Septic Tank Size Calculator

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Sets the published bedroom-count table. Use Typical when your state is not listed, then verify with the local health department.
Codes size residential septic tanks by bedroom count (a proxy for occupancy), not by actual residents. Dens/offices that could be bedrooms often count.
Ground food waste raises solids loading. Many codes require the next size up or +250 gal; applied here as one bedroom-equivalent step.
Codes assume two occupants per bedroom for design flow. Override only for the cross-check - the table minimum still applies.
Per-person daily wastewater design flow. 70 gal/day is a common middle value; some codes use 60, 75 or 110 per bedroom instead.
gal/day

Results

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Minimum tank size(V)
1,000gal
Pass

Controlled by the Typical (state not listed) table minimum.

Larger of the state-table minimum and the daily-flow method - the published table is always a floor.

Also computed

State table minimumPass1,000gal

Typical (state not listed) minimum at 3 bedrooms.

Daily-flow method840gal

2 x 420.0 gal/day; a cross-check, never smaller than the table minimum.

2 x daily design flow (~48 h detention).

Daily design flow(Q)420gal/day

6 occupants x 70.0 gal/person/day.

Method notes 4 notes
  • Table basis - Typical adopted minimum pattern across US onsite codes; your state or county table governs.
  • Garbage disposal is applied as one bedroom-equivalent step. State rules differ: New York codifies exactly this; Minnesota requires 1.5x the base capacity plus screening; Massachusetts requires a two-compartment tank; Pennsylvania already includes grinders in its base flow.
  • The daily-flow method (2 x daily design flow, about 48 h detention) is a cross-check. The published table minimum is authoritative as a floor even when the flow method computes smaller.
  • Tank capacity only - drainfield/soil treatment area is sized separately from soil and percolation testing. Compartments, effluent filters, risers, setbacks and permits come from the locally adopted code; verify with the local health department before design.

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