An Illinois paycheck loses money in four places: federal income tax, Social Security, Medicare, and the Illinois state income tax. The Illinois part is the simplest of the four, because Illinois charges one flat rate of 4.95% on taxable income, with no brackets and no city income tax on top. The federal part is the complicated one, because it has brackets, a standard deduction, and a wage cap on Social Security. This guide takes each deduction in turn, works a complete $85,000 example from gross pay to take-home, and explains the pre-tax deductions and exemptions that change the numbers.

The four deductions, in order

Gross pay is the salary before anything is taken out. Net pay, or take-home pay, is what is left after the deductions. Between the two sit at most four tax withholdings. Federal income tax is computed from the federal brackets after the standard deduction. Social Security takes 6.2 percent of wages up to the annual wage base, which is $184,500 for 2026. Medicare takes 1.45 percent of all wages, plus 0.9 percent on high wages. Illinois tax takes 4.95 percent of wages after the state exemptions. On top of those four come the voluntary pre-tax deductions the worker chooses, such as a 401(k) contribution or a health insurance premium, which reduce some of the taxes and not others.

The order matters, because each deduction is computed on a different base. Federal tax is computed on taxable income, which is gross pay minus the standard deduction and minus pre-tax deductions. Social Security and Medicare are computed on wages, which pre-tax 401(k) contributions do not reduce. Illinois tax is computed on wages after the state exemptions, and because Illinois starts from federal adjusted gross income, a 401(k) contribution does reduce the Illinois base on a real return, even though the calculator on this site keeps the Illinois line on gross wages after exemptions for simplicity. The worked example below follows the calculator convention, and the pre-tax section explains where the real return differs.

Two things never come out of an Illinois paycheck as taxes. No city, county, or other local government in Illinois levies an income tax, so a paycheck in Chicago is taxed exactly like a paycheck in Springfield or Carbondale. And Illinois does not tax most retirement income: Social Security benefits, pensions, and 401(k) and IRA distributions are excluded from Illinois taxable income, which matters for retirees even though it does not change a working paycheck. The rest of this guide is about the four deductions that do apply.

Federal income tax: brackets and the standard deduction

Federal income tax is the only bracketed deduction on the stub. For 2026, a single filer pays 10 percent on taxable income up to $12,400, 12 percent on the slice from $12,400 to $50,400, and 22 percent on the slice from $50,400 to $105,700, with higher rates above that. Taxable income is gross pay minus the standard deduction, which is $16,100 for a single filer in 2026, and minus any pre-tax deductions. The brackets apply to slices, not to the whole income: earning $1 more into the 22 percent bracket costs 22 cents on that dollar, not 22 percent on the whole salary, which is the point the marginal-rate confusion always misses.

Work the federal tax for the $85,000 example. Subtract the $16,100 standard deduction, leaving taxable income of $68,900. The first $12,400 is taxed at 10 percent, which is $1,240. The next $38,000, from $12,400 to $50,400, is taxed at 12 percent, which is $4,560. The remaining $18,500, from $50,400 to $68,900, is taxed at 22 percent, which is $4,070. Add the slices: $1,240 + $4,560 + $4,070 = $9,870 of federal tax for the year. The effective federal rate is $9,870 ÷ $85,000, which is 11.6 percent, well below the 22 percent marginal rate, because most of the income was taxed in the lower brackets.

Withholding spreads that annual figure across paychecks. A biweekly worker is paid 26 times a year, so the employer withholds roughly one 26th of the annual tax each payday, using the withholding tables and the W-4 the worker filed. The withholding is an estimate of the annual tax, not a separate tax, and the difference between what was withheld and what is owed is settled on the tax return. A worker who withholds too little owes at filing time; a worker who withholds too much gets a refund, which is an interest-free loan to the government, not a bonus.

FICA: Social Security and Medicare

FICA is two flat taxes with no standard deduction and no brackets. Social Security takes 6.2 percent of wages up to the annual wage base of $184,500 for 2026, and nothing above it. Medicare takes 1.45 percent of all wages, with no cap, plus an additional 0.9 percent on wages above $200,000 for a single filer. Together they are 7.65 percent on most wages, and they are the same for every worker at the same wage, whatever the filing status or the state.

For the $85,000 example, Social Security is 6.2 percent of $85,000, which is $5,270.00, and Medicare is 1.45 percent of $85,000, which is $1,232.50, for a FICA total of $6,502.50. The wage base matters only above $184,500: a worker earning $250,000 pays Social Security on the first $184,500, which is $11,439, and nothing on the remaining $65,500, while Medicare continues at 1.45 percent on the whole $250,000 plus the 0.9 percent additional tax on the $50,000 above $200,000. The employer pays a matching 6.2 percent and 1.45 percent on top of the wage, which never appears on the stub but is part of the cost of employing the worker.

