Wis. Stat. § 138.04
The rate of interest upon the loan or forbearance of any money, goods or things in action shall be $5 upon the $100 for one year and according to that rate for a greater or less sum or for a longer or a shorter time; but parties may contract for the payment and receipt of a rate of interest not exceeding the rate allowed in ss. 138.041 to 138.056 , 138.09 to 138.14 , 218.0101 to 218.0163 , or 422.201 , in which case such rate shall be clearly expressed in writing.
Wis. Stat. § 138.04
A creditor is entitled to interest on a liquidated claim from the time payment was due by the terms of the contract and, if no time is specified, then from the time demand was made or from commencement of the action. Estreen v. Bluhm, 79 Wis. 2d 142, 255 N.W.2d 473 (1977). A merchant who first informed the customer of the 24 percent interest to be charged on an open account in statements of the account provided after the account was opened violated s. 422.302(2). The merchant was only entitled to interest under this section. Severson Agri-Service, Inc. v. Lander, 172 Wis. 2d 269, 493 N.W.2d 230 (Ct. App. 1992). The writing expressing the interest to be charged need not be subscribed by the party charged. Advance Concrete Forms v. Mc Cann Const. 916 F.2d 412 (1990). Prejudgment interest in Wisconsin personal injury cases. Brennan. WBB Aug. 1983.