If you are married and claim 2 dependents and are paid biweekly what's your federal withholding for $1257.80?

The Federal withholding is based on a tabled format. Your first respondent is close. But the Federal (only) withholding can still vary greatly. For instance, Obama changed the withholding rates during the year last year. Also, the Bush tax cuts expire after last year. So, there are a lot more variables.