One FICA rule surprises people: a 401(k) contribution lowers federal tax and Illinois tax, but never FICA. The contribution comes out of wages before income tax is computed, but Social Security and Medicare are computed on the full wage, including the amount contributed to the 401(k). A worker who contributes $5,000 to a 401(k) saves federal and state income tax on that $5,000 and saves nothing on FICA. The same is true of health insurance premiums paid through a cafeteria plan, which do reduce FICA, unlike 401(k) contributions — one of the few cases where two pre-tax deductions behave differently.

Illinois: one flat rate, after exemptions

Illinois is one of the few states whose constitution effectively requires a flat income tax: its uniformity clause says the tax on income must be at a non-graduated rate, so Springfield cannot build brackets the way the federal government does. Voters were asked to change that in 2020 and rejected the graduated-rate amendment, so one rate for everyone remains the rule. The rate has been 4.95 percent since July 1, 2017, after sitting at 3 percent for two decades, jumping to 5 percent in January 2011, and dropping to 3.75 percent in 2015.

The Illinois calculation is the shortest on the stub. Start with wages, subtract the personal exemption — $2,850 per person for tax year 2025, according to the Illinois Department of Revenue, one for yourself, one for a spouse on a joint return, and one for each dependent, with an extra $1,000 for a person 65 or older or legally blind — and charge 4.95 percent on what is left. Because there are no brackets, the marginal rate and the average state rate are the same 4.95 percent on every dollar above the exemptions. Each exemption is worth exactly $141.08 of tax at the 4.95 percent rate, because $2,850 × 4.95 percent = $141.08.

The exemptions phase out at the top: they disappear entirely if federal adjusted gross income passes $250,000, or $500,000 on a joint return. A high earner therefore pays 4.95 percent on the whole wage, with no exemption shield. The phase-out is the one wrinkle in an otherwise straight-line tax, and it is the reason the calculator on this site asks about dependents and filing status: the exemptions are the only moving part in the Illinois line.

Illinois uses the tax-year 2025 exemption of $2,850, as published by the Illinois Department of Revenue and used by the calculator.

Worked example: $85,000, single, biweekly

Put the four deductions together for a single filer earning $85,000 a year, paid every two weeks, with no dependents and no pre-tax deductions. Gross pay for the year is $85,000. Federal tax, worked slice by slice above, is $9,870.00. Social Security is $5,270.00 and Medicare is $1,232.50, for a FICA total of $6,502.50. Illinois tax is ($85,000 − $2,850) × 4.95 percent, which is $82,150 × 0.0495 = $4,066.43. The four deductions add to $9,870.00 + $6,502.50 + $4,066.43 = $20,438.93. Take-home pay is $85,000 − $20,438.93 = $64,561.07 for the year, and one biweekly paycheck is $64,561.07 ÷ 26 = $2,483.12.

Read the stub the way a payroll department builds it. Gross pay per period is $85,000 ÷ 26 = $3,269.23. Federal withholding is $9,870 ÷ 26 = $379.62. Social Security is $5,270 ÷ 26 = $202.69 and Medicare is $1,232.50 ÷ 26 = $47.40. Illinois withholding is $4,066.43 ÷ 26 = $156.40. The four withholdings add to $786.11 per paycheck, and $3,269.23 − $786.11 = $2,483.12, which matches the annual figure. Small differences of a few cents arise because the employer rounds each pay period separately and uses the withholding tables rather than the annual division; the annual arithmetic is the check, not the per-period rounding.

Change one thing and watch the Illinois line move. Claim two dependents and the exemptions grow from $2,850 to $8,550, the Illinois tax falls from $4,066.43 to $3,784.28, and take-home rises by $282.15 a year — $141.08 per exemption, twice. Contribute 5 percent of pay, $4,250, to a 401(k) and the federal tax falls, because the taxable income falls by $4,250, while FICA does not move at all. The calculator on this site runs all of these changes live, and the garnishment guide explains what happens when a court order takes a further slice.

Pre-tax deductions: 401(k), health premiums, and HSAs

Pre-tax deductions are the worker choices that shrink the taxable base. A 401(k) contribution reduces federal taxable income and, on a real Illinois return, Illinois taxable income, because Illinois starts from federal adjusted gross income, which already leaves out 401(k) contributions. It does not reduce FICA. A health insurance premium paid through the employer cafeteria plan reduces federal tax, Illinois tax, and FICA, which makes it the most tax-efficient deduction on the stub. An HSA contribution reduces federal and Illinois tax, and FICA too when it goes through payroll. The differences are small per paycheck and large over a year, which is why the order of the deductions matters.

The calculator on this site keeps things simple on one point: its Illinois line is figured on gross wages after exemptions, without subtracting 401(k) contributions, so for a heavy contributor its Illinois figure is slightly conservative, and the real saving is the federal saving plus a little Illinois saving on top. The federal line and the FICA line follow the usual rules exactly: a 401(k) lowers federal tax and never FICA. If the 401(k) contribution is large, treat the calculator Illinois line as an upper bound and the real Illinois tax as a little lower.

