my question is more of a real-world scenario question.

how is federal income tax calculated?

i found things online that say its based on annual income, but what about hourly jobs, where the amount of hours a week can vary.

i work for a comoany that constantly takes a flat 10% out every pay period for federal income tax, meaning they assume im in the lower bracket, but all the calculators i found online adjust the FIT based on hours worked and salery as if it applies to some formula.

so then is my company going based off of what i filed as last year?