How this instrument works
Illinois is one of only about a dozen states that charge a single flat rate on individual income rather than climbing through brackets. Every dollar of taxable income — the first and the millionth alike — is taxed at 4.95%, a rate that has held since July 2017. Compare that to a bracketed state like Alabama, which still needs three separate rungs before its top rate applies, or California, which needs nine: Illinois needs exactly one multiplication once the exemption is subtracted, which is the entire arithmetic this instrument performs.
The flatness is not a policy choice a governor could quietly change — it is written into the state constitution's revenue article, which requires a non-graduated tax on individual income. Illinois voters had the chance to remove that requirement in November 2020, when a ballot measure known as the Fair Tax amendment asked whether to allow bracketed rates instead. It was rejected, so the single-rate structure stays locked in place unless a future constitutional amendment succeeds where that one failed.
With no brackets to compute, the only lever left is the personal exemption — a flat dollar amount subtracted from income before the 4.95% rate applies, adjusted periodically rather than indexed to any threshold. This instrument returns state individual income tax only: it excludes federal tax, excludes Illinois's notably high property tax bills, and excludes the fact that the exemption itself phases out entirely for higher earners, so results above roughly $250,000 of adjusted gross income (single) understate what is actually owed.
- Enter your total in Illinois taxable income, $ — the amount left after federal adjustments and Illinois's own additions and subtractions, not gross wages.
- Enter Personal exemption, $ (2023) — the flat per-filer amount Illinois subtracts before the rate applies.
- Read Taxable income after exemption to see what remains once the exemption is subtracted from income.
- Read Illinois state income tax for the flat 4.95% result — no bracket table to look up and no rate to interpolate.
Worked example — $60,000 income, single filer
Take $60,000 of Illinois taxable income against the 2023 personal exemption of $2,775. Subtract the exemption first: $60,000 minus $2,775 leaves $57,225 of taxable income after exemption — there is no bracket boundary to check against, because Illinois only has the one rate.
Multiply that $57,225 by the flat 4.95% rate: 57225 times 0.0495 comes to $2,832.6375, which the tax field rounds to $2,832.64. A California filer with the same $60,000 would need eight separate bracket subtotals added together to reach a comparable figure; here the whole calculation is a single multiplication once the exemption is out of the way.
Questions
Why doesn't Illinois have income tax brackets?
Because the state constitution's revenue article requires a non-graduated tax — every dollar of taxable income must be charged the same rate. Voters had a chance to remove that requirement in a November 2020 ballot measure, the Fair Tax amendment, and rejected it, so the flat structure stays in place unless a future constitutional amendment succeeds.
Is the personal exemption the same for every filer?
The dollar amount here applies to a filer claiming their own exemption, but it phases out for higher earners — filers with adjusted gross income above roughly $250,000, or $500,000 filing jointly, lose the personal exemption entirely, so this instrument understates tax owed above those thresholds.
Does this figure include property tax or other Illinois taxes?
No. This is the state individual income tax only. Illinois carries some of the highest average effective property tax rates in the country, and local governments layer their own sales taxes on top — neither appears in the income tax figure this instrument returns, so budget for them separately.
Why do the marginal and effective rates converge in Illinois?
Because a flat tax has no rungs to climb: every taxable dollar above the exemption is charged the identical 4.95%, so the marginal rate and the effective rate sit close together and grow closer as income rises and the fixed-dollar exemption shrinks as a share of the total. A bracketed state keeps its effective rate below its marginal rate at every income level.
What counts as Illinois taxable income in that field?
It starts from federal adjusted gross income, then applies Illinois's own additions and subtractions — items like federally tax-exempt bond interest get added back, while most retirement income, Social Security benefits, and 529 plan contributions get subtracted — so it will not match a federal return line for line.
Will paycheck withholding match this calculated amount exactly?
Not necessarily. Employers withhold using Illinois's own formula tables and the allowances claimed on Form IL-W-4, which can over- or under-collect relative to the annual liability this instrument computes from full-year taxable income. Reconcile the two on the year-end IL-1040 return rather than paycheck by paycheck.
References
Read this first: This instrument shows arithmetic, not advice. Real offers add fees, taxes and terms that vary by lender and place — verify the figures against your actual paperwork before deciding anything.