5 CFR 831.1003 - Deductions from pay.
(a) Except as otherwise provided in this section, the employing agency, the Secretary of the Senate, or the Clerk of the House of Representatives must withhold 7 percent of an employee's Federal wages to cover both the OASDI tax and the CSRS deduction. The difference between the OASDI tax and the full amount withheld under this paragraph is the CSRS deduction.
(b) For a Congressional employee as defined by 5 U.S.C. 2107 and a law enforcement officer or firefighter as defined by 5 U.S.C. 8331, the appropriate percentage under paragraph (a) of this section is 7 1/2 percent.
(c) For a Member, a judge of the United States Court of Military Appeals, a United States magistrate, and a bankruptcy judge as defined by 5 U.S.C. 8331(22), the appropriate percentage under paragraph (a) of this section is 8 percent.
(d) For any amount of Federal wages paid after reaching the contribution and benefit base calculated including all wages, but before reaching the contribution and benefit base calculated using only Federal wages, the amount withheld under this section is the difference between 7, 7 1/2, or 8 percent, as appropriate, and the OASDI tax rate, even though the Federal wages in question are not subject to the OASDI tax.
(e) For any amount of Federal wages paid after reaching the contribution and benefit base calculated on the basis of Federal wages only, the full percentage required under paragraph (a), (b), or (c) of this section (7, 7 1/2, or 8 percent) must be withheld from Federal wages.