Septic tank size by state / New England
Massachusetts septic tank size
In Massachusetts the minimum septic tank for a three-bedroom house is 1,500 gallons, and four bedrooms need the same 1,500 gallons. The figure comes from 310 CMR 15.223(1) (Title 5, septic tank capacity), which sizes the tank from design flow; the bedroom figures below are that rule's arithmetic. That is 500 gallons more than the 1,000-gallon median of the 47 states listed here.
What the Massachusetts rule says
The rule is 310 CMR 15.223(1) (Title 5, septic tank capacity); design flow 310 CMR 15.203, administered by Massachusetts Department of Environmental Protection (Title 5), administered by local boards of health. In its own words:
15.223(1)(a): "For a single family dwelling unit with a design flow of less than 1,000 gallons per day" … "a minimum effective liquid capacity of 200% of the design flow" … "In no case shall the effective liquid capacity of the tank as measured below the outlet invert elevation be less than 1,500 gallons." 15.203: single family "per bedroom 110", minimum 330 gpd.
Source: 310 CMR 15.223(1) (Title 5, septic tank capacity), read 2026-10-20.
- Title 5 requires a tank of at least twice the design flow (110 gallons a day per bedroom) and never less than 1,500 gallons.
- That 1,500-gallon floor covers every home up to six bedrooms.
- Local boards of health often adopt stricter rules than Title 5.
Massachusetts minimum tank size by bedrooms
| Bedrooms | Minimum tank | Versus the median state |
|---|---|---|
| 1 | 1,500 gal | +500 |
| 2 | 1,500 gal | — |
| 3 | 1,500 gal | +500 |
| 4 | 1,500 gal | +300 |
| 5 | 1,500 gal | same |
| 6 | 1,500 gal | — |
The minimum does not change between one and six bedrooms: every house in that range needs the same 1,500 gallons. These are minimums: the permit can call for more, and a bigger tank needs pumping less often.
How Massachusetts compares with nearby states
Connecticut requires 1,000 gallons for three bedrooms; Maine requires 1,000 gallons for three bedrooms; New Hampshire requires 1,250 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 Massachusetts's three-bedroom figure.
Before you size a tank in Massachusetts
- 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 Massachusetts'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.