How to set extra W-4 withholding to cover an RSU shortfall

Tax year 2026 · Last updated May 1, 2026

The simplest way to plug an RSU withholding shortfall is to update your W-4 so your employer takes a bit more from every regular paycheck for the rest of the year. The W-4 redesign in 2020 made this much easier than the old allowances system.

Step 1: Calculate the shortfall

Use the calculator above. The “suggested extra W-4 per paycheck” line is what to enter on line 4(c).

Step 2: Open Form W-4

Most employers let you update W-4 in their HR portal (Workday, ADP, Rippling, Justworks all support this). Or download the latest paper Form W-4 from the IRS website.

Step 3: Fill line 4(c)

Skip the income/dependents sections if they do not apply. Go to the “Other adjustments” area and put your per-paycheck add into line 4(c) labeled “Extra withholding.” Save and submit.

Step 4: Verify on your next paystub

Federal income tax withheld should jump by the amount you entered. If it does not, your update did not propagate — re-submit and ping payroll.

After year-end, REMEMBER to undo the W-4 change if it was a one-time vest. Otherwise you will over-withhold all of next year and effectively give the IRS an interest-free loan.

Why W-4 beats quarterly estimates for most people

Withholding is treated as paid evenly throughout the year by IRS default — no penalty risk for prior quarters.
No extra forms or check-writing.
Set-and-forget for the duration of the year.

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