How Illinois withholding works
Illinois uses one flat rate for everyone: 4.95%. There are no brackets, so your filing status only matters through the allowances you claim on Form IL-W-4.
The 2026 Booklet IL-700-T gives employers a simple formula:
- Multiply the number of basic allowances (IL-W-4 line 1) by $2,925.
- Multiply the number of additional allowances (line 2) by $1,000.
- Subtract both from your yearly wages, then multiply what's left by 4.95%.
Basic allowances cover you and, if you're married and your spouse doesn't claim it, your spouse. Additional allowances are for things like being 65 or older or blind. This calculator uses one basic allowance (two if married filing jointly) and no additional allowances.
Worked example
A flat tax: what it means for you
With a flat tax, every extra dollar of wages above your allowances costs exactly 4.95% in Illinois tax, whether you earn $40,000 or $400,000. A raise or a bonus is easy to estimate: take 4.95% off for Illinois, then the federal bracket you're in and 7.65% for Social Security and Medicare (if you're under the Social Security wage base).
Because the allowance is small, almost all of your wages are taxed by Illinois. On a $70,000 salary, Illinois tax is about 4.7% of gross pay.
Lowering your Illinois tax
- Pre-tax 401(k) contributions. Each $100 you contribute saves $4.95 of Illinois tax plus federal income tax at your bracket.
- Pre-tax health, dental and vision premiums paid through a cafeteria plan generally reduce Illinois wages too.
- Review your IL-W-4. If you qualify for additional allowances or have other income, updating the form brings withholding closer to what you'll owe.
What's not included
This page doesn't include tax credits such as the Illinois property tax or education expense credits, which you claim on Form IL-1040 rather than through withholding. It also doesn't include after-tax deductions from your pay. Your final Illinois tax is worked out on your IL-1040 return.
The same Illinois salary at other pay frequencies
For the $70,000 example (single, no pre-tax deductions), here's each paycheck at the four common pay frequencies:
| Pay frequency | Gross per paycheck | Illinois tax | Take-home per paycheck |
|---|---|---|---|
| Weekly (52) | $1,346.15 | $63.85 | $1,052.97 |
| Bi-weekly (26) | $2,692.31 | $127.70 | $2,105.96 |
| Semi-monthly (24) | $2,916.67 | $138.34 | $2,281.46 |
| Monthly (12) | $5,833.33 | $276.68 | $4,562.90 |
Filing status and 401(k) in Illinois
Filing status changes your federal tax and, through the IL-W-4 allowance, your Illinois tax a little. On $70,000, married filing jointly with one income means Illinois tax of $3,175.43 (two allowances) and federal income tax of $4,040, for a yearly take-home of $57,429.57. A head of household pays the same Illinois tax as a single filer, $3,320.21, but less federal tax.
Putting 6% ($4,200) into a 401(k) lowers Illinois tax to $3,112.31 and federal income tax to $5,716.