Illinois late fee law

5%per year

Attaches to the debt by operation of law when you agreed no rate, so it accrues before any lawsuit and can be stated on a demand letter.

Creditors are allowed 5% per annum for all moneys after they become due on any bond, bill, promissory note, or other instrument of writing; on money lent or advanced; on money due on the settlement of account from the day of liquidating accounts and ascertaining the balance; on money received to the use of another and retained without the owner's knowledge; and on money withheld by an unreasonable and vexatious delay of payment. Verified via statute text on Justia and FindLaw (multiple corroborating sources); the official ilga.gov site repeatedly refused connections during this research, so confidence is capped at medium.

In written contracts the general agreed rate is capped at 9% (815 ILCS 205/4(1)), but the cap does not apply to business credit: any loan made to a corporation may bear any agreed rate (§ 4(1)(a)), and any business loan to a business association, copartnership, or a person owning and operating a business as sole proprietor is likewise exempt (§ 4(1)(c)) - 'business' means a commercial, agricultural or industrial enterprise carried on for investment or profit. Banks and savings institutions are separately exempt. Net effect: B2B/commercial interest rates are essentially a matter of contract in Illinois.

public+private-construction

Public: State Prompt Payment Act (30 ILCS 540) - late payments by state agencies accrue a 1% per month interest penalty, with 2% per month applying to required pass-through payments to sub-tier construction contractors. Private construction: Contractor Prompt Payment Act (815 ILCS 603) - payments not made in a timely manner accrue interest at 10% per annum. No general private non-construction prompt-payment act was found.

Even absent an agreed clause, 815 ILCS 205/2 gives sellers 5% per annum on liquidated accounts and on money withheld by unreasonable and vexatious delay. Agreed flat late fees on commercial invoices are otherwise governed by the ordinary liquidated-damages/penalty doctrine; agreed interest-type charges to incorporated or business customers are effectively uncapped under the § 4 business exemptions.