After-tax deductions are different again: Roth 401(k) contributions, union dues, and garnishments come out of take-home pay, after all four taxes are computed, and they change no tax line. A Roth contribution is taxed now and grows tax-free, which is a bet on the future tax rate, not a reduction in the current one. The distinction between pre-tax and after-tax is the distinction between deductions that change the tax and deductions that only change the cash, and the stub keeps them separate for exactly that reason.

Why your real paycheck differs slightly

Small differences between the calculator and a real paycheck are normal. The employer withholds from the Illinois IL-W-4 allowances claimed and the state withholding tables, rounds each pay period separately, and applies the federal withholding tables to the W-4 as filed, which may not match the annual division exactly. The employer also subtracts things the calculator does not know about: union dues, garnishments, extra withholding the worker asked for, and benefit premiums. Treat the calculator result as a close estimate, not a payroll guarantee, and check the first real stub against it line by line: the differences will be visible, small, and explained by one of those items.

Two differences are worth checking first. If the federal withholding looks high, the W-4 may still carry the old allowances, or the worker may have asked for extra withholding. If the Illinois line looks low, the IL-W-4 may claim more exemptions than the return will support, or the employer may be using the tables for a different pay frequency. Neither is an error in the calculator; both are differences in the inputs, and the fix is to compare the inputs, not the arithmetic.

Mistakes: the flat rate, the exemption, and the city myth

First, treating Illinois as if it had brackets: applying a higher rate to higher pay, or expecting the state line to climb as the salary climbs. It does not climb; it is 4.95 percent on every dollar above the exemptions, at every income level, and the only thing that changes with income is the phase-out of the exemptions at the top. Second, forgetting the exemption: charging 4.95 percent on the whole wage overstates the tax by $141.08 per exemption, which is exactly the amount one dependent is worth. Third, adding a Chicago income tax that does not exist: no city in Illinois levies an income tax, and Chicago taxes parking, restaurants, and rideshares, not wages.

Fourth, expecting the 401(k) to lower FICA: it never does, and the stub will show the same Social Security and Medicare lines with or without the contribution. Fifth, confusing withholding with tax: the amount withheld per paycheck is an estimate of the annual tax, and the return settles the difference, so a large refund means too much was withheld, not that the tax was low. And sixth, using the wrong year figures: the exemption, the wage base, and the brackets change year to year, and the figures in this guide are the 2026 figures the calculator uses, with the Illinois exemption stated for tax year 2025 as published by the Illinois Department of Revenue.

Frequently asked questions

What taxes come out of an Illinois paycheck?

Four withholdings at most: federal income tax from the 2026 brackets after the standard deduction, Social Security at 6.2 percent up to the $184,500 wage base, Medicare at 1.45 percent, and Illinois tax at one flat 4.95 percent after exemptions. On top of those come any pre-tax deductions you choose, like a 401(k) or health premiums. No city in Illinois adds an income tax.

How much is taken out of an $85,000 salary in Illinois?

For a single filer with no dependents and no pre-tax deductions, the four taxes total $20,438.93 a year: $9,870.00 federal, $5,270.00 Social Security, $1,232.50 Medicare, and $4,066.43 Illinois. Take-home pay is $64,561.07 a year, or $2,483.12 every two weeks. Dependents and pre-tax deductions change the figures; the calculator on this site runs them live.

What is the Illinois state income tax rate?

Illinois charges one flat rate of 4.95 percent on taxable income, the same rate for every worker, at every income level, in every city. The rate has been 4.95 percent since July 1, 2017. There are no Illinois brackets, so unlike federal tax your state rate does not climb as you earn more.

Does a 401(k) lower my Illinois tax?

On a real Illinois return, mostly yes, because Illinois starts from federal adjusted gross income, which already leaves out 401(k) contributions. The calculator on this site keeps its Illinois line on gross wages after exemptions, so its figure is slightly conservative for a heavy contributor. A 401(k) lowers federal tax too, and never lowers FICA.

Does Chicago add its own income tax?

No. No city, county, or other local government in Illinois levies an income tax, so a paycheck in Chicago is taxed exactly like one in Springfield or Carbondale: federal tax, FICA, and the one state flat tax. Chicago has other taxes, on things like parking, restaurants, and rideshares, but none of them come out of your wages.

How much is the Illinois personal exemption?

For tax year 2025 the exemption is $2,850 per person, according to the Illinois Department of Revenue: one for yourself, one for a spouse on a joint return, and one for each dependent, plus $1,000 more if that person is 65 or older or legally blind. Each exemption is worth $141.08 of tax at the 4.95 percent rate. The exemptions phase out entirely above $250,000 of federal AGI, or $500,000 on a joint return.