Idaho late fee law

12%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.

When there is no express contract in writing fixing a different rate, the legal rate of interest is 12% per annum ('twelve cents on the hundred by the year'), applying to money due by express contract, money after it becomes due, money lent, money due on settled mutual accounts from the date the balance is ascertained, and open accounts after 3 months from the last item (Idaho Code § 28-22-104(1)). Judgments bear a separate variable rate (5% plus a Treasurer-set base, § 28-22-104(2)). Verified via full statute text on FindLaw's Idaho Code, corroborated by legislature.idaho.gov search results and the Idaho State Treasurer's site (the official legislature page repeatedly refused direct fetch).

The 12% default applies only absent an express written contract fixing a different rate - parties may agree in writing to another rate (§ 28-22-104(1)). Sources consulted identify no general usury ceiling for negotiated commercial/B2B rates (consumer credit is separately regulated under the Idaho Credit Code, Title 28, ch. 41-46); a specific statutory cap for commercial contracts was not verified - verify with counsel.

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Idaho Code § 67-2302: all bills owed by the State of Idaho or any taxing district must be paid within 60 calendar days of receipt of billing (unless a contract in place at order time sets a longer period); late payments bear interest at the rate provided in Idaho Code § 63-3045 (the variable tax-interest rate). No private-sector or construction-specific prompt-payment interest statute was found.

No Idaho statute specifically governing flat late fees on commercial invoices was found; agreed late fees are generally enforceable subject to the liquidated-damages/penalty doctrine. Absent an agreed rate, 12% under § 28-22-104 is the default on overdue amounts (open accounts start accruing 3 months after the last item). Dishonored checks carry 12% plus limited collection costs (§ 28-22-105).