Updated October 24, 2026. Quick answer: Line 9 of the 2025 Illinois IL-1040 is the end of the base income step. The form reads “Illinois base income. Subtract Line 8 from Line 4.” The instructions add: “This line may not be less than zero. If the result is a negative number, enter “0” (zero).”
What this line asks
The printed label is “Illinois base income. Subtract Line 8 from Line 4.” The step it closes is titled “Step 3: Base Income”. The only instruction printed for the line itself is: “This line may not be less than zero. If the result is a negative number, enter “0” (zero).”
Who it applies to
Line 9 comes after the subtraction lines. It uses the two totals the form defines: line 4, “Total income. Add Lines 1 through 3.” and line 8, “Add Lines 5, 6, and 7. This is the total of your subtractions.” Retirees whose Social Security, pension, IRA or railroad retirement income was subtracted on line 5 see that reflected through line 8; the line is explained on IL-1040 line 5 retirement income subtraction.
What the instructions say to enter
| Step | What the form and instructions say |
|---|---|
| Subtraction | “Illinois base income. Subtract Line 8 from Line 4.” |
| Where line 4 comes from | “Total income. Add Lines 1 through 3.” |
| Where line 8 comes from | “Add Lines 5, 6, and 7. This is the total of your subtractions.” |
| Negative result | “This line may not be less than zero. If the result is a negative number, enter “0” (zero).” |
The figures the line prints
The line 9 instructions print no dollar amounts or percentages. The only number in the rule is the zero floor: “This line may not be less than zero. If the result is a negative number, enter “0” (zero).”
Where the result goes next
The next step is exemptions, which the form titles “Step 4: Exemptions”. Line 11 is then “Residents: Net income: Subtract Line 10 from Line 9.” The tax rate is a separate subject; see Illinois income tax rate and the Illinois IL-1040 line by line page.
Edition and line numbers
This page reads the 2025 Form IL-1040 Instructions from the Illinois Department of Revenue, the current file served on 2026-10-24. Check the edition printed on your own form and use that edition’s line numbers.
Common mistakes
- Entering a negative number. The instructions say “This line may not be less than zero. If the result is a negative number, enter “0” (zero).”
- Subtracting in the wrong direction. The form says “Illinois base income. Subtract Line 8 from Line 4.”
- Using a line 8 total that leaves out one of its parts. The form says “Add Lines 5, 6, and 7. This is the total of your subtractions.”
Related: IL-1040 line 5 retirement income subtraction, Illinois income tax rate, and Illinois retirement taxes.
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Answer againSources
- 2025 Form IL-1040 Instructions, Illinois Department of Revenue: tax.illinois.gov
- 2025 Form IL-1040 Individual Income Tax Return, Illinois Department of Revenue: tax.illinois.gov
Read from the state revenue documents linked above on October 24, 2026. General information, not tax advice. Your own facts decide the outcome, and a preparer or the state revenue department is the right place to confirm anything consequential.