These tables were derived from Publication 15, Circular E, Employer's Tax Guide
from the IRS. If you know the total amount you want to withhold, you can subtract the amount which will be withheld
based on marital status and number of exemptions to give an additional withholding amount. If these tables change in subsequent years
you will be at least partially protected if you don't change your withholding preference. The marginal tax bracket and calculated
withholding are displayed for zero thru seven exemptions for both single and married taxpayers.
All calculations are done at the annual level (allowable per above publication) and may not be exact due to roundoff error. For example
on page 35, a single employee with two exemptions making $600.00 per week gives $53.53 using weekly tables, but $53.51 here.
The following example, also from page 35, is from that publication and reproduces its final answer:
Annual income tax withholding. Figure the income tax
to withhold on annual wages under the Percentage Method for an annual payroll period. Then prorate the tax back to the payroll period.
Example. A married person claims four withholding allowances. She is paid $1,000 a week. Multiply the weekly wages by 52 weeks to
figure the annual wage of $52,000. Subtract $15,200 (the value of four withholding allowances for 2012) for a balance of $36,000.
Using the table for the annual payroll period on page 37, $3,435 is withheld. Divide the annual tax by 52. The weekly income tax to
withhold is $66.06.
If you are experimenting with your annual estimated income from all sources, you may enter it in the "Salary or Pension" field,
but then check "No Multiply".