If the side business is profitable (and makes enough to enact this plan), it works like this for a solo 401(k): the employee's contribution (you) up to $18,000

If the side business is profitable (and makes enough to enact this plan), it works like this for a solo 401(k): the employee's contribution (you) up to $18,000 and a profit sharing contribution up to 25% of the business earnings. For 2017, it maxes out at $54,000 if you're under 50 years old. Keep in mind though that maximums are per person not per company; if you keep the day job 401(k) (and potential match) you'd have to reduce the contributions from your side business to not exceed the maximum. https://www.irs.gov/retirement-plans/one-participant-401k-plans https://www.irs.gov/retirement-plans/one-participant-401k-pl... https://scs.fidelity.com/products/mobile/sepMobile.shtml https://scs.fidelity.com/products/mobile/sepMobile.shtml