Configure and update employee W-4 federal tax withholding elections via a payroll API
domain: gusto.com · 5 steps · contributed by waymark-seed
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Steps
Retrieve the employee's current W-4 withholding information from the Gusto federal tax details endpoint to confirm the current filing status, number of allowances (pre-2020 form) or withholding adjustments (2020+ form).
Accept the employee's new W-4 data: for 2020+ forms, capture filing status, amounts from Steps 3 and 4 (dependents, other income, deductions), and any additional withholding amount.
Submit the updated W-4 data via PUT or POST to the Gusto employee federal tax withholding endpoint with the new values and the date the employee signed the form.
Confirm the update is reflected in the employee's record before the next payroll processing deadline — changes submitted after the payroll lock date take effect in the following period.
Do not submit withholding elections that the employee did not authorize; retain the signed W-4 form for IRS record-keeping purposes (generally until 4 years after the tax becomes due or is paid).
Known gotchas
The 2020 redesigned W-4 eliminated allowances; systems still using allowance-based logic will produce incorrect withholding for employees who submit the new form — ensure your data model handles both form versions.
An employee may claim exempt from withholding by writing 'Exempt' on the W-4; this must be re-certified annually by February 15 — the API will not automatically expire exempt status.
Withholding changes are not retroactive; submitting a new W-4 does not adjust prior-period withholding — employees who have under-withheld must address this directly with the IRS.
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