I need to calculate the tax penalty for early disbursement of my Roth 401K

[karlaferguson]

I have a total of $27,000 in a Roth 401K, and I am changing jobs. I intend to cash it out and pay the tax penalty. (That decision has been made, so this is not the time to talk me out of it.)
The total includes a loan of $11,000, which I do not plan to pay back.
Based on what I have read, I think I will owe 10% on the total amount, taxes on the loan amount, and taxes on the portion of the total that is interest (but not the portion that is direct contribution.) Is this correct?
My main question is: How do I calculate the total tax penalty so that I know how much I need to set aside?
My second question is: What is the correct way to report this on my tax return, so that I get charged the correct amount (and not overcharged)?
Finally: Does the 10% come out immediately at the time of the cash out, or is it deferred until tax time?

Thank you in advance for all of your help!

[Hal_Al]

It's a Roth 401K, so most of it is not taxable or subject to penalty.
You will not owe a 10% penalty on the entire amount. You will only owe tax & penalty on the interest portion. The contributions portion of  a Roth may be withdrawn, at any time, tax & penalty free.

Q. How do I calculate the total tax penalty?
A. Multiply the interest portion by your marginal tax rate + 10%. e.g. if you're in the 15% tax bracket, multiply by 25%.
Q. What is the correct way to report this on my tax return?
A. Enter the 1099-R and Turbotax does the rest after you answer the follow up questions. Total distribution ends up on line 16a of form 1040; taxable amount on line 16b; penalty on for 5329 and line 58 of form 1040.
Q. Does the 10% come out immediately?
A. That's up to you or your plan's rules. But, what ever is withheld is not the real tax but only an estimate. The actual tax is calculated at tax time, as part of the tax on your total income.

[cmgibson1999]

how do you find the interest portion?

[Hal_Al]

From your own records. The plan administrator may or may not  be able to provide you that info. It will probably show in box 2a of the 1099-R that the plan will send you, in January, for the distribution.