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Social Security and Medicare are flat taxes, meaning a specific percentage is taken out of your paycheck. For Medicare in 2011, the rate is 1.45 percent of your gross income. The Social Security tax has an income cap that is adjusted yearly. For example, in 2011 the cap was $106,800. Once you earn that much, your employer stops deducting additional Social Security tax. The usual rate for Social Security tax is 6.2 percent. However, for the 2011 tax year it was lowered to 4.2 percent as part of efforts to stimulate the economy.
Federal Taxable Income
Before you can calculate how much federal income tax is taken out of a paycheck, you have to determine how much of your pay is subject to income tax. That depends largely on how many withholding allowances you claim. One withholding allowance equals $3,700 per year (for 2011). You divide this amount by the number of pay periods per year. For example, if you are paid weekly, you would divide $3,700 by 52 for a weekly withholding allowance of $71.15. Multiply this figure by the number of withholding allowances claimed. Subtract the total from your gross pay.
You also may need to subtract some other items, such as contributions to a retirement plan that are tax-deductible.
Federal Income Tax
Once you know how much of your pay is subject to federal income tax, you must use the tax tables for the current year to calculate the amount of the tax. Federal income tax is progressive, meaning that, as your income increases, the percentage of tax gets higher. For example, suppose you are married, paid weekly and your taxable earnings are $1,000. The tax on the first $152 is zero. You pay 10 percent of the amount from $152 to $479 ($32.70) plus 15 percent of the amount from $479 to $1,000 ($78.15) for total federal income tax of $110.85. The federal income tax rate continues to rise as your earnings go up until it reaches a maximum rate of 35 percent.
State and Local
Most states and some municipalities levy income taxes. Each of these governments has its own formula for calculating how much payroll tax to deduct from your paycheck. To find out how any state or local income taxes are calculated, contact the state or municipal department of revenue or taxation where you work.