The correct amount of income tax withholding is calculated
by taking a flat 25 percent of the supplemental wages. Social security
tax and Medicare tax withholding are calculated at the usual rates and
are in addition to the 25 percent income tax withholding.
This answer was provided by the Internal Revenue Service (IRS). For further guidance/clarification on this issue, agency representatives may contact IRS (Federal, State and Local Governments) at (202) 283-9665.
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