Septic Drainfield Capacity (Flow / Bedrooms)
The design flow and bedroom count a drainfield supports from the trench length a lot can fit, the inverse of the trench-length tile. 300 ft of 3 ft trench at 0.6 gpd/ft^2 serves 540 gpd (3 bedrooms). AHJ governs.
Formula and source
Design 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.
U.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).
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