How your Illinois paycheck is calculated
Illinois income tax withholding
Illinois withholding = 4.95% × (wages − (Line 1 allowances × $2,925 + Line 2 allowances × $1,000) ÷ pay periods per year). Line 1 covers you, your spouse and dependents; Line 2 is for being 65 or older or legally blind.
Federal withholding and FICA
Federal income tax follows IRS Publication 15-T (2026). Social Security is 6.2% of wages up to $184,500, and Medicare is 1.45%, plus 0.9% above $200,000.
Pre-tax deductions
Traditional 401(k) and Section 125 contributions reduce the wages used for federal and Illinois income tax in this calculator; only Section 125 also reduces Social Security and Medicare wages.
Federal income tax, Social Security and Medicare
Federal withholding uses the IRS 2026 percentage method for automated payroll systems (Publication 15-T, Worksheet 1A). Social Security is 6.2% of wages up to the $184,500 wage base, and Medicare is 1.45% of all wages plus 0.9% Additional Medicare Tax on wages above $200,000. Per-paycheck Social Security and Medicare are shown as a yearly average: if you pass the wage base, your later paychecks will show none.
What this calculator does not include
This estimate does not cover bonuses or other supplemental wages, multiple jobs, Roth (after-tax) contributions, HSA payroll contributions, wage garnishments, or tips and overtime deductions unless you enter them on Form W-4 Step 4(b). See the methodology for details.
Frequently asked questions
What is the Illinois income tax rate for 2026?
The individual income tax rate used for 2026 withholding is 4.95% (Illinois Department of Revenue, IL-700-T).
How much is an Illinois withholding allowance worth?
Each allowance on Line 1 of Form IL-W-4 exempts $2,925 a year, and each Line 2 allowance exempts $1,000. On a weekly paycheck, two Line 1 allowances reduce taxable pay by about $112.50.
How is federal income tax withholding calculated?
We follow Worksheet 1A of IRS Publication 15-T (2026) for a 2020-or-later Form W-4: your taxable pay is annualized, adjusted by the Form W-4 entries (Step 2 checkbox, Step 3 credits, Step 4 other income, deductions and extra withholding), run through the 2026 percentage-method table for your filing status, and divided back by your number of paychecks. Our engine reproduces all 4,350 cells of the IRS 2026 wage-bracket tables for weekly, biweekly, semimonthly and monthly pay.
Why does Social Security stop at $184,500?
For 2026 the Social Security (OASDI) wage base is $184,500 (SSA). The 6.2% employee tax applies only to wages up to that amount each year. Medicare (1.45%) has no cap, and employers must withhold an extra 0.9% Additional Medicare Tax on wages above $200,000 in the year, regardless of filing status.
Is withholding the same as the tax I will owe?
No. Withholding is an estimate taken from each paycheck. Your actual tax is settled when you file your return and depends on all your income, deductions and credits, so you may get a refund or owe more.
Official sources
- Illinois Department of Revenue: Booklet IL-700-T (2026) withholding tables
- IRS Publication 15-T (2026), Federal Income Tax Withholding Methods
- IRS Rev. Proc. 2025-32 (2026 tax brackets and standard deduction)
- SSA: 2026 Social Security wage base ($184,500)
- IRS Topic 560: Additional Medicare Tax