How is self-employment tax calculated? - Most W2 employees don’t realize that their employer matches the 7.65% withheld from their paychecks. As I mentioned earlier, we are all required to pay 15.3% of our gross income into Medicare and social security. W2 employees only pay half of the 15.3% while their employer pays the other half, for a grand total of 15.3%. For those of us that are self-employed, we do not have an employer to pay half of our Medicare and social security payments, therefore, we are required to pay the entire 15.3% ourselves. Find out how self-employment tax is calculated and everything else you need to know.