Calculate the amount of income tax withheld from a paycheck by determining the salary deductions, such as health insurance, marital status, earnings and number of exemptions claimed, Washington State University says. The withholding rates for tax year 2015 include 6.2 percent for Social Security from the taxpayer and 6.2 percent from the employer, 1.45 percent Medicare from the employee and 1.45 percent Medicare from the employer, the IRS notes. Employers deduct these amounts from the gross wages.
There is a wage base limit for Social Security withholding that is subject to change from year to year, the IRS notes. For the year 2015, there is a 12.4 percent withholding rate for wages up to $118,500. There is no wage base for Medicare withholding, as all wages are subject to a 2.9 percent Medicare withholding tax.
Employers use a tax table when determining how much money to withhold from an employee's paycheck. The employer takes into account the number of allowances the employee has claimed on his W-4 Certificate of Withholding. The number of allowances has a direct impact on deductions from a paycheck each pay period and also impacts the size of the tax refund that may be due each year to the taxpayer. If the employee claims zero allowances, the taxes withheld are at the maximum amount, RapidTax notes.