The IRS has updated income-tax withholding tables for 2013 in light of the American Taxpayer Relief Act of 2012, signed into law Jan. 2. Also, because the payroll tax holiday hasn’t been extended, employers must withhold Social Security tax at the rate of 6.2% rather than at the 4.2% rate that applied in 2011 and 2012.

According to the IRS, employers should start using the revised withholding tables and withholding the correct amount of Social Security tax “as soon as possible in 2013, but not later than Feb. 15, 2013.”

Employees don’t have to do anything, but you may want to revise your W-4 if you get married or divorced, have a child or buy a home. Revising your W-4 also may be a good idea if you hold multiple jobs or if when you file your 2012 return you have a large balance due or receive a large refund.