Rhode Island · 250-RICR-150-10-6
Rhode Island DEM administers the OWTS program directly for the entire state.
Minimum tank
One to three bedroom dwelling.
Design flow
Held two days for settling.
Cost index
Regional labour and permits.
Sets the design flow and the legal minimum.
Code sizes on bedrooms, not people.
Used to size the matching drainfield. Unsure? Sandy loam is the common default.
Glacial till and outwash, with sandy soils near the coast and shallow bedrock inland. Small lot sizes and proximity to salt ponds and Narragansett Bay drive nitrogen requirements.
Conventional trenches on suitable ground, and advanced nitrogen-reducing treatment in designated critical resource areas — a significant share of the state given how much of Rhode Island is near water.
| Dwelling | Design flow | Minimum tank | With disposal |
|---|---|---|---|
| 1 bedroom | 300 gpd | 1,000 gal | 1,250 gal |
| 2 bedrooms | 300 gpd | 1,000 gal | 1,250 gal |
| 3 bedrooms | 450 gpd | 1,000 gal | 1,250 gal |
| 4 bedrooms | 600 gpd | 1,250 gal | 1,500 gal |
| 5 bedrooms | 750 gpd | 1,500 gal | 1,750 gal |
Rhode Island requires a licensed designer and licensed installer, and DEM reviews applications directly. Denitrification is required in critical resource areas including the salt pond region and around Narragansett Bay.
The state sets a floor. These Rhode Island counties are where the largest septic populations sit, and where local requirements most often exceed it.
| County | What makes it different |
|---|---|
| Washington | South County salt pond region with denitrification requirements |
| Newport | Coastal, small lots, nitrogen sensitive |
| Kent | Suburban, mixed soils |
| Providence | Northern rural areas use on-site systems |
| Bristol | Coastal, dense development |
A three-bedroom home at 150 gallons per bedroom produces 450 gallons per day of design flow. On sandy loam that needs roughly 750 square feet of absorption area. On clay loam it rises to about 2,250 square feet, and on clay a conventional field is usually not permitted at all. Run your own soil class through the drainfield calculator.
Advanced treatment systems require maintenance contracts and reporting to DEM. Use the pump schedule calculator to work out the interval for your tank size and household.
Regional labour and permit costs in Rhode Island run about 128% of the national baseline. Here is what typical installations land at for a three-bedroom home:
| System type | Typical installed cost in Rhode Island | When it applies |
|---|---|---|
| Conventional gravity | $11,500 – $20,500 | Good soil, adequate depth to water |
| Chamber system | $12,700 – $23,600 | Alternative to gravel trenches |
| Mound system | $21,800 – $43,000 | Shallow water table or thin soil |
| Aerobic treatment | $18,400 – $36,900 | Clay soil or small lots |
| Tank pump-out (service) | $410 – $830 | Every 3–5 years |
Replacing an existing system costs more than these figures — old-system removal and re-permitting add to every line. The replacement cost calculator covers that case, and the cost estimator adjusts all of the above for your bedroom count and soil.
Verify before you build. The figures above are state minimums compiled for planning, drawn from 250-RICR-150-10-6, Rules Establishing Minimum Standards Relating to Location, Design, Construction and Maintenance of Onsite Wastewater Treatment Systems. County and health district rules routinely exceed them, code text changes, and only Rhode Island DEM — Onsite Wastewater Treatment Systems or your local health department can confirm what applies to your parcel.
Send your sizing numbers to installers who work under Rhode Island rules every day.