How we calculate: methodology

A paycheck calculator is only as good as the method underneath it, and most of them never say what that method is. Ours is not a proprietary formula or an approximation fitted to sample data. It is the same sequence of steps an employer's payroll system performs, taken from IRS Publication 15-T and Illinois Booklet IL-700-T, implemented once and shared by every tool on the site. When the Illinois paycheck calculator, the bonus calculator and the salary tables agree with each other, it is because they are literally running the same code.

The order of operations

Payroll taxes are not computed in parallel; each one depends on what came before it. The engine follows this order, and the order matters more than any single rate:

1. Gross wages for the period — from an hourly rate and hours, an annual salary divided by pay periods, or a figure you type directly.
2. Pre-tax deductions — traditional 401(k), 403(b), HSA, FSA and Section 125 premiums come off first. Which taxes each one escapes is not uniform, and the next section covers that.
3. Federal income tax withholding — Publication 15-T, Worksheet 1A, on the wages remaining after pre-tax deductions.
4. Illinois income tax withholding — Booklet IL-700-T flat rate on wages after IL-W-4 allowances.
5. FICA — Social Security to the wage base, Medicare on everything, plus the Additional Medicare Tax over the threshold.
6. Post-tax deductions — Roth contributions, union dues, garnishments, after-tax insurance.
7. Net pay — what is left.

Federal withholding, step by step

Publication 15-T offers two methods. The wage bracket tables are printed grids that stop at a fixed wage ceiling and round into wide steps; they exist for employers doing payroll by hand. The percentage method for automated payroll systems is exact at any wage. Real payroll software uses the second, so we do too — this is the single biggest reason our output matches an actual stub when other calculators do not. Worksheet 1A runs like this, for Forms W-4 filed in 2020 or later:

1. Annualize. Multiply the wages for the pay period by the number of periods in the year. Biweekly wages are multiplied by 26, semi-monthly by 24, weekly by 52.
2. Add W-4 Step 4(a) income. Interest, dividends and other income the employee asked to be covered by withholding.
3. Subtract W-4 Step 4(b) deductions. Itemized deductions above the standard deduction, if the employee entered any.
4. Subtract the computational bridge, when Step 2 is not checked. This is the step most calculators get wrong. Publication 15-T subtracts $12,900 for married filing jointly and $8,600 for everyone else, so that a 2020-or-later W-4 lands on the same withholding a pre-2020 W-4 with the equivalent allowances would have produced. Omit it and the calculator overstates federal tax by hundreds or thousands of dollars a year.
5. Apply the rate schedule. The published bracket thresholds shifted by the offset — the standard deduction minus the bridge amount — which is what turns taxable income brackets into wage brackets.
6. Subtract Step 3 credits. $2,200 per qualifying child and $500 per other dependent, taken off the annual tax, not off wages.
7. Divide back down to the pay period and add any extra withholding requested on Step 4(c).

When the Step 2 checkbox is ticked — the "two jobs, similar pay" box — the method changes rather than merely shifting. The bridge is not subtracted at all, the offset becomes half the standard deduction, and the tax is computed by doubling the adjusted wage, running it through the schedule and halving the result. That halved-bracket arithmetic is what makes two similar jobs withhold correctly instead of each behaving as though it were the only income. We implement both branches; you can see the difference by toggling the multiple jobs switch on the withholding checkup tool.

A worked example you can check

A single filer earning $75,000 a year, paid biweekly, no dependents, no pre-tax deductions, Step 2 unticked, tax year 2026. Every figure below is produced by the live engine at the moment this page is built, so it cannot be a stale hand-typed number:

The effective tax rate there is 22.8% — every tax combined, divided by gross. The marginal federal rate is 22%, which is the rate on the next dollar and the number people usually quote when they mean the first one. The gap between those two figures is the single most useful thing a paycheck calculator can show you, and it is explained at length on what gets taken out of an Illinois paycheck.

Illinois withholding

Illinois is a flat-rate state at 4.95%, unchanged since 2017, and there is no municipal income tax anywhere in the state — not in Chicago, not anywhere else. That does not make the calculation trivial. Booklet IL-700-T subtracts the value of the allowances claimed on Form IL-W-4 before applying the rate: $2,925 per basic allowance for 2026, plus $1,000 for each additional allowance claimed for being 65 or older or legally blind. Two basic allowances therefore shield $5,850 of annual wages, worth about $290 a year in withholding. The engine applies allowances annually and divides down to the period, which is what the booklet instructs, rather than pro-rating the allowance value per period — the two approaches differ by a cent or two per check.

