The federal government requires employers in the United States to withhold Social Security tax, Medicare tax and federal income tax from employees' wages. IRS Circular E has the withholding and payment procedures for each of these federal taxes. In most cases, you are mandated to withhold taxes on an employee's earnings before any other deductions are taken out; these earnings are also called gross pay. Each tax on gross income requires a different calculation.
Compute Social Security tax at 4.2 percent of gross pay up to $106,800 for the year, at the time of publication. For example, the employee earns $1,000 semimonthly. The calculation is: $1,000 x .042 = $42 semimonthly. Once an employee earns the maximum wage base for the year ($106,800 for 2011), stop the withholding and resume it at the start of the next year.
Figure Medicare tax at 1.45 percent of all gross pay. For example, an employee earns $420 weekly. Calculation: $420 x .0145 = $6.09 weekly. Withhold Medicare tax from all wages paid to the employee for the year, since no annual wage base applies.
Calculate federal income tax according to the employee's W-4 data and IRS Circular E's tax withholding tables. Obtain the employee's filing status from line 3 of her W-4 form and her allowances from line 5; find the Circular E tax withholding table that matches this information plus the employee's gross wages and pay period. For example, in 2011, the employee earns $500 weekly and claims single with one allowance on her W-4. According to page 38 of the 2011 Circular E, she pays $51 weekly in federal income tax.