My wife and I have a fairly wide discrepancy between our incomes.
1. In theory, what happens if I check box 2c and she doesn't?
2. How much will be withheld if neither of us checks it?
3. How much will be held if both of us check it?
Box 2c is meant to be used only when both parties file a form W-4 with box 2c checked and both incomes are pretty close.
If only one of you checks box 2C, the employer w/o the checked form will withhold as if that were the only income for your filing status (under withhold). Checking box 2c divides the standard deduction and filing brackets in 1/2. See the bolded explanation of what happens if you have disparate incomes and both check box 2c. This is from the IRS directions for the W-4.
BEGIN IRS EXCERPT
If you (and your spouse) have a total of only two jobs, you may instead check the box in option (c). The box must also be checked on the Form W-4 for the other job. If the box is checked, the standard deduction and tax brackets will be cut in half for each job to calculate withholding. This option is roughly accurate for jobs with similar pay; otherwise, more tax than necessary may be withheld, and this extra amount will be larger the greater the difference in pay is between the two jobs.
END IRS EXCERPT
You can also use options 2A and 2B to improve your accuracy or check out Intuit's W-4 withholding calculator here.
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