IRS Sales Tax deduction Calculator

Is is a traditional 401(k) or a Roth? If it's a traditional, then the money you put in won't actually be spendable till you draw it out, and when you do it's part of taxable income, so I'd think it's reasonable to use it to figure the optional sales tax in the year when you withdraw, not the year when you make the contribution.

If it's a Roth, I'm not so sure.

If it were a Roth, wouldn't it come out of post-tax income? So it would already be in the AGI in the contribution year.

In this case, it's a regular 401K, and what you say makes sense. It shouldn't be added to the calculation in the year of the contribution because it will eventually be part of the AGI when it is finally withdrawn.

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When you get to the web page in the calculator that asks for your income range, click on the "income" link. A window will open that lists the types of income that you can use in the calculation. It lists a variety of nontaxable income with the last entry called other nontaxable income. Some of the other items not listed are nontaxable life insurance, nontaxable bequests, and any nontaxable legal settlements (e.g., personal injury). Nontaxable contributions you make are not income for this purpose. Nontaxable distributions are income for this purpose.