Septic tank size by state / Mountain
Idaho septic tank size
In Idaho 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 IDAPA 58.01.03.007.08.a (Individual/Subsurface Sewage Disposal Rules). That matches the 1,000-gallon median of the 47 states listed here.
What the Idaho rule says
The rule is IDAPA 58.01.03.007.08.a (Individual/Subsurface Sewage Disposal Rules); flows IDAPA 58.01.03.007.09, administered by Idaho Department of Environmental Quality (DEQ); permits issued by the seven public health districts. In its own words:
a. Tanks serving single dwelling units. The minimum tank capacity is one thousand (1,000) gallons. For each bedroom over four (4) in a dwelling unit, add two hundred fifty (250) gallons. (7-1-25)
Source: IDAPA 58.01.03.007.08.a (Individual/Subsurface Sewage Disposal Rules), read 2026-10-20.
- Above four bedrooms the minimum rises 250 gallons per bedroom.
- Permits are issued by Idaho's seven public health districts.
Idaho 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 | 1,250 gal | −250 |
| 6 | 1,500 gal | — |
The first step up comes at 5 bedrooms, from 1,000 to 1,250 gallons, and a six-bedroom house needs 1,500 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 Idaho compares with nearby states
Arizona requires 1,000 gallons for three bedrooms; Colorado 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 Idaho's three-bedroom figure.
Before you size a tank in Idaho
- 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 Idaho'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.