Should I make quarterly estimated payments?

For most employees with RSU income above the level where the 22% default rate meaningfully undershoots their marginal bracket, yes. Quarterly estimated payments on Form 1040-ES, and California Form 540-ES, let you pay the actual shortfall as it accrues instead of facing one large balance in April, and they are the most direct way to satisfy the safe harbor rules if your withholding alone will not cover the relevant threshold. The alternative is increasing W-4 additional withholding, which achieves a similar result through your paycheck instead of separate payments.

Can I just increase my W-4 withholding instead of making estimated payments?

Yes, and for some clients it is simpler. Filing a new W-4 with a specific additional dollar amount on Line 4(c) increases the tax withheld from your regular paycheck. Because withholding is treated by the IRS as paid evenly throughout the year no matter when it is actually withheld, increasing W-4 withholding partway through the year can retroactively smooth out an earlier-quarter shortfall in a way a catch-up estimated payment cannot always do as cleanly. The tradeoff is that a W-4 adjustment affects every paycheck, not just the RSU-vest quarters, so it needs to be sized correctly.

Does the withholding gap apply the same way at every company?

The 22%/37% federal supplemental rate and the 10.23% California supplemental rate apply uniformly regardless of employer. What differs is vesting cadence and grant size. Employees at companies with large infrequent vest events see a concentrated shortfall in one quarter, while employees on monthly or quarterly vesting schedules see the shortfall accrue steadily. Either way, the fix, projecting the year and truing up quarterly, is identical.

Bay Area tech employee reviewing RSU vest statement and withholding shortfall with Silicon Valley Tax advisors

RSU Withholding

RSU Withholding & Tax Planning for Tech Employees

Last updated: July 24, 2026 · 9 min read

Every time your RSUs vest, your employer withholds a portion of the shares to cover taxes and hands you the rest. Most people assume that covers the bill. It does not. The default federal supplemental rate is 22%, and most Bay Area tech employees with meaningful equity sit in the 32%, 35%, or 37% bracket. That gap compounds across every vest date and shows up as a five- or six-figure balance due on April 15, plus an underpayment penalty.

This page covers why the gap exists, how large it runs for Bay Area tech compensation, and four fixes: quarterly estimated payments, sell-to-cover adjustments, the prior-year safe harbor, and W-4 additional withholding. It also covers the separate California gap, which stacks on top for anyone vesting equity as a California resident.

Why RSU Withholding Is Always Too Low

RSU income is taxed as ordinary wages at fair market value on the vest date, added to your W-2 for that year. The IRS requires employers to withhold on it using the supplemental wage rules under IRC §3402(g), with mechanics set out in Revenue Procedure 2005-14: a flat 22% federal rate, applied regardless of your actual marginal bracket.

The flat rate exists for administrative simplicity, not accuracy. Payroll systems cannot see total household income, spouse's earnings, other vesting equity, or deductions, so they apply one number. For a senior engineer, director, or VP with RSU income stacked on a six-figure base, 22% is often 10 to 15 points below the marginal rate actually owed, because RSU income lands on top of everything already earned that year and gets taxed at your highest bracket.

A second flat rate applies once aggregate supplemental wages for the year cross $1,000,000: everything above is withheld at a mandatory 37% under IRC §3402(g)(1), the top individual bracket, made permanent by the 2025 One Big Beautiful Bill Act (OBBBA) extension of the TCJA rate structure. Anyone with a very large single-year vest (a founding cliff, a refresh grant, a post-IPO acceleration) should model both thresholds.

How Large the Gap Typically Runs

Marginal Federal Bracket	Default RSU Withholding	Withholding Shortfall (Federal Only)	Shortfall on a $300,000 Annual Vest
24%	22%	2 points	~$6,000
32%	22%	10 points	~$30,000
35%	22%	13 points	~$39,000
37%	22% (37% above $1M aggregate supplemental wages)	15 points on the sub-$1M portion	~$45,000

These figures are illustrative, not a substitute for a calculation against your actual W-4, filing status, and income. A married couple with two RSU-bearing incomes can see the gap roughly double, since each employer withholds independently with no visibility into the other spouse's income.

The Four Ways to Close the Gap

1. Quarterly Estimated Tax Payments (Form 1040-ES)

The most reliable fix: calculate the expected shortfall and pay it directly to the IRS on Form 1040-ES (and California Form 540-ES), timed to each quarter's vest activity. Project the year's vest income, subtract what withholding covers, and send the difference on the quarterly due dates (April 15, June 15, September 15, January 15). Paying as it accrues, rather than one April payment, is what avoids the underpayment penalty.

2. Sell-to-Cover Adjustments

Most equity platforms (Fidelity, Schwab, E*TRADE) let you elect a sell-to-cover percentage above the mandatory 22%, generating cash for an estimated payment, or in some cases withhold at a higher rate outright. This forces the discipline of setting cash aside from each vest instead of spending the full net share count.

