Wis. Stat. § 138.04: Legal rate.

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.