Illinois Paycheck Calculator FAQ

It uses year-based federal IRS withholding tables and Illinois flat-rate withholding for 2025 and 2026, plus standard FICA rules and common deduction assumptions. It is still an estimate because payroll providers may apply employer-specific plan settings, YTD wage history, benefit deduction timing, and local tax handling that this tool cannot see.

Yes, Illinois withholding uses a flat 4.95% rate on taxable wages after exemptions. However, the amount withheld from each paycheck can still vary based on the number of allowances claimed, any additional withholding you request, and changes to the taxable wage base throughout the year.

In advanced mode you can choose either a flat supplemental federal withholding method (22% flat rate) or an aggregate method that blends the bonus into regular wages. Illinois withholding and FICA still use the taxable wage base for the current pay period regardless of which federal method you select.

Yes for hourly workers. You can enter regular hours, overtime hours, and an overtime pay multiplier (typically 1.5x). In this v1 release, salary mode does not model overtime and treats salary as a fixed annual amount spread evenly across the selected pay frequency.

Year-to-date Social Security wages determine whether the Social Security wage base cap ($176,100 for 2025) has already been reached, at which point Social Security withholding stops for the year. YTD Medicare wages help determine whether the Additional Medicare surtax (0.9%) should apply in the current paycheck based on cumulative earnings above the threshold.

Traditional 401(k) contributions reduce both federal and Illinois taxable wages but not FICA in this model. Roth contributions are post-tax and do not reduce current taxable wages. HSA, FSA, and health insurance default to common pre-tax treatment, and advanced mode lets you adjust their tax-base behavior individually when needed.

Illinois allows a standard exemption amount per allowance claimed on your IL W-4. The exemption reduces your taxable wages before the flat 4.95% rate is applied. If you claim zero exemptions, the full amount of your wages is subject to Illinois withholding at the flat rate.