | 750 gal | −250 |
| 2 | 1,000 gal | — |
| 3 | 1,000 gal | same |
| 4 | 1,200 gal | same |
| 5 | 1,500 gal | same |
| 6 | 1,500 gal | — |
The first step up comes at 2 bedrooms, from 750 to 1,000 gallons, and a six-bedroom house needs 1,500 gallons. These are minimums: the permit can call for more, and a bigger tank needs pumping less often.
How New Mexico 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; Utah requires 1,000 gallons for three bedrooms; Wyoming requires 1,000 gallons for three bedrooms. 37 of the 47 states listed here share New Mexico's three-bedroom figure.
Before you size a tank in New Mexico
- 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 New Mexico'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.