一个人可以从Roth 401(k)借钱出来. 还钱的时候需要交interest.
那是不是可以把interest设定的特别高来变相contribute更多钱到Roth 401(k)里呢?
比如我自己开的401(k), 设定interest rate是每天100%…
我借出来50k，第二天就把钱还了，并且还了50k的interest.
账户里就等同于多了50k的gains (感谢@ryd994 提醒 这些是gains所以不到60拿不出来)
也就是说, 一年可以往账户里打18M的gains…
可以通过调整interest rate, 控制整个roth 401(k)账户的大小.
印象中可以，关键challenge在于你的雇主是不是允许借钱
那你得开出你自己能控制的401k，比如solo 401k才行。
但是，既然有solo401k了，你直接存aftertax不就完事了，整这麻烦的作甚。58000还不够？那就再开一间公司，再开一个solo 401k。58000的限制是每plan的。19500的限制是每人的
因为我上面说的方法一年能往roth里存18 million… 提升interest rate可以存任意多…
你这样存进去的可不是contribution。是gain啊。59岁之前不能取出。
会玩, 敲碗等更多大胆的想法.
所有相关的plan在一起有一个58000上限…
所以并不是自己开一堆堆公司都可以… 你要保证公司都不相关…
喔 这的确没错.
可能目标是早早创建一个足够大的基金做tax free investment, 并且并不准备60之前取出来. 
如果立法把Roth给改了, 那…
也许之后不能 after-tax to Roth conversion
以后的事以后再说嘛
每天一个作死小技巧 
interest rate不是你自己随便能设的，要和市场的rate consistent
但是我看借钱相关的规定，都是限制你不能利率太低。这个是利率太高，似乎并没有不可以。
如果是employee的401k plan是plan里设定好的rate，一般是benchmark上加个spread，这也不能自己随便改。要是solo 401k的话我查了下很多都是说要consistent with the interest rate charged by commercial lenders for a loan made under similar conditions
我查了下CFR 1.72(p)-1
§ 1.72(p)-1 Loans treated as distributions
The questions and answers in this section provide guidance under section 72(p) pertaining to loans from qualified employer plans (including government plans and tax-sheltered annuities and employer plans that were formerly qualified). The examples included in the questions and answers in this section are based on the assumption that a bona fide loan is made to a participant from a qualified defined contribution plan pursuant to an enforceable agreement (in accordance with paragraph (b) of Q&A-3 of this section), with adequate security and with an interest rate and repayment terms that are commercially reasonable.
IRS可以有两个点challenge: 是否是bona fide loan，而不是way of 401k excess contribution，以及interest rate是否是commerially reasonable.
bona fide+reasonable 估计应该可以让lz免费领20年盒饭吃了 
再分析一下强行做会有什么后果。
首先是给钱定性的问题。为什么正常的401k loan interest是roth 401k gain而不是contribution？
401k loan是401k的一个投资，所以401k loan interest是这项投资的收益，只不过这次债权人正好是roth 401k的持有人。
但如果取出的钱没有定性为401k loan，那么还回去的18M interest算什么呢？
既不是contribution，也不是employer match，也不是rollover，也不是earning，所以只能是excess deferral，跟每年超过401k 限额的效果是一样的。
根据Sec. 402 (A)(2)(C), excess deferral及其对应的earning在取出时不是qualified distribution
(C) Distributions of excess deferrals and contributions and earnings thereon
The term “qualified distribution” shall not include any distribution of any excess deferral under section 402(g)(2) or any excess contribution under section 401(k)(8), and any income on the excess deferral or contribution.
excess deferral 对应的earning处理在Sec. 402 (A)(3)
(3) Treatment of distributions of certain excess deferrals
Notwithstanding section 72, if any excess deferral under section 402(g)(2) attributable to a designated Roth contribution is not distributed on or before the 1st April 15 following the close of the taxable year in which such excess deferral is made, the amount of such excess deferral shall—
(A) not be treated as investment in the contract, and
(B) be included in gross income for the taxable year in which such excess is distributed.
这就是说这18M即使在60岁之后取，即使是从roth 401k中取，也是要算收入税的。
其对应的earning部分，因为属于non-qualified distribution，则要交收入税加10%的罚金。
能否rollover到roth ira呢？根据CFR 402(g)1, not eligible for rollover
In addition, such distributions cannot be qualified distributions described in section 402A(d)(2) and are not eligible rollover distributions within the meaning of section 402(c)(4).
因此在rollover的时候会被当成unqualified distribution处理。
牢狱之灾在做贷款的那几年不一定，因为excess deferral似乎不算tax fraud。Roth 401k excess deferral 在当年没有少缴税，但是楼主可以说他会在取出时按规定交税，实际上税法在excess deferral 发生后第二年没有纠正的情况下也没有给出其他纠正方法，似乎默认是取出那年再征一次税。可能会出问题的地方是楼主拿18M的interest 去抵税，这样可能当年就会出事。
否则只有在取出的那一年，不按上述规则报税，被irs发现会以tax avoidance起诉。
由于interest rate需要雇主配合，所以这个方案只能在solo 401k里面做。
这时还要考虑楼主作为 solo 401k plan administrator 的责任，不清楚irs会怎么处理。
原来正常401k借款是可以抵税的？！
是我搞错了，绝大部分情况下不行，无论什么用途；但是有一种条件苛刻的罕见情况，即401k中没有elective deferral，
Exceptions for 401(k) and 403(b) plan loans
The law says that interest paid on a loan secured by your account balance is nondeductible if any of the account balance used to secure the loan is attributable to elective deferrals that you made. … So it’s easy to see that this exception virtually prohibits an interest deduction for interest that you pay on loans against your 401(k) or 403(b) plan. It’s possible that the vested balance in your account is totally from non-elective employer contributions, but extremely unlikely. So generally, interest that you pay on your 401(k) or 403(b) loan is nondeductible – regardless of how the loan funds were originally used.
出处是Sec. 72(p) Loans treated as distributions,
(3) Denial of interest deductions in certain cases
(A) In general
No deduction otherwise allowable under this chapter shall be allowed under this chapter for any interest paid or accrued on any loan to which paragraph (1) does not apply by reason of paragraph (2) during the period described in subparagraph (B).(B) Period to which subparagraph (A) applies
For purposes of subparagraph (A), the period described in this subparagraph is the period—
(i) on or after the 1st day on which the individual to whom the loan is made is a key employee (as defined in section 416(i)), or
(ii) such loan is secured by amounts attributable to elective deferrals described in subparagraph (A) or (C) of section 402(g)(3).
看这个原文还不能是key employee。如果既不是key employee，又没有elective deferral，而且是otherwise deductible，那样才可以抵税。