Which IRS form is used by employers to determine withholdings from an employee's paycheck?

Employers utilize the W-4 form to ascertain the appropriate amount of federal income tax to withhold from an employee’s paycheck. When an employee starts a new job, they are required to fill out this form, providing information about their tax situation, including marital status and the number of dependents. This information helps employers calculate the correct withholding based on current tax laws and the employee's specified preferences, ensuring that the right amount is deducted from their earnings.

The other forms serve different purposes: the W-2 form reports the total annual earnings and tax withholdings for employees to both the individual and the IRS; the W-9 form is used by independent contractors to provide their taxpayer identification number to businesses; and Form 1040 is used by individuals to file their annual income tax returns. Thus, the W-4 form is specifically designed for determining paycheck withholdings, making it the correct choice in this context.