Septic Tank Size Calculator
Free septic tank size calculator: bedroom flow, optional occupant flow, detention-day volume hint, illustrative bedroom floor gallons, common precast gallon ladder, scenario rows—plus a plain-English guide. Not for permits.
Permit & code notice: Septic tank size is set by state, county, and soil/site rules—not a generic web page. This tool blends illustrative daily flow assumptions, a detention-day volume check, and a bedroom-count floor table (common U.S. planning conversation ranges) so you can rehearse questions with a designer. It is not stamped engineering, not a substitute for perc/soil evaluation, and not approval for installation.
Summary: Enter bedrooms (proxy for dwelling size), optional occupants for a people-based flow cross-check, gallons-per-bedroom and gallons-per-person factors, and a detention multiplier (days of average daily flow you want represented in liquid volume). The tool reports a planning gallons range, bumps to a common precast size ladder, and lists scenario rows—same spirit as other calculators’ stress checks.
What this tool does — and does not (tap to expand)
- Does: estimates design daily flow from bedrooms and/or occupants, multiplies by detention days for a hydraulic volume hint, compares to a bedroom floor table (illustrative), then rounds up to a standard tank gallon step for discussion.
- Does not: size drainfields, account for garbage grinders, commercial flows, STEP systems, nitrogen limits, pump tanks, dual compartments, setbacks, or inspector checklists.
Septic tank size calculator (planning gallons)
The headline number is a conversation starter with a licensed installer or sanitarian. If your jurisdiction publishes a table, their table wins—use this page to understand why flow and detention show up in those rules.
Assumptions & methodology (short)
- Bedroom flow: bedrooms × “gallons per bedroom per day” (default 150—sometimes cited in older residential design discussions; your authority may use 120, 110, or a different method).
- People flow (optional): if occupants > 0, design flow is the maximum of bedroom-based flow and occupants × gallons per person per day.
- Hydraulic hint: design flow × detention days (default 2). This is not a code statement—some rules fix minimum tank liquid capacity by bedroom count alone.
- Bedroom floor table: illustrative minimum liquid gallons by bedroom count (common retail ladder: 750 → 1000 → 1250 …). Merged with hydraulic hint via max().
- Standard size: first step in [750, 1000, 1250, 1500, 1750, 2000, 2500, 3000, 3500, 4000, 5000, 6000] that meets the merged minimum.