3. The Prior-Year Safe Harbor (IRC §6654(d)(1)(B))

You avoid the underpayment penalty entirely if withholding plus estimated payments equal at least 100% of last year's total tax (110% if your prior-year AGI exceeded $150,000, which covers most Bay Area tech households). This is the strongest lever for income that varies year to year: a large balance can be due April 15 with zero penalty, provided the safe harbor is met. It protects against the penalty; it does not eliminate the balance, so cash flow planning for April still matters.

4. Additional W-4 Withholding

File a new W-4 with a specific extra-withholding amount on Line 4(c) to pull the shortfall out of your regular paycheck instead of separate quarterly payments. The IRS treats withholding as paid evenly through the year regardless of when it was actually withheld, so increasing it partway through the year can retroactively smooth an earlier shortfall.

The California Layer: A Second, Separate Gap

California runs its own parallel system and does not conform to the federal supplemental rate. The default California supplemental withholding rate on RSU income is 10.23%, set by the EDD. California's top rate is 13.3% (income over $1 million, with the Mental Health Services Tax surcharge), and most Bay Area tech employees sit at or near the 11.3% to 13.3% brackets. That's a second shortfall, typically 1 to 3 points, stacking on the federal gap, requiring its own California Form 540-ES payment or DE 4 adjustment. We calculate both gaps together in the same quarterly check-writing exercise.

The scale and cadence of the shortfall differs by employer, though the mechanics are identical everywhere. Apple and Netflix employees typically see large, infrequent vest events that concentrate the shortfall in a single quarter. Google and Meta employees on quarterly or monthly schedules see it accrue steadily, easy to underestimate if no one tracks the running total. Nvidia and Airbnb employees with significant share-price appreciation often see the gap widen mid-year as vest-date value outpaces what was modeled in January. The fix is the same regardless: project in January, true up each quarter.

How Silicon Valley Tax Runs an RSU Withholding Engagement

For clients with RSU income, we run this as a standing annual process, not a one-time fix:

January projection. Model the full year's vest schedule against household income and filing status to calculate the federal and California gap.
Quarterly true-up. Recheck after each vest against actual values and share-price movement, adjusting remaining estimates.
Safe harbor check. Confirm whether the 110% prior-year safe harbor already covers the shortfall before recommending additional cash payments.
Platform coordination. Set additional withholding elections where the equity platform allows it, so the fix is partly automated going forward.

This connects to our broader equity compensation tax practice (RSU, ISO, NSO, ESPP together) and our QSBS / Section 1202 planning page for founders whose equity is qualifying stock rather than RSUs. For deeper California-specific mechanics, see California RSU tax planning.

FAQ: RSU Withholding

Why is my RSU withholding not enough?

Your employer withholds federal tax on RSU vest income at a flat 22% supplemental rate under IRC §3402(g), regardless of your actual bracket. At 32%, 35%, or 37% marginal, that leaves a 10 to 15 point gap on every dollar. California adds its own gap, withholding 10.23% against a 13.3% top rate. Neither system sees your full household income picture, so the flat rate is a floor, not an estimate of what you owe.

Do I owe an underpayment penalty?

Possibly, unless you meet a safe harbor. Under IRC §6654(d)(1)(B), you avoid the penalty if withholding plus estimated payments equal at least 100% of last year's tax, or 110% if your prior-year AGI exceeded $150,000. A large balance can still be due on April 15 with zero penalty, if the safe harbor is met.

Should I make quarterly estimates?

For most RSU earners above the level where 22% meaningfully undershoots their bracket, yes. Form 1040-ES and California Form 540-ES let you pay the shortfall as it accrues rather than facing one balance in April, and satisfy the safe harbor directly. The alternative is W-4 additional withholding, achieving a similar result through your paycheck.

Can I increase W-4 withholding instead?

Yes, and it's simpler for some clients. Line 4(c) additional withholding is treated by the IRS as paid evenly across the year regardless of when withheld, so a mid-year increase can smooth an earlier shortfall. It affects every paycheck, not just vest quarters, so size it correctly.

Does the gap apply the same at every company?

The 22%/37% federal and 10.23% California rates apply uniformly. What differs is cadence: large infrequent vests concentrate the shortfall in one quarter; monthly or quarterly vesting lets it accrue steadily. The fix, project and true up, is the same either way.

Get Ahead of Your Next Vest Date

The RSU withholding gap is predictable, calculable, and entirely avoidable with a January projection and a quarterly true-up. It is not predictable if you wait until your CPA is preparing your return in March to look at it for the first time. Cooper Hathaway and Alfonso Nuñez work with Bay Area tech employees across every major local employer to project the federal and California withholding gap before it becomes a surprise balance due. Book a complimentary consultation or call us at (408) 383-9870 before your next vest date.