How much will I lose if I withdraw my 401K?

You will have to pay the 10% early withdrawal penalty (unless you are at least 55 years old or permanently and totally disabled), and also regular income tax on the withdrawal. If you live in a state or municipality that has income tax you will have to pay that as well. Your income tax rate depends on your particular situation - adjusted gross income and filing status - so it's impossible to say how much you will lose without more info. But typically you would owe perhaps 25% (or more) for federal income tax, 5% for state income tax, and 10% penalty, for a total bill of around 40% of the amount of the withdrawal.