The 20% withholding on your 401k distribution is just an ESTIMATE of the taxes you may owe due to this distribution. At the end of the year, you make a final accounting with the IRS and if you overpaid your taxes, you get a refund, or possibly underpaid and owe the IRS. The 401k distribution gets added to your other income and the total tax obligation is calculated.
There is also an early withdrawal penalty of 10% unless you meet specific criteria, the software should step you through the exceptions to this penalty.
In the end, the total taxes on an early 401k distribution can range from zero to 40+ percent depending upon your total income, marital status, dependents, and exceptions to the early withdrawal penalty. There is no set tax rate on this distribution.
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