If you are under age 59 1/2, you must pay a 10% early withdrawal penalty. The regular taxes on your 401K withdrawal will depend on how much total income you have for the year. You will be taxed at the marginal tax rate. If you're in the 15% tax bracket, the tax will be 15% plus the 10% early withdrawal penalty.
The 401K plan administrator will probably withhold a certain percentage of taxes. You will get a credit for that withholding when you file your tax return. The actual taxes may be more or less than the amount of the withholding.
