wages as defined for the QBI is a little complicated but doesn't include the employer's share of payroll taxes
(2) Definition of W-2 wages—(i) In general. Section 199A(b)(4)(A) provides that the term W-2 wages means with respect to any person for any taxable year of such person, the amounts described in section 6051(a)(3) and (8) paid by such person with respect to employment of employees by such person during the calendar year ending during such taxable year. Thus, the term W-2 wages includes the total amount of wages as defined in section 3401(a) plus the total amount of elective deferrals (within the meaning of section 402(g)(3)), the compensation deferred under section 457, and the amount of designated Roth contributions (as defined in section 402A). For this purpose, except as provided in paragraphs (b)(2)(iv)(C)(2) and (b)(2)(iv)(D) of this section, the Forms W-2, “Wage and Tax Statement,” or any subsequent form or document used in determining the amount of W-2 wages, are those issued for the calendar year ending during the individual's or RPE's taxable year for wages paid to employees (or former employees) of the individual or RPE for employment by the individual or RPE. For purposes of this section, employees of the individual or RPE are limited to employees of the individual or RPE as defined in section 3121(d)(1) and (2). (For purposes of section 199A, this includes officers of an S corporation and employees of an individual or RPE under common law.)
