Wait until you choose retirement plans to pay the military deposit. You'll have an opportunity to pay your military deposit once you've received detailed information about your benefits options and made your choice. It won't matter that you're a retiree, and no longer a Federal employee. Before you're asked to choose retirement plans, OPM will give you information about your benefits under both CSRS and FERS. They will tell you how much you owe under both. They will also explain how payment of a deposit will affect your benefit, and let you know whether you will be able to receive credit for the service by taking an actuarial reduction in your annuity.
If, in the meantime, you want to know how much you'll owe so you can put the money aside, ask your agency to estimate the deposit under both CSRS and FERS rules.
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