Not enough if your marginal rate is above 22%. A penalty applies only if your withholding for the year falls below the safe harbor, so check that number first.
Employers withhold on RSU vests at the flat supplemental wage rate, 22% on the first $1 million of supplemental wages in a calendar year and 37% above that. The vest itself is ordinary income taxed at your marginal rate. In the 32%, 35%, or 37% bracket, every vest is under-withheld by 10 to 15 points, and the gap shows up as a balance due in April.
A balance due is not a penalty. The underpayment penalty applies only when withholding and estimated payments for the year fall below the required amount, and for most RSU holders that is the safe harbor, 110% of last year's total tax. If regular paycheck withholding plus the 22% on vests reaches that figure, there is no penalty. You owe the difference in April and nothing more.
Form 2210 treats withholding as paid in four equal parts on the due dates no matter when it was taken, so a large vest in November does not create a late installment the way a late estimated payment would.
If the total is short, there are two fixes. Submit a new W-4 with an extra dollar amount per paycheck, which also counts as paid evenly, or make an estimated payment for the shortfall by the next quarterly deadline. The RSU tax calculator shows the gap between 22% and your actual rate on a vest, and the RSU guide covers the full year.
Related
Get your exact quarterly number
Answer a few questions and see what you owe each quarter, federal and state, under both official IRS methods, free.
Sources: IRS Publication 15, supplemental wages· IRS Publication 505