An old valid form can continue
A Form W-4 from 2025 or an earlier year generally remains effective for 2026 until the employee submits a replacement. The payroll system must preserve the correct form design and computational method.
Form W-4 payroll timing
A replacement W-4 has a latest effective payroll period
For a replacement Form W-4, an employer must begin using the form no later than the first payroll period ending on or after the 30th day after receipt. A new hire's valid W-4 applies to the first wage payment; an earlier-year valid form generally remains in effect until replaced.
This is the federal payroll timing rule. Invalid forms, lock-in letters, exempt claims, electronic signature systems, state withholding certificates, and payroll cutoffs require their own procedures.
Separate new hires from replacement forms
A new employee should give the employer a signed W-4 when work starts, and the employer makes it effective with the first wage payment. If a new employee provides no completed 2026 form, Publication 15 generally instructs the employer to treat the worker as single or married filing separately with no entries in Steps 2, 3, or 4.
Replacement received May 6
Assume biweekly payroll periods end May 15, May 29, and June 12. The 30th day after May 6 is June 5. The first payroll period ending on or after that day ends June 12, so the employer must use the replacement no later than that period. The employer may implement it earlier if payroll processing allows, but should not retroactively recalculate periods before the effective date.
A W-4 changes withholding
The form does not amend wages, Social Security tax, Medicare tax, or the employee's final income-tax liability. It changes federal income tax withheld from future wage payments.
Common processing errors
Do not backdate the form, wait indefinitely for a convenient quarter, use a verbal request in place of a valid form, or apply the federal form automatically to state withholding.
Official sources checked 2026-09-03: IRS Publication 15 (2026), Form W-4 section, IRS Topic 753, and IRS Publication 15-T (2026).