Because there is no local wage tax, a Chicago paycheck and a Rockford paycheck with identical gross and identical forms are identical to the penny. Local cost differences in Illinois come through sales tax and property tax, never through withholding, which is why we do not publish separate city paycheck calculators that would return the same answer under different headings.

FICA and the two thresholds

Social Security is 6.2% of wages up to the annual wage base, which is $184,500 for 2026. Above that, the tax stops entirely for the rest of the calendar year and restarts in January. Medicare is 1.45% with no ceiling. The Additional Medicare Tax adds 0.9% on wages above $200,000 for single filers, $250,000 for married filing jointly and $125,000 for married filing separately — and it is an employee-only tax, so the employer match stops at the base rate.

One detail worth stating because it trips up so many calculators: an employer is required to begin withholding the Additional Medicare Tax once an individual employee's wages from that employer pass $200,000, regardless of filing status. A married couple each earning $150,000 will therefore have nothing withheld and will owe it at filing time. We show it on the filing-status threshold, which is the figure that matters for what you actually owe, and flag the discrepancy in the results panel when it applies.

How pre-tax deductions are treated

Not every pre-tax deduction escapes the same taxes, and treating them as interchangeable is a common source of error:

The retirement line is the one that surprises people. A 401(k) contribution reduces income tax immediately but never reduces FICA, which is why a $500 contribution does not increase net pay by $500 minus your income tax rate alone. Illinois goes further than most states at the other end: retirement distributions — 401(k) withdrawals, IRA distributions, pensions and Social Security benefits — are subtracted from Illinois taxable income entirely, which is covered on the Illinois income tax rate page.

Where the numbers come from

Every rate, threshold, wage base and allowance lives in a single data file, keyed by tax year, and is entered from the primary bulletin that published it. Nothing is copied from another calculator or from a news article summarizing a change. The sources page maps each figure to the exact document, and the annual calendar those documents follow is set out on the about page. Static tables inside our articles are generated from the engine at build time rather than typed by hand, which eliminates the most common failure on sites like this one: a calculator that is current sitting next to an article table that is two years stale.

What we deliberately do not model

An honest methodology page has to say where it stops. The engine does not attempt:

• Multi-state allocation. If you live in Illinois and work in another state, or work in Illinois while living in Iowa, Kentucky, Michigan or Wisconsin under the reciprocal agreements, the withholding split depends on facts a calculator cannot see. The Illinois versus Indiana comparison covers the most common cross-border case in detail.
• Year-to-date effects. Every per-period calculation assumes the period you entered is typical of the whole year. A mid-year raise, a bonus month or an unpaid leave will move your real stubs away from the model even though the annual total stays close.
• Non-cash and imputed income. Group term life over $50,000, personal use of a company car, and similar imputed wages raise taxable wages without raising cash pay.
• Equity compensation. RSU vesting and stock option exercises are supplemental wages with their own withholding treatment; the bonus and supplemental wage calculator handles the flat-rate arithmetic but cannot know your vesting schedule.
• Court-ordered and agency deductions beyond the statutory ceilings modeled in the wage garnishment calculator.

Where a tool cannot model something, it says so on the page rather than quietly returning a confident wrong number.

Testing

The engine is checked against worked examples published in Publication 15-T and the Illinois withholding booklet, against IDES benefit tables for the unemployment calculator, and against the statutory formulas in 735 ILCS 5/12-803 for garnishment. Because every tool shares one engine, a regression in any of them shows up everywhere at once rather than hiding on a page nobody checks. Every calculator is also exercised in a real browser before release — inputs typed, results read back — rather than assumed to work because the arithmetic is right.

Corrections

If a figure here does not match your stub or a published table, that is worth reporting and we would rather hear it. Send us the page, the figure and what you believe it should be; we check it against the source document, correct it if it is wrong, and record the change in the updates log with the date. Our approach to corrections, independence and how content is produced is set out in the editorial policy.