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How to calculate tax amounts withheld on mixed pre-tax and Roth 401(k) contributions, and match?
Your 401k IRA will now have three different sub-accounts, the one holding your Traditional (pre-tax) 401k contributions, the one holding your Roth 401k contributions, and the one holding the employer match contributions (which, as has been pointed out to you, cannot be considered to be Roth 401k contributions). That is, it is not true that So my next month's check shows $500+$500 going to the regular 401k, and $82+$82 going to the Roth 401k. Your next month's paystub will show $500 going into the regular 401k, $100 going into the Roth 401k, and if employer matching contributions are listed on the paystub, it will still show $600 going into the employer match. If you have chosen to invest your 401k in mutual funds (or stocks), shares are purchased when the 401k administrator receives the money and are also segregated in the three subaccounts. If you are paid monthly, then you will know on a month-by-month basis how many shares you hold in the three  separate subaccounts, and there is no end-of-year modification of how many shares were purchased with Roth 401k contributions versus how many were purchased with  pretax contributions or with employer matching funds as you seem to think.