Does Illinois have a state income tax?

Yes—Illinois has a state income tax system described on IL-1040. Flat 4.95% on base income after exemptions.

What form do I file for Illinois?

Most residents use IL-1040 for the state return, subject to exceptions for part-year residents and special situations. Always confirm the latest state instructions each filing season.

Does BasilTax calculate Illinois state tax?

Yes—BasilTax includes Illinois rules in the computation engine so your state return can align with your federal return when you upload your documents.

Illinois State Income Tax (2026) — Guide

Illinois state income tax

Last updated: 2026-04-27

TL;DR / ELI5

Illinois taxpayers generally file IL-1040 and pay state income tax based on a flat rate on taxable income after state-specific adjustments. Flat 4.95% on base income after exemptions. BasilTax can compute this state end-to-end when you upload your documents so you can verify withholding and key lines against your W-2 and other forms.

Primary form: IL-1040

BasilTax engine: Yes — we calculate this state for you

Structure: flat

Flat 4.95% on base income after exemptions.

Illinois income tax at a glance

This guide explains how Illinois fits into your overall filing picture. Your federal Form 1040 is only half the story—states add their own definitions, subtractions, credits, and withholding rules.

Most Illinois residents will prepare IL-1040 alongside the federal return. Flat 4.95% on base income after exemptions.

Withholding and W-2 boxes

If you work in Illinois (or your employer withholds for Illinois), compare boxes 15–17 on your Form W-2 to what you expect on the state return. Mismatches here are a common source of balance due surprises.

Credits, deductions, and quirks

State returns often start from federal adjusted gross income (AGI) and then apply state-specific subtractions, additions, credits, and alternative minimum taxes. Read your state instructions for the exact list—this page is a hub, not a substitute for official guidance.

Verification workflow

Upload your PDFs, assign them to a return, and run compute. When BasilTax supports Illinois end-to-end, you can trace state lines to the same documents you used for federal. When support is still in progress, use BasilTax to lock down federal and broker schedules first, then complete Illinois manually with confidence.

Ready to tie every line to your documents?

Upload your W-2s and 1099s—BasilTax extracts fields, computes federal and state, and explains each number with citations.

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Frequently asked questions

Does Illinois have a state income tax?: Yes—Illinois has a state income tax system described on IL-1040. Flat 4.95% on base income after exemptions.
What form do I file for Illinois?: Most residents use IL-1040 for the state return, subject to exceptions for part-year residents and special situations. Always confirm the latest state instructions each filing season.
Does BasilTax calculate Illinois state tax?: Yes—BasilTax includes Illinois rules in the computation engine so your state return can align with your federal return when you upload your documents.