The IRS instructions for 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, page 5:

Getting the Right Amount of Tax Withheld

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.

• You accurately complete all the Form W-4 worksheets that apply to you.
• You give your employer a new Form W-4 when changes occur.

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.

• You are married and both you and your spouse work.
• You have more than one job at a time.
• You have nonwage income, such as interest, dividends, alimony, or unemployment compensation.
• You will owe additional amounts with your return.
• Your withholding is based on obsolete Form W-4 information for a substantial part of the year.
• You work only part of the year.
• You change the amount of your withholding during the year.
• You are subject to Additional Medicare Tax or NIIT. If you anticipate liability for Additional Medicare Tax or NIIT, you may request that your employer withhold an additional amount of income tax withholding on Form W-4.

If any of these situations apply to you, you can use the IRS Tax Withholding Estimator 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 webpage or document at the time it is accessed.