You have to file state taxes in Illinois if you lived in the state (full-year or part-year) or earned Illinois-source income as a non-resident, and your Illinois base income exceeds your exemption allowance of $2,925 per exemption. You also have to file if you were required to file a federal return, even when your Illinois income falls below that threshold. Illinois charges a flat 4.95% tax on net income, reported on Form IL-1040, and the Illinois Department of Revenue (IDOR) splits filers into three groups with slightly different rules: full-year residents, part-year residents, and non-residents.1Illinois Department of Revenue. Filing Requirements
Full-Year Residents
If you lived in Illinois for the entire year, you need to file Form IL-1040 in either of two situations: you were required to file a federal return, or your Illinois base income is greater than your exemption allowance even if you weren’t required to file federally.1Illinois Department of Revenue. Filing Requirements
The exemption allowance equals $2,925 multiplied by the number of exemptions you claim. A single person with one exemption has to file once their Illinois base income tops $2,925. A married couple filing jointly with four exemptions has a threshold of $11,700.2Illinois Department of Revenue. What Is the Illinois Personal Exemption Allowance Base income starts with your federal adjusted gross income and then reflects Illinois-specific additions and subtractions.3Illinois Department of Revenue. Taxable Income
If someone else claims you as a dependent, the rules tighten. You have to file if your Illinois base income exceeds $2,925, or if you want a refund of Illinois tax withheld from your pay.4Illinois Department of Revenue. Illinois Form IL-1040 Instructions – General Information
Part-Year Residents
If you moved into or out of Illinois during the year, you’re a part-year resident. You’re taxed as a resident for the months you lived in the state and as a non-resident for the rest. Your filing threshold is based on your total federal gross income for the whole year, not just what you earned while living in Illinois. If you meet it, you file Form IL-1040 along with Schedule NR, which calculates how much of your income Illinois can actually tax.5Illinois Department of Revenue. IL-1040 Schedule NR Instructions
Non-Residents With Illinois Income
If you don’t live in Illinois but earned money from Illinois sources, you only need to file if your Illinois base income is high enough to create a tax liability, meaning it exceeds your exemption allowance, or if you want a refund of Illinois tax that was withheld in error.1Illinois Department of Revenue. Filing Requirements Common Illinois-source income includes wages for work physically performed in the state, rental income from Illinois property, and business income from Illinois operations.
There’s one exception worth knowing. If your only Illinois income comes from partnerships, S corporations, or trusts that already withheld enough Illinois tax to cover your liability, you don’t have to file at all.4Illinois Department of Revenue. Illinois Form IL-1040 Instructions – General Information
If You Live in a Reciprocal State and Work in Illinois
Illinois has reciprocity agreements with Iowa, Kentucky, Michigan, and Wisconsin. If you live in one of those four states and commute to Illinois for work, your wages are only taxable in your home state, and you don’t file an Illinois return for them. File a withholding exemption form with your Illinois employer so they stop withholding Illinois tax from your paycheck.6Legal Information Institute. Illinois Code tit 86 100.7090 – Reciprocal Agreement
Reciprocity only covers wages and salary. A Wisconsin resident who commutes to Chicago doesn’t need to file an Illinois return for those wages, but if that same person also owns rental property in Illinois, the rental income is still taxable by Illinois and triggers an Illinois filing obligation under the non-resident rules.5Illinois Department of Revenue. IL-1040 Schedule NR Instructions
The reverse also applies. If you’re an Illinois resident working in Iowa, Kentucky, Michigan, or Wisconsin, you owe Illinois tax on those wages, not the other state. If you work in a state that isn’t on the reciprocity list, you’ll likely owe tax to both states on that income, and you claim a credit on Schedule CR for the tax paid to the other state to avoid being taxed twice.7Illinois Department of Revenue. 2025 IL-1040 Schedule CR Instructions
Filing Just to Get a Refund
Even if your income falls below the filing threshold, file an IL-1040 if Illinois tax was withheld from your pay. That’s the only way to get it back. This matters most for students, part-time workers, dependents, and non-residents whose employers withheld Illinois tax in error. The refund-only trigger shows up in each of the three categories above, and it’s often the practical reason a lower-income filer still ends up submitting a return.1Illinois Department of Revenue. Filing Requirements
What About Retirement Income
Retirement income can change the filing picture. Illinois subtracts virtually all retirement income from your federal AGI when calculating your Illinois base income. That includes Social Security benefits, distributions from 401(k)s, traditional and Roth IRAs, pensions, government retirement plans, and railroad retirement income.8Illinois Department of Revenue. Does Illinois Tax My Pension, Social Security, or Retirement Income The subtraction covers the federally taxed portion of those distributions, which effectively means Illinois doesn’t tax retirement income at all.9Illinois Department of Revenue. Publication 120 – Retirement Income
So a retiree whose only income is Social Security and pension distributions may have a federal filing requirement (which pulls them into an Illinois filing requirement too) but owe no Illinois tax, because the base income after subtractions sits at or near zero. You still file the return; you just don’t owe.
When the Return Is Due
If you do have to file, the deadline for your 2025 Illinois return is April 15, 2026, matching the federal due date.10Illinois Department of Revenue. Due Date/Extension to File Income Tax Return If that date falls on a weekend or holiday in a given year, the deadline shifts to the next business day.11Illinois Department of Revenue. When Is My Individual Income Tax Return Due and Where Do I Mail It
Illinois automatically grants a six-month extension to file, pushing your filing deadline to October 15. You don’t have to request it. The catch that trips people up every year: the extension only covers your paperwork, not your payment. If you owe tax, the full amount is still due by April 15. Use Form IL-505-I to calculate and remit what you owe by the original deadline, even if you haven’t finished the return.12Illinois Department of Revenue. IL-505-I Automatic Extension Payment You can file electronically through IDOR’s free MyTax Illinois portal or commercial tax software, or mail a paper return to the department’s Springfield address.