RSU Withholding: How to Fix the W-4 Gap Before April

Your employer withholds 22% in federal income tax when your RSUs vest. For most tech employees in California or New York earning $200K+, the real combined marginal rate is 42–51%. The difference — often $30,000–$100,000 per year in taxes that weren't withheld — hits all at once in April, with possible underpayment penalties on top. This guide explains why the gap exists, how to calculate your specific shortfall, and how to fix it via your W-4 before the problem compounds.

Why 22% is almost always wrong for you

The IRS requires employers to withhold federal income tax on supplemental wages — including RSU vests — at a flat 22% rate. This is called the supplemental withholding rate.¹ The rule is straightforward for the employer: apply 22%, remit, move on.

The problem is that 22% is the 22% bracket threshold — a marginal rate that only applies to income roughly between $48,000 and $103,000 for a single filer in 2026. If you're a software engineer earning $250,000 in salary with $200,000 of annual RSU vesting, your marginal federal rate on the RSU income is 37%, not 22%. Your employer doesn't know or care — it's required by law to use 22%.²

Here's what the full withholding picture looks like at vest, and what you actually owe:

For a California tech employee with $400,000 in W-2 income plus RSU vesting, the gap is typically 10–18 percentage points. On a $300,000 RSU vest year, that's $30,000–$54,000 in taxes not withheld — all due by April 15, plus potential underpayment penalties if you didn't pay quarterly estimated tax through the year.

Three ways to close the gap

You have three options, and they're not mutually exclusive. Most people with significant RSU income use a combination of all three.

Option 1: Additional withholding via W-4 (simplest, lowest penalty risk)

IRS Form W-4 Step 4(c) allows you to request a specific dollar amount of additional withholding per paycheck — on top of what the employer calculates from your regular salary. The amount comes out with every paycheck, so by year-end your total withholding is higher without you writing quarterly checks.

The advantage: the IRS treats employer withholding as evenly distributed throughout the year even if it actually came out of late-year paychecks. This means you can increase your W-4 withholding in October after a large Q3 vest and still satisfy the safe harbor as if it had been withheld all year — eliminating underpayment penalties retroactively for earlier quarters.³ Quarterly estimated tax does not have this property; it must be paid on time each quarter to avoid penalties.

How to calculate your W-4 additional withholding amount:

1. Estimate your total RSU vest income for the year (share count × current stock price × number of vests remaining — note this will fluctuate with the stock price).
2. Estimate your total tax liability for the year using your tax software or the IRS Tax Withholding Estimator. Include salary, RSU income, any investment income, and all deductions.
3. Subtract what will be withheld through normal payroll (salary withholding + RSU supplemental withholding).
4. Divide the shortfall by the number of remaining paychecks in the year. That is your Step 4(c) additional withholding amount.

Option 2: Quarterly estimated tax payments (more flexible, timing-sensitive)

If your RSU income is lumpy — a large cliff vest in Q1, smaller quarterly vests thereafter — you may prefer quarterly estimated tax payments over a blanket W-4 increase. Estimated tax is paid to the IRS via EFTPS (Electronic Federal Tax Payment System) and to your state's revenue agency separately.

The 2026 federal estimated tax due dates are:

• Q1 (Jan 1–Mar 31): April 15, 2026
• Q2 (Apr 1–May 31): June 16, 2026
• Q3 (Jun 1–Aug 31): September 15, 2026
• Q4 (Sep 1–Dec 31): January 15, 2027

Important: each quarterly payment must cover the taxes owed for that quarter's income to avoid underpayment penalties on that quarter. Unlike employer withholding, late estimated payments are penalized quarter-by-quarter, not just at year-end.

See RSU Estimated Tax: Safe Harbor Rules and Quarterly Deadlines for the complete safe harbor calculation and EFTPS mechanics.

Option 3: Prior-year safe harbor (simplest penalty avoidance, but not ideal long-term)

The IRS waives underpayment penalties if you pay at least 100% of last year's tax liability through withholding and/or quarterly payments (110% if your prior-year AGI exceeded $150,000).⁴ If your RSU income is spiking this year but last year's liability was lower, safe harbor limits penalties — but you still owe the full balance plus interest when you file.

Safe harbor is a penalty shield, not a planning strategy. It does not prevent a large April balance due; it only prevents penalties on top of that balance.

The aggregate method: can you ask your employer to change how they withhold?

The IRS allows employers to use either the flat 22% supplemental rate or the aggregate method, which calculates withholding on supplemental wages as if they were added to your most recent regular paycheck and the combined amount were your regular pay.⁵ The aggregate method usually produces a higher withholding amount — closer to your actual marginal rate.

