I have considerable Roth assets because my employer's 401k allows for the Mega-backdoor, which means I can put $30k+ per year of after-tax income into 401k (beyond the normal pre-tax contributions) perform a Roth-in-plan-conversion on the after-tax assets, and then roll it out into a Roth IRA.

You can’t personally contribute to a 401k beyond the limit, even after tax. Your employer can add extra via match or profit sharing contributions. What are you talking about?

For 2026, the 401k limits are $72,000 overall, and $24,500 for pre-tax employee contributions.

Assume an employer who matches 50% up to pre-tax employee contribution max, the result is this:

$24,500 pre-tax employee contribution $12,250 employer match

This leaves $35,250 to the $72k limit.

Roth MegaBackdoor enabled plans allow the employee to put $35,250 of _after tax_ contributions in to fill that window, and to convert them to Roth assets. They can even be rolled out into a Roth IRA while the 401k is still active.

I have no clue why you think this relatively common plan option is, somehow, impossible.

That's what the https://www.investopedia.com/terms/b/backdoor-roth-ira.asp is for, assuming you don't have any existing traditional IRA balances

You can also roll your traditional IRA into an employers 401k (if the plan allows this) to zero out your traditional IRA balance.

You can convert your 401k to an IRA when you leave an employer. Some employers also offer in service rollovers (I think these mostly have minimum age restrictions on them though)

That’s a traditional IRA, not Roth.

Traditional 401k balances transfer to traditional IRAs, and Roth 401k balances transfer to Roth IRAs.

Right but once your traditional IRA is fully rolled over to a 401k, you can take full advantage of backdoor Roth IRA contributions regardless of income.

But you can do a backdoor Roth IRA by maxing out your 401k and then rolling it over into a Roth IRA with recharachterization. You lose the tax benefits of the 401k but gain the tax benefits of the Roth IRA which can be a good trade depending on your tax situation.

No, this isn't how a backdoor Roth IRA works.

Backdoor Roth IRAs involve making a traditional IRA contribution and not taking the deduction at tax filing time (because you can't), but then rolling over (not recharacterizing, that's something else) the traditional IRA contribution into a Roth IRA. It's completely tax free, assuming you have a $0 traditional IRA balance once the rollover is complete. The usual way to accomplish this is to roll all traditional deductible IRA balances into a traditional 401k first.

What you're kind of thinking of, but also not quite right, is called a 'mega backdoor Roth', which involves contributing to a 401k via a non-deductible contribution (which is not part of 24.5k/yr limit), then immediately rolling it over into a Roth 401k. It has to be allowed by the plan, but some plans even offer to do the rollovers for you automatically.

The Mega Backdoor basically lets you get an extra ~40k/yr of Roth contributions, if you can afford it.