Fedweek

The interest paid in the voluntary contribution retirement savings program available to CSRS (but not FERS) employees will rise to 2.75 percent in calendar year 2019 from 2.125 percent in 2018.

That rate also applies as the interest rate for required deposits and redeposits into the retirement fund to get credit for service for which no contributions were taken or for which refunds were received at a break in service, as well payments as to capture credit toward federal retirement for military service time, for those eligible.

Read more on Redeposits for Service Credit CSRS and FERS at ask.FEDweek.com
Some federal employees leave the government, withdraw their retirement contributions, and then return to work for the government. In those cases, the prior service time is counted toward years of service for determining retirement eligibility and the salary paid is counted for the high-3 calculation if pertinent. However, for that service to count in the calculation of an annuity or a survivor annuity, a redeposit with interest must be made.