For pass-through businesses, the employee and employer portion of the Solo 401k contribution is reported on line 15 of Schedule 1. There is a direct connection from Schedule C to Schedule 1. For example, you report business (earned income) from Schedule C on line 3 of Schedule 1. Then, as part of the Schedule 1 calculation, the employer contribution to your Solo 401k becomes part of your adjusted income. Afterward, that is subtracted out of your taxable income. Finally, report adjusted income on line 8a of Schedule 1 on your 1040 form (see line 8b subtraction calculation).
