How do you report 401k on w2?

The amount contributed to a 401(k) plan is reported in Box 12 of your W-2 form, typically using the code "D" for elective deferrals to a 401(k) plan. This amount is not included in your taxable wages in Box 1, as it is pre-tax. Employers may also report any matching contributions separately in Box 12 with different codes. Always review your W-2 to ensure accuracy and consult a tax professional if you have questions.