When calculating your maximum solo 401(k) contribution, TurboTax is incapable of taking into account your elective deferrals or Roth contributions at your W-2 employer. This is a published limitation of TurboTax.
You could manually calculate and enter your maximum permissible regular elective deferral or Roth contribution, the tell TurboTax to maximize a Profit Sharing Keogh contribution to allow TurboTax to automatically calculate the employer contribution. However, in your example your elective deferrals to the solo 401(k) are limited by your net earnings from self-employment such that the total of your elective deferrals to the W-2 employer's 401(k) plan and your solo 401(k) plan will not exceed the $18,000 regular elective deferral plus Roth contribution limit, so you can simply use TurboTax's 401(k) maximize function to determine your maximum permissible elective deferral. The result is the amount that you must contribute to the solo 401(k) as elective deferral. No employer contribution to the solo 401(k) will be possible because all of your available net earnings from self-employment will have gone to make the elective deferral.
If you are under the AGI limit for being able to deduct a traditional IRA contribution, you might consider making a portion of what would otherwise be a solo 401(k) elective deferral be a traditional IRA contribution instead. There are fewer restrictions to accessing funds in a traditional IRA and, by reducing the amount contributed to the solo 401(k), you reduce the chance that your balance in the solo 401(k) will reach the $250,000 threshold where Form 5500EZ reporting is required of the solo 401(k).
