Illinois Late Fee Calculator
Calculate late fees and interest on overdue invoices in Illinois — pre-set to the state's 5% statutory default rate.
Late fee rules in Illinois
Statutory interest rate
5% per annum prejudgment on money due under written instruments, settled accounts, and money withheld by unreasonable and vexatious delay; 9% per annum post-judgment (with lower rates for governmental and small consumer-debt judgments).
815 ILCS 205/2 (prejudgment 5%); 735 ILCS 5/2-1303 (judgment 9%)
Can you charge more? (contract & caps)
No cap for business debts — 815 ILCS 205/4 permits any rate of interest on business loans and loans to corporations, so the contract controls, subject to the common-law penalty doctrine.
Prompt payment rules
The Contractor Prompt Payment Act (815 ILCS 603) makes delinquent parties on private construction projects (excluding small residential) liable for 10% per annum interest when payment due within the Act's 15-day deadlines is late.
Worth knowing
Illinois splits rates sharply by context: unwritten commercial accounts reach the 5% prejudgment rate only via the 'unreasonable and vexatious delay' standard.
This page is general information, not legal advice. Statutory rates change — some reset annually or quarterly — and how they apply depends on your contract and situation. Verify current rates against the cited statutes or consult a lawyer before charging interest.
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