Hawaii · Hawaii Administrative Rules Chapter 11-62
The Hawaii Department of Health Wastewater Branch administers the program statewide.
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.
Volcanic. Young lava flows on Hawaii Island can be so porous that effluent moves without treatment; older weathered soils on Oahu and Kauai behave more conventionally. Proximity to the ocean and to drinking water sources drives most restrictions.
Septic tanks with absorption beds or trenches, and aerobic units where sensitive. Hawaii has been converting cesspools statewide, which is the dominant regulatory story in the state.
| 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 |
Hawaii's cesspool conversion mandate requires existing cesspools to be replaced by 2050, and this drives most residential onsite work in the state. New construction requires DOH approval, with stricter treatment near shorelines and drinking water sources.
The state sets a floor. These Hawaii counties are where the largest septic populations sit, and where local requirements most often exceed it.
| County | What makes it different |
|---|---|
| Hawaii | The largest cesspool population; porous lava creates treatment concerns |
| Maui | Coastal nutrient concerns drive advanced treatment |
| Honolulu | Mostly sewered, on-site systems in outlying areas |
| Kauai | Rural onsite systems with conversion obligations |
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 units require ongoing maintenance under DOH permit conditions. Use the pump schedule calculator to work out the interval for your tank size and household.
Regional labour and permit costs in Hawaii run about 155% of the national baseline. Here is what typical installations land at for a three-bedroom home:
| System type | Typical installed cost in Hawaii | When it applies |
|---|---|---|
| Conventional gravity | $14,000 – $24,800 | Good soil, adequate depth to water |
| Chamber system | $15,400 – $28,500 | Alternative to gravel trenches |
| Mound system | $26,600 – $52,100 | Shallow water table or thin soil |
| Aerobic treatment | $22,400 – $44,600 | Clay soil or small lots |
| Tank pump-out (service) | $500 – $1,010 | 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 Hawaii Administrative Rules Chapter 11-62, Wastewater Systems. County and health district rules routinely exceed them, code text changes, and only Hawaii DOH — Wastewater Branch or your local health department can confirm what applies to your parcel.
Send your sizing numbers to installers who work under Hawaii rules every day.