An employer must withhold income taxes from compensation paid to common-law employees (but not from compensation paid to independent contractors). The amount withheld from an employee’s wages is determined in part by the number of withholding exemptions and allowances the employee claims. Note that although the Tax Code and regulations distinguish between “withholding exemptions” and “withholding allowances,” the terms are interchangeable. The amount of reduction attributable to one withholding allowance is the same as that attributable to one withholding exemption. Form W-4 and most informal IRS publications refer to both as withholding allowances, probably to avoid confusion with the complete exemption from withholding for employees with no tax liability.

An employee may change the number of withholding exemptions and/or allowances she claims on Form W-4, Employee’s Withholding Allowance Certificate. It is generally advisable for an employee to change his or her withholding so that it matches his or her projected federal tax liability as closely as possible. If an employer overwithholds through Form W-4 instructions, then the employee has essentially provided the IRS with an interest-free loan. If, on the other hand, the employer underwithholds, the employee could be liable for a large income tax bill at the end of the year, as well as interest and potential penalties.

How allowances affect withholding

For each exemption or allowance claimed, an amount equal to one personal exemption, prorated to the payroll period, is subtracted from the total amount of wages paid. This reduced amount, rather than the total wage amount, is subject to withholding. In other words, the personal exemption amount is $4,000 for 2015, meaning the prorated exemption amount for an employee receiving a biweekly paycheck is $153.85 ($4,000 divided by 26 paychecks per year) for 2015.

In addition, if an employee’s expected income when offset by deductions and credits is low enough so that the employee will not have any income tax liability for the year, the employee may be able to claim a complete exemption from withholding.

Changing the amount withheld

Taxpayers may change the number of withholding allowances they claim based on their estimated and anticipated deductions, credits, and losses for the year. For example, an employee who anticipates claiming a large number of itemized deductions and tax credits may wish to claim additional withholding allowances if the current number of withholding exemptions he is currently claiming for the year is too low and would result in overwithholding.

Withholding allowances are claimed on Form W-4, Employee’s Withholding Allowance Certificate, with the withholding exemptions. An employer should have a Form W-4 on file for each employee. New employees generally must complete Form W-4 for their employer. Existing employees may update that Form W-4 at any time during the year, and should be encouraged to do so as early as possible in 2015 if they either owed significant taxes or received a large refund when filing their 2014 tax return.

The IRS provides an IRS Withholding Calculator at that can help individuals to determine how many withholding allowances to claim on their Forms-W-4. In the alternative, employees can use the worksheets and tables that accompany the Form W-4 to compute the appropriate number of allowances.

Employers should note that a Form W-4 remains in effect until an employee provides a new one. If an employee does update her Form W-4, the employer should not adjust withholding for pay periods before the effective date of the new form. If an employee provides the employer with a Form W-4 that replaces an existing Form W-4, the employer should begin to withhold in accordance with the new Form W-4 no later than the start of the first payroll period ending on or after the 30th day from the date on which the employer received the replacement Form W-4.