Septic tank size by state / East North Central
Illinois septic tank size
In Illinois the minimum septic tank for a three-bedroom house is 1,000 gallons, rising to 1,250 gallons at four bedrooms. The figure comes from 77 Ill. Adm. Code 905.40(c) and Part 905 Appendix A, Illustration F (Private Sewage Disposal Code). That matches the 1,000-gallon median of the 47 states listed here.
What the Illinois rule says
The rule is 77 Ill. Adm. Code 905.40(c) and Part 905 Appendix A, Illustration F (Private Sewage Disposal Code), administered by Illinois Department of Public Health (IDPH). In its own words:
Septic tanks for individual residences shall be sized in accordance with Appendix A, Illustration F. ... NUMBER OF BEDROOMS | MINIMUM LIQUID CAPACITY | WHEN GARBAGE GRINDER IS USED: 2 or less 750 1125; 3 1000 1500; 4 1250 2000; 5 1500 2200; 6 1750 2600; 7 2000 3000
Source: 77 Ill. Adm. Code 905.40(c) and Part 905 Appendix A, Illustration F (Private Sewage Disposal Code), read 2026-10-20.
- Two bedrooms or fewer may use a 750-gallon tank.
- Illinois has a separate, larger column for homes with a garbage grinder: 1,500 gallons for three bedrooms.
- Some counties adopt stricter local ordinances.
Illinois minimum tank size by bedrooms
| Bedrooms | Minimum tank | Versus the median state |
|---|---|---|
| 1 | 750 gal | −250 |
| 2 | 750 gal | — |
| 3 | 1,000 gal | same |
| 4 | 1,250 gal | +50 |
| 5 | 1,500 gal | same |
| 6 | 1,750 gal | — |
The first step up comes at 3 bedrooms, from 750 to 1,000 gallons, and a six-bedroom house needs 1,750 gallons. These are minimums: the permit can call for more, and a bigger tank needs pumping less often.
How Illinois compares with nearby states
Indiana requires 1,000 gallons for three bedrooms; Michigan sets no statewide figure; Ohio requires 1,500 gallons for three bedrooms. 37 of the 47 states listed here share Illinois's three-bedroom figure.
Before you size a tank in Illinois
- 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 Illinois'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.
Figures are the state's minimum as written on 2026-10-20. Local health departments can require more; the permit issued for your property governs. See the methodology for how each state was read.