When your RSUs vest, your broker doesn’t deliver every share you earned. A chunk gets sold automatically to cover federal tax, Social Security, Medicare, and state income tax — that’s “sell-to-cover” withholding. Most employees are surprised by how many shares disappear and by how the math leaves them under-withheld at filing time. Here’s exactly how it works.
The mechanic
Sell-to-cover happens in three near-simultaneous events on the morning of vest:
- Vesting — the company delivers the full vested share count into your brokerage account. The fair market value of those shares is your ordinary-income event for the day.
- Mandatory withholding calculation — payroll computes your statutory tax on the vest income (federal supplemental + FICA + state).
- Automatic share sale — the broker sells just enough shares at the vest-day price to fund step 2’s tax bill, plus a small buffer to round up to whole shares. The remaining shares stay in your account.
Your net-share count is whatever’s left after the broker has sold enough to cover the withholding.
Why your net-share count looks low
Take a 1,000-share vest at $100 FMV — a $100,000 vest event for a single filer in California. At the typical 22% federal supplemental rate plus FICA + 9.3% CA state:
| Item | Rate | Amount |
|---|---|---|
| Federal income tax (supplemental) | 22% | $22,000 |
| Social Security (if under wage base) | 6.2% | $6,200 |
| Medicare | 1.45% | $1,450 |
| Additional Medicare (if applicable) | 0.9% | $0 |
| California income tax | 9.3% | $9,300 |
| Total withholding | 38.95% | $38,950 |
The broker sells 390 shares at $100 to cover $39,000 of withholding (rounding up). You end up with 610 shares out of the 1,000 that vested.
If you mentally penciled in 1,000 shares × $100 = $100,000 of new wealth, you actually have 610 shares × $100 = $61,000. The other $39,000 went to taxes — appropriately. But that’s only the withholding — not necessarily the actual tax you’ll owe.
The supplemental withholding gap
The 22% federal supplemental rate is a flat statutory rate (37% on amounts above $1M in a calendar year, applied to the marginal portion). It does not reflect your actual marginal bracket.
If your total ordinary income (salary + RSU vest + bonuses) puts you in the 32% / 35% / 37% federal bracket, your withholding is short by 10–15 percentage points × the vest amount. On the 1,000-share vest above, that’s an extra $10,000–$15,000 you’ll owe at filing time — and possibly an underpayment penalty.
The IRS underpayment safe harbor wants you to either pay 90% of current-year liability through the year, or 100% / 110% of prior-year liability (110% if AGI > $150K). Sell-to-cover at the supplemental rate often falls short of this on its own.
How to bridge the gap
Three options. Pick one before year-end:
- Make a Q4 estimated tax payment to cover the shortfall. Form 1040-ES or pay through IRS Direct Pay. Due January 15 of the following year.
- Increase W-4 withholding for the rest of the year on regular paychecks. Payroll will spread extra withholding across remaining checks.
- Sell additional vested shares immediately and apply the proceeds via Q4 estimated tax. Useful if you want to diversify anyway.
The W-4 Withholding Optimizer and Quarterly Estimated Tax Calculator can size the gap for your specific situation.
Sell-to-cover vs. same-day sale vs. cash withholding
Your company’s stock plan typically lets you pick one of three withholding methods:
Sell-to-cover (default)
What everyone gets if they don’t choose. Shares are sold to fund withholding; you keep the rest.
- Pros: No cash out of pocket, you stay invested in some shares
- Cons: Concentration in employer stock; supplemental-rate gap; small odd-share rounding
Same-day sale (cashless)
The entire vest is sold automatically. You receive net cash equal to (FMV × shares) − (withholding) − (transaction fees).
- Pros: Full diversification, no employer-stock concentration risk, simplest tax planning
- Cons: Removes any upside on the shares; ordinary tax already paid on the FMV regardless
- When: Use when you’d rather have cash than employer stock — usually the right call once total company-stock exposure exceeds 10–15% of net worth
Cash withholding (less common)
You wire your employer cash equal to the withholding before vest, and keep all the shares.
- Pros: Maximum share retention, useful if you’re confident the stock will appreciate
- Cons: Cash flow burden, increases concentration risk
- When: Rarely the right call unless you have very high confidence in the stock and ample diversified savings elsewhere
Most large employers default to sell-to-cover, but the choice is yours and can be changed via your equity portal between vests.
The 22% trap and the AMT-adjacent surprise
A subtle issue: the 22% supplemental rate applies up to $1 million of supplemental wages in a calendar year. Above $1M the rate jumps to 37%. If you have a $1.2M vest, the first $1M is withheld at 22% and the next $200K at 37%.
For executives with multi-million-dollar single vests, the bigger issue is that 37% withholding is exactly the top federal marginal rate — so the gap closes at the very top. Where the gap is most painful is the $200K–$1M ordinary-income range, where actual marginal is 32%–35% but withholding is stuck at 22%.
There is no AMT trap on RSUs (unlike ISOs). RSUs are ordinary income at vest by definition; they don’t generate an AMT preference item.
State-sourcing for mobile employees
If you vest while a California resident and sell after moving to Texas, the vest-day ordinary income is sourced to California — California will assess income tax on the full vest FMV regardless of where you live at sale. The post-vest appreciation is sourced to your residence at the time of sale.
Keep records of every vest date, the vest-day FMV, and your state of residence on each date. The state-tax surprise on a multi-million-dollar exit is one of the most common — and avoidable — tech-employee tax mistakes.
Common questions
“Why did my W-2 show so much more income than I received in cash?” Because the full vest FMV is your ordinary income for tax purposes, even though the broker sold a chunk to cover withholding. The W-2 box 1 number is correct; what you “received” was shares plus tax payments to the IRS on your behalf.
“Do I owe FICA on RSU vest income?” Yes — 6.2% Social Security up to the SSA wage base, 1.45% Medicare uncapped, plus 0.9% additional Medicare on wages above the filing-status threshold. Sell-to-cover handles all of this in the same share-sale.
“Can I avoid sell-to-cover by deferring the vest?” No — vesting follows the schedule in your grant agreement. The only way to defer ordinary-income recognition is a §409A-compliant deferred compensation plan, which is rare for employee RSUs.
“Is the broker’s selling price the same as the W-2 FMV?” Usually yes — most plans use the vest-day closing price or VWAP. Check your equity portal’s vest report to confirm. If the broker sells at a slightly different intra-day price, that creates a tiny short-term capital gain or loss reported on Form 1099-B, separate from the ordinary income.
Key takeaways
- Sell-to-cover automatically sells shares at vest to fund federal supplemental + FICA + state withholding
- The 22% supplemental rate often under-withholds your actual marginal bracket — bridge the gap with Q4 estimates or W-4 increases
- Same-day-sale (cashless) is the simplest path if you don’t want concentration in employer stock
- Cash withholding maximizes share retention but is rarely the right call given concentration risk
- All three methods produce the same vest-day ordinary income; they differ only in how withholding is funded
- State-sourcing matters: vest income follows residence on vest day, not residence at sale
Run your specific numbers in the RSU Tax Calculator — it models all three withholding methods and shows the actual federal + FICA + state liability, the withholding gap, and the net-share count you’ll see in your account after each vest.