What is a mega backdoor Roth through my 401(k) and who can actually do it?

The mega backdoor Roth lets high earners get far more into Roth than the normal limits allow. It requires a plan that offers both after-tax (non-Roth) contributions and either in-plan Roth conversions or in-service withdrawals to a Roth IRA. You max your regular deferrals, receive the match, then contribute after-tax dollars up to the overall Section 415(c) combined limit, and immediately convert those after-tax dollars to Roth.

Because you convert right away, little or no growth is taxed on conversion. This can add tens of thousands in tax-free Roth savings each year. The catch is that both features must exist in your specific plan, and many plans lack them. Ask your plan administrator whether after-tax contributions and prompt conversions are available before counting on this strategy.