If it's a sizable sidegig, i'd say more than about ten % of what you're making at your a regular job, i'd probably plan on paying quarterly estimated payments. They gi give me a little bit of a break the first year, but after that, they're not very merciful on these at all. So make sure you're paying em ok,. how much can go into a solo one, four, one k. As an employer contribution, if you've used your maximum employee contribution, which is 20 thousand, 500 for those of us under 50, this year, then you can put in is the employer contribution - essentially 20% of your net income.