Mo. Rev. Stat. § 288.124
The term "average annual payroll" as used in sections 288.113 to 288.127 means one-third of the total amount of taxable wages paid for insured work during the thirty-six consecutive months' period immediately preceding any July first calculation date; provided, however, if an employer during any one calendar half year in such period paid no wages for insured work, whether taxable or not, such employer's average annual payroll shall be twice the total amount of taxable wages paid by him in the calendar half year of such period wherein the taxable wages paid by him were highest, and except that for an employer who has been subject to the Missouri employment security law less than thirty-six months the term shall mean the total amount of taxable wages paid by him during the twelve-month period immediately preceding any July first calculation date. If an employer during any one calendar half year in such period paid no wages for insured work, such employer's average annual payroll shall be twice the total amount of taxable wages paid by him in the calendar half year during the twenty-four consecutive months preceding the July first calculation date wherein taxable wages paid by him were highest.
§ 288.124, RSMo