1 Best answer
Are you a W-2 employee? If so, your 401(k) contributions are already shown in box 12 of your W-2 and the amount is not included in box 1 of your W-2. So you don't report it anywhere else.
The only other thing that is possible, is if you made qualified contributions to your 401(k) yourself personally, by writing a check and depositing to the account. That most likely would not be included in box 12 - but it could be.