There's also the self employment tax to consider for contractors working for themselves in the US.
Employers pay 7.65% and you pay 7.65%. If you're on your own in addition to the income tax you have to pay you also need to pay the whole 15.3% of it. To compare salaries between a person working for themself and someone working for a company it should be taken into account since it's a big chunk of the money you're making.