When you file your return for the year, based on your taxable income any excess amounts withheld previously with be refunded at that time. (highinterestsavings.ca)
To make sure you are having the correct amount withheld from your pay, the IRS has developed a withholding calculator designed to help individuals avoid having too little tax withheld. (bcmpayroll.com)
If you are under age 59 1/2, you will have to pay the 10 % IRS penalty tax on early distributions for any distribution from the Plan (including amounts withheld for income tax) that you do not roll over, unless one of the exceptions listed below applies: (franklintempleton.com)