Mo. Rev. Stat. § 287.690
Prior to December 31, 1993, for the purpose of providing for the expense of administering this chapter, every person, partnership, association, corporation, whether organized under the laws of this or any other state or country, the state of Missouri, including any of its departments, divisions, agencies, commissions, and boards or any political subdivisions of the state who self-insure or hold themselves out to be any part self-insured, company, mutual company, the parties to any interindemnity contract, or other plan or scheme, and every other insurance carrier, insuring employers in this state against liability for personal injuries to their employees, or for death caused thereby, under this chapter, shall pay, as provided in this chapter, tax upon the net deposits, net premiums or net assessments received, whether in cash or notes in this state, or on account of business done in this state, for such insurance in this state at the rate of two percent in lieu of all other taxes on such net deposits, net premiums or net assessments, which amount of taxes shall be assessed and collected as herein provided. Beginning October 31, 1993, and every year thereafter, the director of the division of workers' compensation shall estimate the amount of revenue required to administer this chapter and the director shall determine the rate of tax to be paid in the following calendar year pursuant to this section commencing with the calendar year beginning on January 1, 1994. If the balance of the fund estimated to be on hand on December thirty-first of the year each tax rate determination is made is less than one hundred ten percent of the previous year's expenses plus any additional revenue required due to new statutory requirements given to the division by the general assembly, then the director shall impose a tax not to exceed two percent in lieu of all other taxes on net deposits, net premiums or net assessments, rounded up to the nearest one-half of a percentage point, which amount of taxes shall be assessed and collected as herein provided. The net premium equivalent for individual self-insured employers shall be based on average rate classifications calculated by the department of commerce and insurance as taken from premium rates filed by the twenty insurance companies providing the greatest volume of workers' compensation insurance coverage in this state. For employers qualified to self-insure their liability pursuant to this chapter, the rates filed by such group of employers in accordance with subsection 4 of section 287.280 shall be the net premium equivalent. Any group of political subdivisions of this state qualified to self-insure their liability pursuant to this chapter as authorized by section 537.620 may choose either the average rate classification method or the filed rate method, provided that the method used may only be changed once without receiving the consent of the director of the division of workers' compensation. Every entity required to pay the tax imposed pursuant to this section and section 287.730 shall be notified by the division of workers' compensation within ten calendar days of the date of the determination of the rate of tax to be imposed for the following year. Net premiums, net deposits or net assessments are defined as gross premiums, gross deposits or gross assessments less cancelled or returned premiums, premium deposits or assessments and less dividends or savings, actually paid or credited.
§ 287.690, RSMo