How much tax per week
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Calculate the amount of your Social Security and Medicare withholding, which are fixed at 6.20 percent and 1.45 percent of your weekly salary, respectively. Express these percentages as decimals (.062 and .0145) and add them together (.0765). Multiply your weekly salary by this sum. For example, if your weekly salary is $1,000, you will have $76.50 withheld for Social Security and Medicare during this period.
Consult IRS Publication 15. Go to the 2010 Income Tax Withholding Tables, Tables for Percentage Method of Withholding. Use Table 1, Weekly Payroll Period, to calculate your tax withholding. Rates are divided by income bracket in the table, with a fixed rate assessed for each bracket, plus an additional percentage of any amount over the
lower limit of the bracket. Assuming the individual in the example who earns $1,000 per week is single, his or her rate will be 25 percent of the amount over $693, which is $307, plus a fixed amount of $82.35.
Convert the percentage listed for your income bracket into decimal form. Multiply the amount of your salary that exceeds the lower limit of your bracket by this decimal. For the worker in the example, this will be $76.75 per week (.25 X $307).
Add the fixed rate from Step 3 ($82.35) to the amounts calculated in Step 1 ($76.50) and Step 4 ($76.75) to determine your weekly federal withholding. For the single taxpayer in the example who earns $1,000 per week, this amount is $235.60.Source: ehow.com