Late filing: small, and only in one situation
The Illinois late-filing penalty is the lesser of $250 or 2% of the tax required to be shown, reduced by anything paid by the original due date and by credits. It is owed only when a processable return is filed after the extended due date and the tax was not paid by the original due date. Illinois grants an automatic extension to file, so filing late by itself costs nothing if the tax was paid. A second tier applies only if IDOR sends a notice of nonfiling and no return arrives within 30 days: the greater of $250 or 2% of the tax shown, up to $5,000, whether or not tax is due.
Late payment: two rates, by days
Tax not paid by the original due date is charged 2% if it is paid 1 to 30 days late and 10% if it is paid 31 days or more late. The rate is applied to each payment separately, by that payment's own lateness, and any amount never paid is charged 10%. There is no monthly accrual and no 25% cap as there is federally; a payment 400 days late costs the same 10% as one 31 days late. After an audit or investigation begins, amounts still unpaid are charged 15%, and 20% if not paid within 30 days after an audit-prepared amended return.
Interest: simple, daily, semiannual
IDOR sets one rate for all the taxes it administers, reviewed every January 1 and July 1 from the federal underpayment rate. Interest is simple: tax times the rate divided by 365 (366 in a leap year) times the number of days from the day after the due date through the payment date. No interest is charged on interest. Publication 103 states the method for tax; it does not say interest is charged on the penalties, so the calculator does not compute it on them.
Returns this page does not cover
Motor fuel use tax, the coin-operated amusement device tax and zero-liability transaction returns have their own statutory penalties. Withholding income tax payment schedules (Form IL-941) are not modeled. Choose one of the four income tax returns; anything else is declined with the reason.
Official sources used on this page
- Publication 103, Penalties and Interest for Illinois Taxes: R-09/25
- 2025 Form IL-2210 Instructions (late-filing and late-payment penalty worksheets): R-12/25
- 2025 Form IL-1040 Instructions (due date, automatic extension): R-12/25
- IDOR: Interest Rates: page read 2 Sep 2026
Every rule record names its source, revision and the date it was last verified; the results panel prints the ones your calculation used. Rules last verified 2 September 2026.