RSU taxation catches people at the exact moment they feel richest. Your shares vest, a chunk gets sold automatically to cover taxes, and you assume the IRS is handled. It usually isn't. The default withholding rate on vested RSUs is a flat 22% federal rate, and for anyone earning above roughly $170,000 a year, that rate is too low. The gap between what got withheld and what you actually owe shows up as a surprise bill every April, sometimes with an underpayment penalty attached.
Restricted stock units get taxed when they vest, full stop. Not when they're granted. Not when you sell. The moment a batch of shares becomes yours, the fair market value on that day counts as ordinary income, reported on your W-2 just like your salary. If 250 shares vest at $80 each, that's $20,000 of taxable compensation, added to your regular pay for the year.
This surprises people who think of stock compensation as investment income taxed at capital gains rates. It isn't, not at vesting. Capital gains only apply to the difference between the value at vesting and the value when you eventually sell. Hold the shares for a year after vesting and sell at a gain, and that additional gain gets long-term capital gains treatment. But the vesting-day value itself is ordinary income, taxed at your marginal rate, no exceptions.
Companies handle this by withholding shares or cash at vesting, treating the event like a bonus payment. That's where the trouble starts, because bonus payments have their own withholding rules, and those rules were built for smaller amounts than most RSU grants generate.
The IRS classifies RSU income as a supplemental wage, the same bucket as bonuses and commissions. Employers can withhold supplemental wages at a flat 22% rate instead of running them through your W-4 elections. Almost every payroll system defaults to this flat rate because it's simple to automate.
The problem: 22% is the marginal rate for income between about $47,000 and $100,000 for a single filer in 2024. If your combined salary and RSU income lands you in the 32%, 35%, or 37% bracket, the 22% withholding covers only part of your real liability. Someone earning $250,000 in salary who vests $100,000 in RSUs is likely in the 32% or 35% bracket for that stock income, meaning 10 to 13 percentage points go unwithheld on every vest. On $100,000 of vested value, that's $10,000 to $13,000 short, before state taxes even enter the picture.
This isn't a bug your company will proactively fix. It's how the supplemental wage rule works by design, and it assumes you'll true up the difference yourself through estimated payments or a bigger paycheck withholding elsewhere. Most people don't realize they need to, until they file.
There's a second rule that matters if your vests are large or frequent. Once your supplemental wages exceed $1 million in a calendar year, the mandatory withholding rate jumps to 37%, the top federal bracket, on the amount above that threshold. This hits senior employees at high-growth companies where a single vesting event, especially right after an IPO or a big stock price run, can push total supplemental income past seven figures.
Below $1 million, employers can choose 22% flat or use an aggregate method that factors in your W-4. Almost none use the aggregate method because it requires more calculation per employee. So the practical reality is binary: 22% withholding until you cross $1 million in supplemental wages for the year, then 37% on the overage. Neither number is calibrated to your actual bracket, which is the whole point of understanding RSU taxation before your first big vest, not after.
If you've worked in more than one state during the vesting period, RSU taxation gets a second layer of complexity. Many states tax the portion of RSU income earned while you were a resident or working within their borders, calculated by the ratio of days worked in-state during the vesting period to total days in that period.
Someone who vested shares granted four years ago, but moved from California to Texas eighteen months into that vesting schedule, may owe California tax on roughly 37.5% of that vest's value, even though they no longer live there and Texas has no income tax at all. California is especially aggressive about pursuing this allocation, and it applies even after you've left the state permanently.
Employers' payroll withholding rarely gets this allocation right. Most systems either withhold based on current state of residence, or apply the state where the company is headquartered, neither of which matches the legally correct multi-state formula. If you've changed states during a vesting period, assume your withholding is wrong and check the math yourself, or with a preparer who handles equity compensation regularly.
Once you understand the gap, closing it is mechanical. Four approaches work, and most people need at least one.
The underpayment penalty itself is small, usually a few percentage points annualized on the shortfall. It's not the penalty that hurts. It's writing an unplanned five-figure check in April because nobody told you the withholding was designed to be wrong.
There's a behavioral trap layered on top of the tax trap. People see the number on their equity portal at vest and treat it as spendable income, forgetting that a third or more of it belongs to the government before they've spent a dollar. Then the stock drops before they sell, and they're paying tax on value that no longer exists in their portfolio.
The fix is procedural, not clever. Separate the tax-reserve decision from the investment decision. Decide what percentage of every vest you're setting aside for taxes, based on your actual marginal rate, not the 22% withheld. Do that math before you look at the account balance and start planning what to do with it. RSU taxation isn't complicated once you accept the two basic facts: vesting is the taxable event, and the default withholding almost never matches what you owe.
No, but it can look that way. Vesting-day value is taxed once as ordinary income. If the stock price rises after vesting and you later sell at a profit, only that additional gain gets taxed again, as a capital gain. If you sell immediately at vest, there's usually little to no additional gain to tax.
The IRS classifies RSU income as a supplemental wage, and employers are allowed to withhold supplemental wages at a flat 22% federal rate instead of using your W-4 elections. That rate matches the marginal bracket for moderate incomes, so anyone in a higher bracket gets underwithheld automatically.
Once your supplemental wages exceed $1 million in a calendar year, the IRS requires mandatory withholding of 37% on the amount above that threshold. This applies regardless of your actual marginal bracket and often overwithholds slightly for that portion.
Often yes. Many states tax the portion of RSU income earned while you were a resident, based on the ratio of days worked in-state during the vesting period. This can create a tax bill in a state you no longer live in, particularly with California.
Calculate your actual marginal federal and state bracket for the year, then add that percentage to whatever your employer already withholds, since the default 22% federal rate is usually short. High earners often need to set aside 35% to 45% of vesting-day value in total to cover federal, state, and any additional Medicare surtax.