How to determine the cost basis of over contribution to 401k on 1099-R?

Work for two employers in 2023 and over contributed above the 401k limit. Received two 1099-R forms and it seemed TurboTax treat the distribution as income but asks for cost basis that is not found in 1099-R form, any suggestion on how to calculate or where to find the cost basis?

posted

To clarify, what is the code in box 7?

Generally, you must include the excess deferral in your wages in the year the excess deferral happened. Any earnings will be include in the year of the distribution. Please see Pub 525 for details.

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