If you make a deposit for your active duty service, it will add service time towards your retirement for both eligibility and computation. So if you have 5 years of active duty, you will add 5% to your to your annuity computation. Depending on your agency, you need to complete RI-20-97, attach all copies of your DD-214s for all periods of active duty service and DFAS will send you a statement of your earnings during those periods of active duty. You then take that to your HRO and they will calculate what you owe which is 3% of that amount plus interest if you have more than 3 years of civilian service. FYI, there is a 2 year interest free period to make the deposit. If the deposit is not satisfied during that period, interest begin on your third anniversary on the unpaid balance.