How the Illinois Tax Calculator works
Illinois taxes individual income at a single flat rate rather than graduated brackets. This calculator applies that flat rate to your net taxable income — gross income minus any non-taxable subtractions (such as exempt retirement or Social Security income) and your personal exemption allowances — to estimate your state income tax liability.
The formula
Illinois net taxable income equals gross income minus non-taxable subtractions minus total exemption allowance:
Taxable income = Gross income − Subtractions − (Exemptions × Allowance per exemption)
The state tax is then:
Illinois tax = Taxable income × 4.95%
The number of exemptions is one for the taxpayer, plus one more if filing jointly with a spouse, plus one for each dependent claimed. Illinois has used a single flat rate of 4.95% for all individual income since July 1, 2017; there are no separate brackets for different income levels or filing statuses.
Worked example
A single filer with $65,000 of gross income, no dependents, no subtractions, and the default $2,775 personal exemption has taxable income of $65,000 − $2,775 = $62,225. At 4.95% that produces an estimated Illinois tax of about $3,080, an effective rate of roughly 4.74% of gross income.
Common mistakes
- Using gross income instead of Illinois base income: some income (like most retirement income and Social Security benefits) is not taxed by Illinois and should be entered as a subtraction, not included in taxable income.
- Forgetting dependent exemptions: each qualifying dependent adds another exemption allowance, which lowers taxable income.
- Confusing the flat rate with the effective rate: because exemptions reduce taxable income before the 4.95% rate applies, your effective rate (tax divided by gross income) is always somewhat below 4.95%.
What this calculator does not include
This is a simplified estimate of the Illinois flat-tax calculation. It does not account for the Illinois property tax credit, K-12 education expense credit, earned income credit, the additional exemption for taxpayers age 65+ or legally blind, local taxes, or federal income tax. Use it for planning, and confirm final figures with the Illinois Department of Revenue or a tax professional.
When to re-run this calculation
Re-run the estimate whenever your income, filing status, or number of dependents changes, or at the start of a new tax year when the Illinois Department of Revenue publishes an updated exemption amount.