I took an early withdrawal on my 401k of 10,000, when filling out box 2a on 1099 what is the taxable amount ?

[DoninGA]

You do not complete the Form 1099-R manually without receiving the Form 1099-R from the plan administrator for the 401(k) tax deferred retirement account.

If the withdrawal was in 2019 then you will not receive the 1099-R until January 2020 for entry on the 2019 tax return in 2020.

If the withdrawal was in 2018 then you should have received the 1099-R in January 2019.  If you do not have the 1099-R then contact the plan administrator or sign onto the plan website and get the 1099-R.

[dmertz]

The taxable amount is usually the entire amount, $10,000 in this case, except when some portion of the distribution is from after-tax money in the 401(k).  Most people don't have any after-tax money in their 401(k).

(I'm assuming that this is not a lump-sum distribution involving a distribution of Net Unrealized Appreciation of employer securities, otherwise you probably would have mentioned that.)