Septic tank size by state / East North Central
Wisconsin · rule checked 2026-10-21
Wisconsin septic tank size
Wisconsin has no statewide minimum septic tank size. Wisconsin's private sewage code sets how much wastewater a house is assumed to produce, not how big its septic tank must be. SPS 383.43 counts 100 gallons a day per bedroom and sets the design flow at no less than 150% of that, but it prints no minimum tank volume, and neither does the state's in-ground soil absorption component manual, which requires tanks "approved by the department". The county office that issues the sanitary permit is the place to ask what size it will accept. This page quotes the state rule and shows what neighbouring states require for comparison.
- 1Statewide minimum None, set locally
- 2Who sets it Wisconsin Department of Safety and Professional Services
What the Wisconsin rule says
The rule is Wis. Admin. Code SPS 383.43(2) and (3)(a) (design wastewater flow); no statewide tank volume, administered by Wisconsin Department of Safety and Professional Services (DSPS); POWTS permits issued by counties. In its own words:
Estimated daily flow for a dwelling: "100 gallons x B = F" (SPS 383.43(3)(a)), where "B = number of bedrooms, based on 2 persons per bedroom"; "the design wastewater flow of a POWTS shall equal at least 150% of the estimated daily flow" (SPS 383.43(2)).
Source: Wis. Admin. Code SPS 383.43(2) and (3)(a) (design wastewater flow), read 2026-10-21. Also: Wisconsin DSPS, In-Ground Soil Absorption Component Manual for POWTS, Version 2.1 (May 2022), SPD-10705, read 2026-10-21.
- Wisconsin's code (SPS 383.43) assumes 100 gallons a day per bedroom, two people to a bedroom, and designs for at least 150% of that: 150 gallons a day per bedroom.
- Neither SPS 383.43 nor the state's In-Ground Soil Absorption Component Manual (version 2.1, May 2022) prints a minimum septic tank volume; the manual requires tanks "approved by the department".
- Graywater and blackwater kept apart are counted separately: 60 gallons a day per bedroom of graywater and clear water, 40 of blackwater.
How Wisconsin compares with nearby states
Illinois requires 1,000 gallons for three bedrooms; Indiana requires 1,000 gallons for three bedrooms; Michigan sets no statewide figure; Ohio requires 1,500 gallons for three bedrooms.
Before you size a tank in Wisconsin
- Bedrooms, not people, set the size in most rules. Rooms that could be used as bedrooms are usually counted, so check how your permitting office counts a den or a finished basement.
- Garbage disposals, grinder or ejector pumps and large tubs change the answer in many states; the points above list Wisconsin's adjustments.
- The tank is only half the system. The drainfield is sized separately from soil tests, and it is usually the bigger cost.
- A private well on the same lot has its own setbacks from the tank and field; the well pump and pressure tank calculators size the water side.
Every state side by side for one house size: 2 bedrooms · 3 bedrooms · 4 bedrooms · 5 bedrooms · 6 bedrooms.
Figures are the state's minimum as written on 2026-10-21. Local health departments can require more; the permit issued for your property governs. See the methodology for how each state was read.