How much tax will be taken out from your 401k if you withdraw it all?

Mandatory 20% when you withdraw. There could also be a mandatory state tax withholding as well depending on which state you live. However, that may not be all the taxes you owe. The 20% could just be a down payment to the IRS. If you are in the 25% tax bracket then you would owe the extra 5% at tax time. If you are under age 59.5 then you would owe an additional 10% early withdraw penalty.