Arkansas late fee law

6%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.

By statute, "the rate of interest under a contract in which a rate of interest is not specified is six percent (6%) per annum" (§ 4-57-101(d), text verified via FindLaw). This operates within the constitutional framework of Amendment 89 (effective Jan. 1, 2011), which replaced the old Federal-Reserve-discount-rate formula in art. 19, § 13: the maximum lawful rate on loans or contracts not otherwise described is 17% per annum. Arkansas is unusual in that its interest ceilings are constitutional, not merely statutory.

Parties may agree in writing to any rate up to the Amendment 89 ceiling of 17% per annum for general (including commercial/B2B) contracts; there is no broad business exemption from the constitutional cap (federal preemption applies to many regulated lenders, e.g. banks, and Amendment 89 expressly yields to applicable federal law). Consumer loans and credit sales above 17% are void as to principal and interest. Agreed B2B late-payment interest above 17% therefore carries real risk.

public

Public projects only (state construction; State Highway Commission contracts excluded). Ark. Code Ann. § 19-4-1411 sets 5-working-day processing windows for design professionals, agencies, and DFA, with an 8% per annum penalty payable to the contractor by the party responsible for delay; secondary sources report wrongfully withheld public-construction payments accrue 10% per annum under the related public-works provisions (Ark. Code Ann. § 22-9-205) - verify exact rate with counsel. No private-commercial prompt-payment act found.

Caution: because Amendment 89's 17% ceiling applies to "contracts" generally, a commercial late charge computed as (or recharacterizable as) interest exceeding 17% per annum may be unenforceable; flat late fees are otherwise assessed under standard liquidated-damages doctrine. No Arkansas statute specifically authorizing or capping flat late fees on commercial invoices was found.