In 2023, if a plan provides this option, a participant can contribute up to $22,500 ($30,000, if age 50 or older) in pretax compensation and as much as $43,500 as an after-tax contribution to a 401(k) plan for a total of $66,000 ($73,500, including the $7,500 catch-up contribution if age 50 or older).
Then, the participant can execute a transfer—a conversion—of those monies to a Roth 401(k). When untaxed regular 401(k) assets are transferred to the Roth 401(k) account and subjected to the tax on their transfer, the subsequent earnings in the account accumulate tax-free. Distributions from Roth 401(k)s are tax-free and generally must begin when the owner reaches age 72.
Internal Revenue Service. "Retirement Topics - 401(k) and Profit-Sharing Plan Contribution Limits."