Each of your employers is obligated to withhold social security taxes from their wages, even if the total they withhold exceeds the maximum amount of tax that can be imposed for the year—$6,622 in 2010 (equal to 6.20% of the $106,800 wage base); $4,486 in 2011 (4.20%, the reduced rate in effect for 2011 only, of the $106,800 wage base). If they do withhold in total more than the maximum, you can recover the excess by claiming a credit for a payment of taxes on your tax return for the year.
Although you can (and should) recover the excess amount withheld when you file your tax return for the year as described above, you won’t get the benefit of the credit until you file that return, probably in April of the next year. IRS isn’t required to pay you interest on this amount, so you will lose out on any earnings you might have had on the excess amount withheld. In effect you will have made an interest-free loan to the IRS. If you are separately making estimated tax payments for the year, you may be able to avoid this loss by reducing your estimated payments to reflect the overwithholding. Essentially, you may be able to turn the overwithholding into an estimated payment.
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