Montana late fee law

10%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.

Unless there is an express written contract fixing a different rate, interest is payable at 10% per annum after the money becomes due on written instruments (other than judgments), stated accounts, and settled balances. Verified on the official Montana Code Annotated site (mca.legmt.gov).

Parties may agree in writing to a rate not exceeding the greater of 15% per annum or 6 percentage points above the prime rate published by the Federal Reserve System (Mont. Code Ann. § 31-1-107, verified on mca.legmt.gov). Exceptions for regulated lenders exist elsewhere in Title 31; no general blanket corporate-borrower exemption from the § 31-1-107 cap was verified - treat the 15%/prime+6 ceiling as the operative cap for ordinary B2B contract rates and verify any claimed exemption with counsel.

public+private-construction

Construction contracts (including private): Mont. Code Ann. § 28-2-2103 - owner approval deemed 21 days after invoice receipt, payment due 7 days after approval; amounts delayed more than 30 days from the contractual due date accrue interest at 1.5% per month (prorated), owner→prime and prime→sub; contractor may suspend or terminate for nonpayment (§ 28-2-2104 area). Public: § 17-8-242 - state agencies pay within 30 days and owe simple interest of 0.05% per day (~18.25%/yr) on late amounts for supplies and services; § 18-2-306 covers interest on final payment for public works. No general prompt-payment statute for ordinary private non-construction B2B invoices was found (10% under § 31-1-106 applies by default).

No Montana statute specific to flat late fees on B2B trade invoices was found; agreed fees are enforceable subject to the liquidated-damages/penalty doctrine. A late charge functioning as interest should stay within the § 31-1-107 cap (greater of 15% or prime+6) unless a specific lender exemption applies.