Wis. Stat. § 138.045: Method of calculating interest.

Interest on any note, bond, or other instrument computed on the declining unpaid principal balance from time to time outstanding may be computed and charged on actual unpaid balances at 1/360 of the annual rate for the actual number of days outstanding if the use of this calculation method is disclosed in the note, bond, or other instrument. This section does not apply to pawnbrokers’ loans under s. 138.10.