The IRS instructions for completing Form W-4 Employee’s Withholding Certificate to calculate your allowances does not account for all possible situations. See the IRS information below relating to withholding.
Per IRS Publication 505 Tax Withholding and Estimated Tax, starting on page 5:
In most situations, the tax withheld from your pay will be close to the tax you figure on your return if you follow these two rules.
But because the worksheets and withholding methods don’t account for all possible situations, you may not be getting the right amount withheld. This is most likely to happen in the following situations.
If any of these situations apply to you, you can use the Tax Withholding Estimator at IRS.gov/W4App to see if you need to change your withholding.
If you have self-employment income or owe self-employment tax, you should use the worksheets in this publication to determine if you should pay estimated tax.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.