Wisconsin 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.

The legal rate of interest on the loan or forbearance of money, goods, or things in action is $5 upon $100 for one year (5% per annum) unless a different rate is clearly expressed in writing. The official docs.legis.wisconsin.gov site was unreachable from this environment; text corroborated via search excerpts of the official statute plus secondary sources - verify against docs.legis.wisconsin.gov with counsel.

Parties may contract in writing for a different rate (§ 138.04, subject to the caps in ss. 138.041-138.056 etc.). The general 12% usury ceiling in § 138.05(1) expressly does not apply to loans to corporations or limited liability companies (§ 138.05(5)), so most B2B/entity credit is effectively uncapped; credit to individuals/sole proprietors may still fall under § 138.05 or the Wisconsin Consumer Act (consumer purposes only) - verify with counsel.

public

State agencies must pay within 30 days of receipt of a properly completed invoice (or of the goods/services, if later); late payments accrue interest at the rate specified in § 71.82(1)(a) (12% per year), compounded monthly (Wis. Stat. § 16.528). The same rate applies to late payments by municipalities and local governmental units under § 66.0135. No general private-commercial prompt-payment act found.

No distinctive Wisconsin statute on flat late fees for commercial invoices was found; default principles apply - agreed late fees enforceable if reasonable, subject to the liquidated-damages/penalty doctrine. The corporate/LLC exemption in § 138.05(5) largely removes usury-recharacterization risk for late charges on invoices to entity customers.