This is the critical thing (italicized) that I was unaware of. I thought the limit was 20/25% regardless of pre or post tax. I had read the NerdWallet link previously and, unless I'm missing it, it does not explicitly discuss contribution limits or separate pre and post tax limitations beyond the $57K total. Does this different income limit apply only to the non-prototype plans? Simply put, could a Fidelity prototype solo 401(K) holder make their 20/25% pre-tax limit, then make a solo roth 401(k) contribution up to the lesser of their remaining net business income or $57K total?
A nitpick question: doesn't the $19,500 reduce the maximum profit sharing below $15K?
And finally, the MySolo401K.net link you provided, under Impact of Voluntary After-Tax Solo 401k Contributions by Day Job Contributions, it says the $57K 415c limit is on a per employer basis. Is that correct? Does that mean if one person works a W2 day job (assuming MBR is available and income is high) and has self-employed income, he/she could be putting $114K/year into his/her 401(k)s? Even more if one had, say, W2 plus two separate companies?
Thanks for explaining this more clearly.
Someguy, I'm continually surprised by a layer in this topic I was not aware of, so take everything I say with a grain of salt. I'll do my best to answer your questions.
The 415(c) limits are general plan limits as established by law. However, a specific 401k plan may effectively be more restrictive by not allowing certain contributions at all. For example the Fidelity prototype plan may not allow aftertax contributions (and I don't think it does). In that case, the limit of all contributions in such a plan is no higher than the sum of the income-eligible tax deferred contributions.
My understanding is that the employer profit sharing contribution is based on wages plus employee deferrals. Therefore, employee deferral contributions do not reduce the maximum employer profit sharing contribution.
Regarding participating in multiple 401k plans my understanding is the 415(c) limit is per plan. However, before business owners get too creative, the plans may not be affiliated entities. Also, the total employee deferral limit is combined across plans. To the extent one's employee deferral is limited, however, one can make it up with employer contributions up to the 415(c) limit per plan.
I'd like to address something you said earlier about generally maximizing tax deferred contributions in earning years. Since the 199A deduction has come into play, it adds an interesting wrinkle to this. 401(k) tax deferred contributions reduce QBI and therefore the 199A deduction, whereas aftertax contributions do not reduce QBI. If one is below phase-out threshold for QBI deduction, in effect one therefore gets a "discount" on aftertax contributions to the extent they reduce tax deferred contributions. For example, if one is in a marginal federal income tax bracket of 24%, the real cost of an aftertax contribution made in lieu of, rather than in addition to, a pretax contribution may only be around 19%. So it may actually be reasonable under such circumstances to consider prioritizing Roth contributions over tax deferred contributions within a 401k plan. It basically moves the needle a bit. In fact, this year I am going to reduce my tax deferred 401k contributions and make up the difference with additional aftertax contributions, as I believe with the added effect of the QBI deduction, I'm likely to pay less tax now than once RMD's begin. I may be wrong, of course.
Disclaimer-I'm not attempting here to provide a definitive explanation of the MBR. I am not an expert and the topic is nuanced and whether it is appropriate for any person depends on many moving parts. My intent primarily is to make the point that if one has a sole proprietor business, or if your employer allows aftertax 401k contributions with in service distributions, and your tax deferred 401k contribution limits are below the overall 415(c) limits, the MBR is highly likely to be a very advantageous tax strategy for you and you should look into it with your human resources department if you are an employee, or by researching online and contacting a professional such as Employee Fiduciary if you have your own business. Another point I would like to emphasize is that all solo 401(k) plans are not equal by any means and so one should try to evaluate at the outset what features may be useful to you, such as the ability to make MBR, rollovers of IRA's, loans, and distribution rules. This may avert the need to transfer your 401(k) to a different plan provider in the future.
Final observation-accountants may or may not have the expertise in this to advise you optimally. I have yet to speak to an accountant with a working knowledge of the practicalities of the MBR for a sole proprietership.