<prompt> Human: Assume married filing jointly with only one W-2. Spouse with W-2 has a 401k at work. Income too high to deduct IRA contribution or use Roth IRA. Also Backdoor Roth unavailable due to tax consequences to existing traditional IRAs.Non-W-2 spouse earns approximately 35k in self employment income. What is the best way to shield the most income from tax? I assume contributions to a normal IRA result in no deduction due to combined income being too high on married filing jointly. Could a portion of the income be shielded from taxation by establishing a SEP IRA or a Solo 401k? What is the max amount that could be shielded for instance assuming a full match? Can those be set up anytime before April 15 the same as an IRA could? Would the employer of the self employed spouse needgp to have a separate EIN and, if so, would it need to be established before December 31 of the tax year in question?  <response>: Solo 401k can have employee contributions of up to 18K more if youre over 50, plus it can also have employer contributions equivalent to 20 of SE profit 12 SE tax. The sum of those two amounts has to remain under 53K, which it definitely would with 35K in SE income. You need to elect to make contributions during the tax year, though the actual contributions can occur in the following year before tax deadline. The Solo 401k needs to have been set up by Dec 31 of the tax year. So too late to do this for year 2015.I believe Pub 560 chapter 5 has the details.A calculator can help you with this: If you enter 35000, it confirms 24505 as the maximum you could tuck into a Solo 401k.That is: SE tax 35000 0.9235 0.153 494512 SE tax 2473Profit 12 SE tax 35000 2473 3252720 of that 0.20 32527 6505 employer contributionPlus 18000 employee contributionTotal is 18000 6505 24505 