Question: My wife has been hired as an independent contractor to give language lessons. She has other teaching jobs where she receives W-2s. She contributes to a 403(b) through those jobs, but is below the $18,000 elective deferral limit. She would like to contribute as much as possible of her 1099-MISC income to a Solo 401(k). Contributing 100% of this income would still keep her below the $18,000 limit, and I'm aware that the $18,000 applies across any combination of 401(k)-like accounts.

How can we correctly calculate the maximum amount she can contribute? By this, I mean how do we calculate the amount she needs to exclude from her earnings for taxes or other purposes so that she can contribute the maximum amount of her earnings?

Answer: 
[The maximum you can contribute to both the 403(b) and 401(k) is $18,000](https://www.irs.gov/retirement-plans/how-much-salary-can-you-defer-if-you-re-eligible-for-more-than-one-retirement-plan). Take the amount you already contributed and subtract it from $18,000. That's how much you have left to contribute before maxing out.
Read this. <https://www.irs.gov/retirement-plans/one-participant-401k-plans> 

The example makes it very clear.