You can't contribute the full $5,000. You can contribute $5,000 minus half your self-employment tax, assuming you made no contributions to the employer 401 (k) in 2017. It will reduce your AGI. I also am assuming you are not planning to take a self-employed health insurance deduction in 2017, as that also reduces the amount you can contribute to a solo 401 (k). The total of Lines 27, 28, and 29 on Form 1040 can total no more than your net profit from self-employment.
