Septic Tank Sizing

Minimum tank gallons from bedrooms or daily flow.

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Example

You enter

Bedrooms 4

You get

Daily flow 600 gpd
Minimum tank gallons 1200 gal
Floor gallons 1000

Checked against published worked examples from: USEPA.

Details, formula, and sources

Formula or tableBy bedrooms: EPA/625/R-00/008 Table 4-13 volumes (750 / 750 / 1,000 / 1,200 / 1,425 / 1,650 / 1,875 / 2,100 gal for 1-8 bedrooms, plus 225 gal per bedroom past 8), not less than 1,000 gal. By daily flow: 2 x design flow (gpd), not less than 1,000 gal.

Source and editionU.S. EPA Onsite Wastewater Treatment Systems Manual (EPA/625/R-00/008); state-published per-bedroom rules.

Free accessFree at epa.gov/septic. State rules 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 namesbedrooms, daily_flow_gpd, minimum_tank_gallons, floor_gallons

Per-bedroom rule from data/plumbing/septic-rules.json keyed to state when supplied; defaults to EPA manual minimums otherwisedata/plumbing/septic-rules.json