Septic tank size by state / New England
New Hampshire septic tank size
In New Hampshire the minimum septic tank for a three-bedroom house is 1,250 gallons, and four bedrooms need the same 1,250 gallons. The figure comes from N.H. Code Admin. R. Env-Wq 1010.01 (septic tank sizing). That is 250 gallons more than the 1,000-gallon median of the 47 states listed here.
What the New Hampshire rule says
The rule is N.H. Code Admin. R. Env-Wq 1010.01 (septic tank sizing); design flow Env-Wq 1008.03, administered by New Hampshire Department of Environmental Services (NHDES), Subsurface Systems Bureau. In its own words:
Env-Wq 1010.01: single-family up to 4 bedrooms, the "minimum septic tank liquid capacity shall be 1,250 gallons." For each bedroom over 4 (up to 10), "the liquid capacity of the septic tank shall be increased by 250 gallons per bedroom." Garbage grinder: "the liquid capacity of the septic tank shall be increased by 50%."
Source: N.H. Code Admin. R. Env-Wq 1010.01 (septic tank sizing), read 2026-10-20.
- New Hampshire's floor is 1,250 gallons for anything up to four bedrooms.
- A garbage grinder adds 50%; pumping raw sewage into the tank doubles the size.
New Hampshire minimum tank size by bedrooms
| Bedrooms | Minimum tank | Versus the median state |
|---|---|---|
| 1 | 1,250 gal | +250 |
| 2 | 1,250 gal | — |
| 3 | 1,250 gal | +250 |
| 4 | 1,250 gal | +50 |
| 5 | 1,500 gal | same |
| 6 | 1,750 gal | — |
The first step up comes at 5 bedrooms, from 1,250 to 1,500 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 New Hampshire compares with nearby states
Connecticut requires 1,000 gallons for three bedrooms; Maine requires 1,000 gallons for three bedrooms; Massachusetts requires 1,500 gallons for three bedrooms; Rhode Island requires 1,000 gallons for three bedrooms; Vermont requires 1,000 gallons for three bedrooms. 1 of the 47 states listed here share New Hampshire's three-bedroom figure.
Before you size a tank in New Hampshire
- 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 Hampshire'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.