To calculate your FERS retirement, you need to know the amount of your high-3 average salary and the number of years you have worked in a covered position. Your high-3 average salary is calculated by taking the highest three consecutive years of basic pay from your last five years of service. The number of years you have worked in a covered position will determine how much creditable service time you receive when calculating your FERS benefit. Once these two items are determined, use an online calculator or contact the Office of Personnel Management (OPM) to calculate your estimated monthly annuity payment upon retirement.