Self Employment tax is in lieu of Social Security tax and Medicare tax. If you are employed by someone else, they take 7.65% out of your paycheck and they also have to kick in another 7.65% on top of that which you never saw. If you are self employed, then you have to pay the entire 15.3% yourself because you’re both the employer and the employee.
Let’s suppose you earned $7,000 mowing lawns for the summer, working for yourself. That money isn’t all profit; you had expenses. So you figure how much you spent on things like fuel, advertising, travel to and from jobs, repairs on your mower, et cetera and it comes to $621. So your net profit is $6,379. You don’t pay SE tax on the expenses, just on the profit.
SE tax is calculated this way: multiply your net profit times 92.35% and then take that result and multiply it by 15.3%. So 6379.00*92.35% is $5,891.01 and then we figure 15.3% of $5,891.01 which is $901.32. The part you get to keep is $5,477.68
This is exactly the same amount of tax that Uncle Sam would have taken if you’d gotten a summer job where your boss promised to pay you $5,891.01. You would see a pay stub that says your wages are $5,891.01 and your employer took out $365.24 for Social Security and $85.42 for Medicare, total deductions $450.66, and the part you get to keep is $5,477.68.
But what you don’t see is that your boss also had to kick in another $450.66 and send it to Uncle Sam, so really the total that your boss paid you plus Uncle Sam together comes to $6,379 and Uncle Sam’s portion comes to $901.32. It’s exactly the same.
And yes, you are right that it doesn’t matter that whether you’re below the poverty line or not. In that way, Social Security tax and Medicare tax are like sales tax.
The only exceptions I know are that 1) if you make less than $400 then you don’t have to fill out the forms or pay the tax at all (which is great news for all those kids out there with lemonade stands), and 2) if you make more than $117,000 (either as a salary or being self-employed) then you don’t have to pay Social Security tax on the portion which is above $117,000.