Timing of Mega Backdoor Roth

I thought I remember reading that the Mega Backdoor Roth contributions had to be made early in the year (I think it was 30 days after the close of the year).  I just wanted to double-check to see if that is accurate?  We have a client asking about it, so before I confirmed that they could start it for 2025 (and not 2024) I just wanted to be sure.

Thanks in advance!

You have 30 days after the year ends for after-tax amounts to count as annual additions for the prior year.  So definitely too late for 2024 but perfectly timed to do 2025 amounts currently.

What about LLC Members in an Owner only plan?  They have combo CB/401k plans and will not know what their net LLC income will be until much later in the year.  Since the CB/401k PS contributions are backed out we have no way of knowing how much can be contributed as Voluntary after-tax to convert to Roth until the CPA calculates the net LLC income and the Actuary calculates the CB contribution.

Suggest they can make the best estimate in January for the prior year and make the deposit by 30 days after the plan year ends or convert to a corp with W-2 wages and you can tell them the number for sure if they give you all the data timely.

If they overshot what they could have done, apply what you can to the prior year, what you can't to the current year and make sure you adjust for that when you do the estimate again the next January.  Let them know at some point they may run into a problem with that and may have to do a 415 refund due to estimating compensation if you can't apply all the overage in following year.

Another route: Evaluate ways to order the retirement plans' limitation years, the employer's tax-accounting year, and the individual's tax year to discern whether that might get helpful outcomes.