26 C.F.R. § 31.3402(h)(2)-1
An employer may determine the amount of tax to be deducted and withheld upon a payment of wages to an employee by taking the following steps:
Step 1. Multiply the amount of the employee's wages for the payroll period by the number of such periods in the calendar year.
Step 2. Determine the amount of tax which would be required to be deducted and withheld upon the amount determined in Step 1 if that amount constituted the actual wages for the calendar year and the payroll period of the employee were an annual payroll period.
Step 3. Divide the amount of tax determined in Step 2 by the number of periods by which the employee's wages were multiplied in Step 1.
Example. On July 1, 1970, A, a single person who is on a weekly payroll period and claims one exemption, receives wages of $100 from X Co., his employer. X Co. multiplies the weekly wage of $100 by 52 weeks to determine an annual wage of $5,200. It then subtracts $650 for A's withholding exemption and arrives at a balance of $4,550. The applicable table in section 3402(a) for annual payroll periods indicates that the amount of tax to be withheld thereon is $376 plus $314.50 (17 percent of excess over $2,700), or a total of $690.50. The annual tax of $690.50, when divided by 52 to arrive at the portion thereof attributable to the weekly payroll period, equals $13.28. X Co. may, if it chooses, withhold $13.28 rather than the amount specified in section 3402 (a) or (c) for a weekly payroll period.
26 C.F.R. §31.3402(h)(2)-1