Drive up the 5 through Sorrento Valley on a weekday morning and you get a pretty good picture of how this region builds wealth. Wireless and semiconductor campuses, biotech labs in Torrey Pines, defense and aerospace contractors scattered from Kearny Mesa to Point Loma. A large share of the people living in Carmel Valley, Del Mar, La Jolla, Scripps Ranch and Rancho Santa Fe are paid partly in restricted stock units that vest on a schedule, often quarterly, sometimes tied to specific dates set years earlier when the grant was issued.
Most people know their company withholds tax when shares vest. What fewer people realize is that withholding and your actual tax bill are two different systems, and the gap between them shows up as a penalty, not just a balance due.
Here is the mechanism, because it matters more than the headline number.
When RSUs vest, your employer is generally required to withhold at a flat statutory rate on the value of the shares, 22% on amounts up to $1 million of supplemental wages in a year, 37% above that threshold. That flat rate has nothing to do with your actual marginal tax bracket. If your household income already puts you well into the 32% or 35% federal bracket before any RSU income is added, a 22% withhold on a large vest is quietly short, sometimes by a lot. The IRS explains the mechanics of supplemental wage withholding in Publication 15, if you want the source document rather than a summary.
That gap alone would just mean writing a bigger check at filing. The part that catches people off guard is what happens when the shortfall triggers the underpayment penalty rules, which run on a quarter-by-quarter basis rather than settling everything at year-end. The IRS expects most taxpayers to pay in roughly as they earn, through withholding or estimated payments, not to true everything up in April. If a large vest lands in, say, September and pushes that quarter's effective tax liability well above what was withheld, you can owe a penalty for that specific quarter even if your total withholding for the year eventually looks reasonable once averaged out. The rules and the safe harbor thresholds, generally 90% of the current year's tax or 110% of last year's tax for higher earners, are laid out in the instructions for Form 2210.
Multiple vest dates in a single year compound this. Say you have quarterly vests in February, May, August and November. Each one adds a chunk of supplemental income withheld at 22%, and each one lands in a different quarter for penalty-calculation purposes. If your income is lumpy that way, the IRS's default assumption, that you owed roughly a quarter of your annual tax in each quarter, may not match reality at all. There is a way to correct for this, the annualized income installment method on Schedule AI of Form 2210, which lets you show the IRS your income actually arrived unevenly and calculate the penalty accordingly rather than accepting the default even-quarters assumption. Most people have never heard of it, because most tax software does not surface it unless you go looking.
One client I work with, described here as an anonymized composite, is a Qualcomm engineer with quarterly vests who was surprised two years running by a few hundred dollars of underpayment penalty on top of an already large tax bill. Not a huge sum in isolation, but frustrating precisely because it was avoidable. Once we mapped her vest calendar against her actual bracket and set up a quarterly estimated payment for the gap between 22% withholding and her real marginal rate, the penalty stopped recurring. The fix was not complicated. It just required someone to look at the calendar before the vest happened, not after.
A few things worth doing if you are in a similar position. Pull your vesting schedule for the year and estimate what your marginal rate will be once RSU income is added on top of your salary, not calculated in isolation. Compare that rate to the 22% flat withholding on each vest and estimate the shortfall. If the gap is meaningful, either adjust your W-4 withholding elsewhere in the year to cover it, or make an estimated payment for the quarter in which the vest occurred. And if your income genuinely arrives unevenly across the year because of vesting, know that the annualized method on Form 2210 exists specifically for situations like yours.
None of this changes how much tax you ultimately owe. It just keeps the IRS from charging you extra for the timing of when you owed it, which is a cost with no offsetting benefit to you at all.
This kind of calendar-and-bracket coordination is one piece of a larger picture for HENRYs building wealth around equity compensation. If you want to see how it fits with the rest of the plan, the San Diego HENRY strategy lays out how the pieces connect.
If you would like your own vest calendar checked against your withholding before your next vest lands, you can book a complimentary review and we can map out where things stand.