Any money that is withdrawn from the 401(k) is subject to regular income tax, because it was not taxed when it was deposited. Your regular income tax rate is probably between 15 and 28%, plus something for state taxes.
Then, early withdrawls are subject to an additional 10% penalty, unless the account holder qualifies for an exception. Unfortunately, paying past debts due to under employment is not an exception that allows you to escape the 10% penalty.
The plan trustee is required to withhold some money from the withdrawl at the time you make it. The actual total tax and penalty dur is not calculated until you prepare your tax return. Then, you get credit for the amount that was withheld previously, which goes into the computation of whether you owe more or get a refund.
