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What are FICA taxes for a sole proprietor in the United States
FICA taxes are separate from federal and state income taxes. As a sole proprietor you owe all of those.  Additionally, there is a difference with FICA when you are employed vs. self employed.  Typically FICA taxes are actually split between the employer and the employee, so you pay half, they pay half. But when you're self employed, you pay both halves. This is what is commonly referred to as the self employment tax. If you are both employed and self employed as I am, your employer pays their portion of FICA on the income you earn there, and you pay both halves on the income you earn in your business. Edit: As @JoeTaxpayer added in his comment, you can specify an extra amount to be withheld from your pay when you fill out your W-4 form. This is separate from the calculation of how much to withhold based on dependents and such; see line 6 on the linked form. This could allow you to avoid making quarterly estimated payments for your self-employment income. I think this is much easier when your side income is predictable. Personally, I find it easier to come up with a percentage I must keep aside from my side income (for me this is about 35%), and then I immediately set that aside when I get paid. I make my quarterly estimated payments out of that money set aside.  My side income can vary quite a bit though; if I could predict it better I would probably do the extra withholding. Yes, you need to pay taxes for FICA and federal income tax.  I can't say exactly how much you should withhold though. If you have predictable deductions and such, it could be lower than you expect. I'm not a tax professional, and when it comes doing business taxes I go to someone who is. You don't have to do that, but I'm not comfortable offering any detailed advice on how you should proceed there. I mentioned what I do personally as an illustration of how I handle withholding, but I can't say that that's what someone else should do.