The example I will present is a childless couple. One person works part time while the other attends school. Early in June the school person gets a full time job at a high tech production company[working only part of the year] where there are a few weeks when the production line is down. The 1st person withholds as married but withholding at the single rate [which is the same as Married filing separately - one size fits all] claiming only themself as a deduction. We calculate the withholdings based on steady hours and working every single week - the only type of worker well served by the standard IRS/state withholding tables; the actual withholdings/earnings will result in overwithholding. In this case the actual federal taxes are $1279 with withholdings of $1282[303+979) with the Oregon tax bill at $1036(315+721) with withholdings of $1036. If they had followed the suggested withholding rates that the IRS recommends they would have overwithheld by at least $400 in federal and $200 in state taxes. Only by careful calculation [that of both a math and tax expert] would they come closer using the standard withholding tables - and then they would have to change exemptions claimed to avoid underwithholding the next year. Apparently, the IRS and your state assume that it is in your best interest to allow them use of your money - interest free!