The choice of method belongs to the employer, not the employee. Most large tech companies use the flat supplemental rate because it's simpler to administer at scale. Some companies (particularly those with sophisticated payroll operations) offer employees the option to opt into the aggregate method, or automatically use it for employees who request it. It is worth asking your payroll or HR team whether the aggregate method is available to you — a single email can potentially fix the withholding gap at the source.

State withholding: California, New York, and Washington

California

California's Employment Development Department (EDD) allows employers to use a flat supplemental withholding rate of 10.23% (2026) on supplemental wages, or the aggregate method.⁶ For California filers at the top income levels, the actual marginal rate is 12.3% (above ~$677,000, it reaches 13.3%). Like the federal gap, the CA supplemental rate of 10.23% systematically under-withholds for high earners by 2–3 percentage points on top of the federal gap.

California requires estimated tax payments in the same quarters as the IRS (though the exact payment amounts differ because CA uses its own income tables). If you owe more than $500 in CA tax, you're expected to make quarterly payments. CA FTB Form 5805 is how CA assesses underpayment penalties — the same structure as federal Form 2210.

New York

New York uses the aggregate method by default for supplemental wage withholding — your employer must add the supplemental wage to your annualized regular wages, calculate withholding on the combined amount, and back out the regular withholding. In practice this means NY withholding on RSU vests is typically closer to your actual marginal NY rate. For employees in New York City, the NYC local income tax (up to 3.876% in 2026) is layered on top. NY estimated tax is due on the same April/June/September/January schedule as federal.

Washington

Washington has no state income tax on ordinary income. However, Washington's capital gains tax (7% on gains above $278,000; 9.9% above $1M, effective 2024) applies when you sell RSU shares after vesting — not at vest, which is an ordinary income event.⁷ Washington does not have estimated tax withholding for this; you pay via an annual return. If you're a Washington employee with significant RSUs, plan for a Q4 WA capital gains liability if you've sold shares during the year above the threshold.

Worked example: how much to add to your W-4

Assume you're a software engineer in California:

• Salary: $250,000
• RSU vesting in 2026: $350,000 (four quarterly vests)
• Filing status: single
• Standard deduction, no other significant income or deductions
• Social Security wage base already met by salary

Total income for 2026: $600,000

Federal tax calculation (approximate):

• Ordinary income tax on $600,000 (single, 2026 brackets): ~$181,000
• Additional Medicare (0.9% on $600K − $200K = $400K): ~$3,600
• Total federal: ~$184,600

California tax (approximate):

• CA income tax on $600,000 (12.3% marginal, effective rate ~10.5%): ~$63,000
• CA SDI (at 1.1%, capped at salary level): ~$2,750

Total liability: ~$250,350

Expected withholding without W-4 adjustment:

• Salary withholding (24–35% effective on $250K): ~$67,000
• RSU supplemental withholding (22% federal + 10.23% CA × $350K): ~$113,000
• Medicare on RSU vests (1.45% + 0.9%): ~$8,200
• Total expected withholding: ~$188,200

Gap: ~$62,000 not withheld

W-4 fix: If you add this adjustment after the Q1 vest (paychecks remaining: ~20 biweekly), you need approximately $3,100 per paycheck in Step 4(c) additional withholding to cover the full gap. Adjust upward if stock price rises during the year; downward if it falls.

Timing your W-4 adjustment

You can submit a new W-4 to your employer at any time; it takes effect on the next payroll run after your employer processes it (typically the following pay period). There is no limit on how many times per year you can update your W-4.

Practical timing considerations:

• After each large vest: Recalculate your year-to-date withholding position and adjust accordingly. A large Q1 vest followed by quarterly vests means your under-withholding compounds through the year unless you act early.
• After a stock price spike: If your employer's stock price rose 30% since you set your W-4 additional withholding amount, your vests are generating 30% more income than you planned for — and the gap has widened correspondingly.
• Year-end catch-up: Because employer withholding is treated as uniformly distributed throughout the year, a large W-4 adjustment in Q4 retroactively covers any earlier-quarter gaps. If you've been behind all year, a large catch-up W-4 adjustment in November can eliminate underpayment penalties for all four quarters.

W-4 vs quarterly estimated tax: which is right for you?

For most W-2-only tech employees with regular quarterly RSU vests, increasing W-4 withholding is the simpler and lower-risk approach. For employees with significant investment income, side businesses, or very large one-time vests, quarterly estimated tax (possibly combined with W-4) gives more flexibility.

Don't over-withhold either

The goal is to match your withholding to your actual liability — not to overshoot it significantly. Every dollar you over-withhold is an interest-free loan to the government. At a 5% opportunity cost, over-withholding by $50,000 costs you roughly $2,500 in foregone returns for a year. A large refund in April isn't "found money" — it's money you already earned that sat in the government's account. Model to aim for a small balance due (~$3,000–$5,000) or a small refund, not a four-figure overpayment.