Sources
Every rate, threshold and benefit figure used anywhere on this site, mapped to the government document that published it. If a number here has no source, it does not belong on the site.
Most tax calculators cite nothing, and the ones that do usually cite a homepage rather than a document. That distinction matters: "irs.gov" tells you nothing about whether a bracket was read from the right publication for the right year. Below is the full list of primary sources behind the site, what each one is used for, and how often it changes. The way those figures are then combined into a paycheck is documented separately on the methodology page, and the standards governing what gets published are on the editorial policy page.
Federal
| Document | What we take from it | Updated |
|---|---|---|
| Publication 15-T, Federal Income Tax Withholding Methods Internal Revenue Service |
Worksheet 1A percentage method, the annual rate schedules, the $12,900 / $8,600 computational bridge amounts, and the Step 2 checkbox half-bracket schedules | Annually, December |
| Publication 15, Employer’s Tax Guide (Circular E) Internal Revenue Service |
Supplemental wage rules, the 22% flat rate and the 37% rate above $1,000,000, deposit rules and FUTA mechanics | Annually |
| Form W-4, Employee’s Withholding Certificate Internal Revenue Service |
Step 2 multiple jobs, Step 3 dependent credits of $2,200 and $500, Step 4 other income, deductions and extra withholding | Annually |
| Annual inflation adjustments (Revenue Procedure) Internal Revenue Service |
Tax brackets and the standard deduction for each tax year | Annually, October |
| Schedule SE and Publication 334 Internal Revenue Service |
Self-employment tax: the 92.35% net earnings factor, 12.4% Social Security, 2.9% Medicare and the employer-half deduction | Annually |
| Contribution and Benefit Base Social Security Administration |
The Social Security wage base — $184,500 for 2026 | Annually, October |
| Additional Medicare Tax guidance Internal Revenue Service |
The 0.9% surcharge and the $200,000 / $250,000 / $125,000 filing-status thresholds | As amended |
Illinois — income tax and withholding
| Document | What we take from it | Updated |
|---|---|---|
| Booklet IL-700-T, Illinois Withholding Tax Tables Illinois Department of Revenue |
The mechanics of Illinois withholding: the flat 4.95% applied after IL-W-4 allowances, and the annualized allowance treatment | Annually, December |
| Form IL-W-4 and its instructions Illinois Department of Revenue |
Basic allowance value ($2,925 for 2026) and the additional allowance for age 65+ or legal blindness ($1,000) | Annually |
| 35 ILCS 5/201, Illinois Income Tax Act Illinois General Assembly |
The statutory 4.95% individual rate and the 1.5% personal property replacement tax on partnerships and S corporations | On legislation |
| Form IL-1040 and Schedule IL-E/EIC instructions Illinois Department of Revenue |
Personal exemption, retirement subtraction, property tax credit, K-12 education expense credit, Illinois EITC and the child tax credit | Annually |
| Form IL-1040-ES, Estimated Income Tax Payments Illinois Department of Revenue |
Quarterly due dates and the safe harbor thresholds used by the estimated tax tool | Annually |
These feed the Illinois income tax calculator, the refund calculator, the IL-W-4 guide and the estimated payment tool.
Illinois — sales, use and vehicle tax
| Document | What we take from it | Updated |
|---|---|---|
| Tax Rate Finder and Publication 130 rate bulletins Illinois Department of Revenue |
Combined state, county, municipal and special district sales tax rates for every city in the sales tax tool | 1 January and 1 July |
| Form RUT-50 and RUT-25 tax charts Illinois Department of Revenue |
Private party vehicle use tax by purchase price and by model year, plus the motorcycle and ATV rates | On legislation |
| Vehicle title and registration fee schedule Illinois Secretary of State |
Title fee, standard plate registration and the electric vehicle registration surcharge | On legislation |
Two 2026 changes are worth calling out because most competing sites still have them wrong. The state's 1% grocery tax ended on 1 January 2026, and municipalities were simultaneously authorized to impose their own 1% grocery tax — so in much of Illinois the shelf price did not change while the recipient of the money did. And the Northeastern Illinois Transit Authority increase took Chicago's combined rate to 10.50% on 1 August 2026. Both are reflected in the sales tax calculator.
Illinois — employment, wages and benefits
| Document | What we take from it | Updated |
|---|---|---|
| Form CLI110L, Table of Weekly Benefit Amounts Illinois Department of Employment Security |
Weekly benefit percentages (47% of average weekly wage), dependent allowances and the maximum weekly benefit | Annually, January and July |
| Form EA-50 employer contribution rates Illinois Department of Employment Security |
The unemployment insurance taxable wage base ($14,250), the new employer rate and the fund building rate | Annually, December |
| Minimum Wage Law, 820 ILCS 105 Illinois Department of Labor |
The $15 state minimum wage, the tipped minimum of $9 and the youth rate | Annually, January |
| Chicago and Cook County minimum wage ordinances City of Chicago / Cook County |
Local minimum wages above the state floor, adjusted each July | Annually, July |
| 735 ILCS 5/12-803, Wage Deduction Illinois General Assembly |
The garnishment ceiling: the lesser of 15% of gross or the excess over 45 times the minimum wage | On legislation |
| Wage Payment and Collection Act and the Pay Stub Protection Act Illinois Department of Labor |
Pay frequency requirements, itemized pay stub content and the employer record retention duty | On legislation |
| Secure Choice Savings Program Act, 820 ILCS 80 Illinois State Treasurer |
Employer participation thresholds and the default employee contribution rate | On legislation |
Wage and salary statistics
| Document | What we take from it | Updated |
|---|---|---|
| Occupational Employment and Wage Statistics (OEWS) Bureau of Labor Statistics |
Median and mean wages by occupation for Illinois and its metropolitan areas | Annually, spring, for the prior May |
| Quarterly Census of Employment and Wages Bureau of Labor Statistics |
Average weekly wage by county and industry | Quarterly |
| Current Population Survey and ACS median household income U.S. Census Bureau |
Household income context used on the salary comparison pages | Annually |
| Employee Database salary lookup Illinois Comptroller / Secretary of State |
State employee salary data referenced on the state employee pay page | Periodically |
Wage statistics carry an unavoidable lag: OEWS data published in one year describes the previous May. Where that matters — on the average salary page in particular — the reference period is labelled rather than presented as a current-year figure.
How to verify a number yourself
Pick any figure on the site and it should be traceable in three steps. Find the tax year the page is set to; open the document listed above for that figure; check the value. If they disagree, we want to know — report it and the correction, with its date, will appear on the updates log. That log is also the fastest way to see what has changed since you last visited.