Reference · Illinois · 2026
Illinois late payment interest rate
When your contract sets no rate, the Illinois figure for interest on an overdue B2B invoice is 5%, under 815 ILCS 205/2.
On instruments of writing, on settlement of account from the day the balance is ascertained, and on money withheld by unreasonable and vexatious delay.
- Rate
- 5%
- Statute
- 815 ILCS 205/2
- Type
- Fixed by statute.
- Source
- ilga.gov · verified July 9, 2026
What 5% comes to on an unpaid invoice
Simple interest at 5% a year on a 365-day year. Count the days from when interest starts to run in Illinois, which is not always the due date.
| Invoice | 30 days | 60 days | 90 days |
|---|---|---|---|
| $5,000 | $20.55 | $41.10 | $61.64 |
| $10,000 | $41.10 | $82.19 | $123.29 |
| $50,000 | $205.48 | $410.96 | $616.44 |
Work out the interest on your invoice
Prefilled with Illinois's legal rate of 5%. If your contract sets a rate, use that instead.
Interest you can claim
$82.19
$10,082.19 with the invoice · $1.37 a day
The ceiling
How high can a late fee go in Illinois?
Your contract decides. The rate above only applies when your terms are silent. Between businesses, an agreed late-interest rate or late fee replaces it, within two limits: the state's ceiling on agreed rates, and the rule that a late fee must compensate you rather than punish the customer.
Illinois's ceiling on an agreed rate wasn't part of our verified research, so check the state's usury law, or ask a lawyer licensed in Illinois, before writing the common 1.5%-a-month (18%) rate into your terms.
Late payment wording for a Illinois invoice
On your invoice
Payment is due by the date shown on this invoice. Late payments are subject to interest at 5% a year, the legal rate under 815 ILCS 205/2.
This restates what Illinois law applies when your contract is silent. If your contract sets a rate, quote that instead. The wording doesn't say when interest starts, because that differs by state; the note at the top of this page covers it where Illinois law sets it.
How Illinois compares
Of the 46 states with a rate that applies on its own, 42 set a higher one and 0 a lower one. The middle of the range is 8%.
Interest is the fallback. Getting paid on time is the plan.
DueTrail turns overdue invoices into managed cases with reviewable reminders and recorded promises to pay, so invoices get settled before anyone reaches for 815 ILCS 205/2.