How much tax is withheld from income?
There is no specific fixed amount or percent. Two people working at the same job, making the same wage may (an almost always do) have much different amounts required to be withheld. THE AMOUNT WITHHELD IS DETERMINED BY YOU...NOT YOUR EMPLOYER, THE IRS OR ANYONE ELSE.
It depends on many, many things...not the least of which is what you consider tax. Many people group all their withholdings as a type of tax, but many may not be. Workers Comp, Unemployment, even FICA are all really more an insurance payment than a withholding against an income tax.
The amount of tax withheld depends obviously o which state (or even city) your in, the amount of income your projected on earning over the year, (which helps determine your tax bracket and the percent that may be required), as well as your filing status, number of dependents and other deductions (like interest on a mortgage) or contributions to 401K, or medical and other benefits you selected, etc., etc.
All these things can be adjusted for your circumstances by properly and completely filling out (or changing) the Form W-4 all employers ask you to. The variations are so numerous that again, it is fair to say that it would be uncommon for 2 people, working at the same job making the same salary would have the same amount withheld.
There are even a number of different legal ways for the payroll provider to calculate the amount to withhold considering all the above...but overall they make only a small difference.
Remember, anything withheld is just being done as an estimated installment payment toward whatever tax, if any, you do ultimately owe. If too much is withheld, it is refunded. (Too little, and you could pay a penalty and interest charges). Again, adjusting your W-4 is the way to correct for any of these circumstances. Just follow the instructions and examples for that form and you should have a very close amount for what is needed withheld for your situation...if for any number of reasons including those above, the situation changes... you will need to change the W-4.