Enterprise
Individual & Small Business
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Federal 2026 estimated tax due dates, for comparison: April 15, 2026, June 15, 2026, September 15, 2026, January 15, 2027.
Who must pay Illinois estimated tax?
You must make estimated income tax payments if you reasonably expect your 2026 tax liability to exceed $1,000 after subtracting your Illinois withholding, pass-through withholding, and tax credits for income tax paid to other states, Illinois property tax, K-12 education expenses, volunteer emergency worker, earned income tax credit (EITC), child tax credit, pass-through entity tax, and Schedule 1299-C credits.
How do you avoid a Illinois underpayment penalty?
If you pay at least 90% of this year's tax or at least 100% of last year's tax in four equal timely installments, you may not be subject to the late-payment penalty (no 110% high-income rule stated).
How much should you pay?
Start with your federal estimate: the free Quarterly Estimated Tax Calculator works out your 2026 federal payments and safe harbor. Illinois figures its own tax, so the state amount is separate.