Septic Drainfield Capacity (Flow / Bedrooms)
The design flow and bedroom count a drainfield supports, from the trench length a lot can fit.
Example
You enter
- Available trench length (ft) 300
- Application rate (gpd / ft²) 0.6
- Trench width (ft) 3
- Design flow per bedroom (gpd) 150
You get
- Design flow supported 540 gpd
- Absorption area 900 ft²
- Bedrooms permitted 3
Checked against published worked examples from: North Carolina Department of Health and Human Services, Division of Public Health...
Details, formula, and sources
Scope300 ft of 3 ft trench at 0.6 gpd/ft^2 serves 540 gpd (3 bedrooms). AHJ governs.
Formula or tableDesign flow supported = available_trench_ft × trench_width_ft × application_rate_gpd_per_ft² (the inverse of the required-area sizing). Bedrooms = floor(design_flow / gpd_per_bedroom); EPA baseline 150 gpd/bedroom.
Source and editionU.S. EPA Onsite Wastewater Treatment Systems Manual (EPA/625/R-00/008) by name; per-state and per-county application rates and per-bedroom design flows set by local code (not bundled).
Free accessFree at epa.gov/septic. State application-rate and per-bedroom-flow tables free on each state department of health / DEQ site.
What governsEstimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.
API field namesavailable_trench_ft, application_rate_gpd_per_ft2, trench_width_ft, gpd_per_bedroom, design_flow_gpd, absorption_area_ft2, bedrooms
- Application rate user-supplied from local code (this tool deliberately does not bundle a per-state shard)local AHJ
- Per-bedroom flow EPA baseline 150 gpd/bedroom; states vary (often 120)local AHJ