I have a 1099 R that has a Gross amount for Box 1, but Box 2a, the taxable amount, says UNKNOWN. What is my taxable amount?

[cjensen]

All of this Gross amount (in box 1) is from a CSRS pension annuity.

[AmyC]

The taxable portion is the difference between your box 1 income and your box 5 contributions.

Publication 721 (2024), Tax Guide to U.S. Civil Service Retirement Benefits states:

Part of the annuity benefits you receive is a tax-free recovery of your contributions to the CSRS or FERS. The rest of your annuity benefits are taxable.