If at one point in your career you left the government, took out your retirement contributions, and then returned, in general, a deposit is required to get credit in your retirement benefit for that earlier service time.
That deposit is the amount of retirement deductions, plus interest, that would have been withheld from your pay if you had been covered by the Civil Service Retirement System (CSRS) or Federal Employees Retirement System (FERS) during a period of employment when retirement deductions were not withheld from your salary. You are not required to make this type of payment, but failure to make it will affect the amount of your annuity.
You can make a deposit if you are working and covered by the FERS or CSRS systems. Widows, widowers, or former spouses of deceased employees who are eligible to receive a monthly death benefit can also make a deposit.
Interest is charged on any outstanding deposit balance. The computation of interest charges is based on your retirement coverage and when the service for which the deposit is being made was performed.
To apply use Standard Form (SF) 2803, “Application to Make Deposit or Redeposit (CSRS),” or Standard Form (SF) 3108, “Application to Make Service Credit Payment (FERS).” If you are currently employed, give your completed application to your agency first because they must certify it. If you no longer work for government, send your completed application to: Office of Personnel Management, Post Office Box 45, Boyers, PA 16017.
After OPM receives your application, it will send you instructions for making payment. You can make installment payments of at least $50, but interest accrues on any unpaid balance. Once you have made a deposit payment, it cannot be withdrawn unless you get a refund of all your retirement contributions.