Can I make after-tax (non-Roth) contributions to my 401(k) beyond the normal limit?

Some plans allow after-tax contributions, a third bucket distinct from pre-tax and Roth. These let you contribute beyond your personal deferral limit, up to the overall combined Section 415(c) ceiling after accounting for your deferrals and the employer match. The contributions are made with already-taxed dollars, and only the growth is taxable later, unless you convert them to Roth.

The reason people use this bucket is the mega backdoor Roth: contribute after-tax dollars, then convert them to Roth (via in-plan conversion or rollover to a Roth IRA) so future growth is tax-free. Without a prompt conversion, after-tax money grows tax-deferred but is less efficient than Roth. Not all plans offer after-tax contributions or in-service conversions, so confirm both features exist before planning around this strategy.