Not a week goes by without someone asking me how their annuity will be affected if they have a period or periods of part-time employment at some time in their careers. In years gone by, I had to ask them whether they were covered by CSRS or FERS. That’s because the rules were different for those two retirement systems. That all changed with Public Law 111-84. As a result, all CSRS and FERS employees who retire on or after October 28, 2009, have their annuities calculated the same way.

If you have any part-time service in your work history, here’s how your annuity will be calculated:

1. Your actual time worked will be compared with the number of full-time hours you could have worked. That includes any time you worked in excess of your official schedule tour-of-duty, as long as it doesn’t exceed the amount of hours you could have worked part-time.

2. Divide the actual hours worked by the total full-time hours available. This will give you the proration factor, rounded to the nearest percent.

3. Now compute the amount of annuity you would have received if you worked full time, using either the CSRS or FERS formula that applies to your period of part-time service.

4. Finally, multiply the result by the proration factor. This will tell you how much annuity you are entitled to.

Obviously, the more part-time service you have in your career, the greater the reduction will be versus having spent those years as a full-time employee.