opm form 1099r box 2a shows amount as unknown,how do i calculate the taxable amount?

After you enter the info from your 1099-R, TurboTax asks follow-up questions to determine the taxable amount (it's usually the amount in Box 1).

 

If no after-tax contributions were made to the pension plan before distribution (Box 5), the entire amount is generally included in taxable income.

 

Here's more info on Is My Pension Distribution Taxable?

 

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