5 CFR §831.1005

1. (a)OPM will reduce the annuity of an individual who has performed Federal service, if the individual is entitled, or on proper application would be entitled, to old-age benefits under title II of the Social Security Act.
2. (b)The reduction required under paragraph (a) of this section is effective on the 1st day of the month during which the employee—
   1. (1)Is entitled to an annuity under CSRS; and
   2. (2)Is entitled, or on proper application would be entitled, to old-age benefits under title II of the Social Security Act.
3. (c)Subject to paragraphs (d) and (e) of this section, the amount of the reduction required under paragraph (a) of this section is the lesser of—
   1. (1)The difference between—
      1. (i)The Social Security old-age benefit for the month referred to in paragraph (b) of this section; and
      2. (ii)The old-age benefit that would be payable to the individual for the month referred to in paragraph (b) of this section, excluding all wages from Federal service, and assuming the annuitant was fully insured (as defined by section 215(a) of the Social Security Act (42 U.S.C. 414(a)); or
   2. (2)The product of—
      1. (i)The old-age benefit to which the individual is entitled or would, on proper application, be entitled; and
      2. (ii)A fraction—
         1. (A)The numerator of which is the annuitant's total Federal service, rounded to the nearest whole number of years not exceeding 40 years; and
         2. (B)The denominator of which is 40.
4. (d)Cost-of-living adjustments under 5 U.S.C. 8340 occurring after the effective date of the reduction required under paragraph (a) of this section will be based on only the annuity remaining after reduction under this subpart.
5. (e)The amounts for paragraphs (c)(1)(i), (c)(1)(ii), and (c)(2)(i) of this section are computed without regard to subsections (b) through (l) of section 203 of the Social Security Act (42 U.S.C. 403) (relating to reductions in Social Security benefits), and without applying the provisions of the second sentence of section 215(a)(7)(B)(i) or section 214(d)(5)(ii) of the Social Security Act (42 U.S.C. 415(a)(7)(B)(i) or 415(d)(5)(ii) (relating to part of the computation of the Social Security windfall elimination provisions).
6. (f)OPM will accept the determination of the Social Security Administration, submitted in a form prescribed by OPM, concerning entitlement to Social Security benefits and the date thereof.