26 C.F.R. § 31.3201-1
Measure of employee tax
The employee tax is measured by the amount of compensation received for services rendered as an employee. For provisions relating to compensation, see § 31.3231(e)-1. For provisions relating to the circumstances under which certain compensation is to be disregarded for the purpose of determining the employee tax, see paragraphs (b)(1) and (2) of § 31.3231(e)-1.