Posted on Monday, 6th August 2012 by

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Many federal employees had some part time work experience during their careers.  Part-time employees receive full-time credit for retirement eligibility (years of service), but the impact on the annuity can be significant. To determine how your part time service will affect your annuity you have to understand the differences between pre and post April 7, 1986 rules.

Part-time calculations are complex and the computation rules changed on April 7th 1986.  Unless you dig, the OPM website is extremely vague on this topic.

If you have part-time service before and after April 7th 1986, the retirement calculator used by most agencies computes the figure separately for these periods of time.  The combined basic annuity of a CSRS employee who has any part-time service on or after April 7, 1986, is the sum of two separate computations:

  • A pre-April 7, 1986, basic annuity, using the employee’s total creditable service through April 6, 1986 (plus unused sick leave as of the date of the employee’s separation); and
  • A post-April 6, 1986, basic annuity, using the employee’s service from April 7, 1986, through the date of separation, and leftover days from the length of service used to compute the pre-April 7, 1986, basic annuity. The result of this computation is prorated to reflect the difference between full-time and part-time service.

The CSRS proration factor is a fraction, expressed as a percentage rounded to the nearest percent. It is used in the computation of the post-April 6, 1986, annuity benefit to reflect the difference between full-time and part-time service performed after April 6, 1986.

Compute it as follows:

Actual Hours Worked from 4-7-86 to Date of Separation
Total Full-Time Hours Possible from 4-7-86 to Date of Separation

What basically happens with the proration factor is they compute the annuity as if you worked full-time, however  the annuity is reduced for the period after 4-6-1986 by the amount of hours you worked in comparison to a full-time worker.  If you worked 50% of the time, you will receive 50% of the annuity for that period of time.

FERS Proration

The FERS proration factor is used to compute FERS and FERS component annuities that include credit for part-time service. The factor reflects the difference between full-time and part-time service for the entire period of covered FERS service (including military service credited under FERS). Compute it as follows:

The actual Hours Worked During All Creditable FERS Service
Total Full-Time Hours Possible During All Creditable FERS Service

Visit for more information and a direct link to OPM guidance.  The OPM guide provides detailed examples for calculating part time service annuity impact. The CSRS part time example is located on page 6 of this guide, see page 21 for a detailed FERS analysis.

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The information provided may not cover all aspect of unique or special circumstances, federal regulations, and financial information is subject to change. To ensure the accuracy of this information, contact your benefits coordinator and ask them to review your official personnel file and circumstances concerning this issue. Retirees can contact the OPM retirement center. Our article is not intended nor should it be considered investment advice. Our articles and replies are time sensitive. Over time, various dynamic economic factors relied upon as a basis for this article may change.

Last 5 posts by Dennis Damp


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One Response to “Annuity Calculations for Part Time Service”

  1. Samuel Randall Says:

    The good thing about this article is not just about calculations. Giving some informative sites and retirement planning tools are very helpful. Thank you!