RSU Withholding Calculator: What Federal Rate Should You Choose at Each Vest?

Your RSUs are not taxed at 22%. They are often withheld at 22%. Those are not the same thing, and the gap between them is why so many tech workers get a surprise tax bill in April. This guide explains how to pick the right federal withholding rate for your next vest, using the 2026 brackets, the 2026 Social Security wage base of $184,500, and the IRS safe-harbor rules. An interactive calculator below turns your YTD numbers into a per-vest recommendation.

What actually happens at vest

When an RSU vests, the fair market value of the shares is treated as ordinary W-2 wages on the vest date. It stacks on top of your salary, counts for federal income tax, state income tax, Social Security, Medicare, and Additional Medicare. Your employer typically sells a portion of the shares to cover withholding and delivers the rest to your brokerage account.¹

Federal withholding on an RSU vest is almost always done under the IRS supplemental wages rule. When your employer separately identifies supplemental wages like RSU income, they are allowed to withhold a flat 22% for supplemental wages up to $1 million in a year, and 37% for any portion above $1 million. That 22% is the statutory optional flat rate; no other percentage is allowed under that method.²

What brokers usually show on your equity portal is a choice of rates (typically 22%, 24%, 32%, 35%, or 37%) that your employer has agreed to support. Fidelity notes explicitly that the method and rate choices available depend on the employer and grant.³ Your plan may offer a menu, a single 22% default, or a binary 22%/37% toggle. Check before the vest.

Why 22% is often too low for high-income tech workers

The 22% supplemental rate was built for average employee bonuses. It is not calibrated to someone whose total W-2 income puts them in the 24%, 32%, 35%, or 37% bracket. Here are the 2026 bracket thresholds:⁴

• Single filers: the 24% bracket starts at $105,700 of taxable income; 32% at $201,775; 35% at $256,225; 37% at $640,600.
• Married filing jointly: 24% at $211,400; 32% at $403,550; 35% at $512,450; 37% at $768,700.

Consider a single tech worker with a $200,000 base salary and $100,000 of RSUs vesting in the year. Total W-2 income of $300,000, after the 2026 standard deduction of $16,100, puts taxable income at $283,900. That lands them in the 35% bracket. Every dollar of that RSU vest is being taxed at a 32%-35% federal marginal rate, but only 22% is being withheld at vest. On $100,000 of RSU income, the federal withholding shortfall is roughly $10,000-$13,000 before you even think about state tax or Additional Medicare.

For married couples with two tech earners, the gap is even more common. Combined income of $350,000 to $500,000 is routine in Silicon Valley, Seattle, and the NYC tech corridor, and that combined income is almost always in the 32% or 35% federal bracket. If both employers are withholding RSUs at 22%, both are under-withholding.

Why 35% is sometimes too high

The reverse case exists too. If your plan offers 22%, 24%, 32%, and 35%, picking 35% by default is a free loan to the U.S. Treasury that you get back at refund time with zero interest. Some cases where 35% is the wrong choice:

• Small vest, modest total income. A $20,000 RSU vest on a $140,000 base salary keeps a single filer in the 24% bracket. 22% is close to right; 35% over-withholds by roughly $2,600.
• Heavy tax-advantaged contributions. Maxing 401(k), HSA, and dependent-care FSA can shave $30,000+ off taxable income, pushing you down a bracket.
• Large itemized deductions or charitable giving. A donor-advised fund contribution in the same year reduces your effective marginal rate on the vest.
• Early-year vests. A vest in Q1 sees a lot of remaining paychecks, which gives you time to adjust W-4 withholding later in the year if needed.

Tax liability versus tax withholding

This is the distinction most online RSU content glosses over. Your tax liability is the number that appears on Form 1040 line 24: what you actually owe for the year based on your bracket. Your tax withholding is what your employer (and your broker, on RSU vests) already sent to the IRS through the year. The refund or balance due at filing is just the difference.

That means your vest election is not setting your tax rate. It is setting how much you pre-pay against a liability that is determined entirely by your full-year income and filing status. The right question is not "what bracket am I in?" but "is what I'm pre-paying enough?" That question has two answers depending on what you optimize for: avoiding an April balance due (and possibly an underpayment penalty), or not over-paying.

Social Security, Medicare, and Additional Medicare by vest date

Federal income tax is the biggest piece, but payroll taxes change the answer at the margin, and they change by vest date. For 2026:

• Social Security tax is 6.2% up to the wage base of $184,500 in 2026, then stops.⁵ A vest in January on a $200,000 salary burns through your SS cap fast; by summer, new vests are SS-tax-free. Maximum employee SS tax for 2026 is $11,439.
• Medicare tax is 1.45% on all wages, no cap.
• Additional Medicare tax is 0.9% above $200,000 for single filers and $250,000 for married filing jointly, measured at the tax-return level. Employers are required to withhold the extra 0.9% once wages from that employer exceed $200,000 in the year, regardless of filing status.⁶

That last bullet creates a mismatch for many households. A married joint filer earning $150,000 from one employer plus $150,000 from a spouse has $300,000 combined wages but zero employer-withheld Additional Medicare, because neither employer individually crosses $200,000. At filing time, that household owes the 0.9% on $50,000 of combined wages above the MFJ $250,000 threshold, or $450, that was never withheld. A higher RSU withholding election can absorb this.

Portal election or extra W-4 withholding?

There are two knobs you can turn to close a withholding gap:

1. Change the federal rate on your next RSU vest in the broker portal. This only helps on future vests, not past ones.
2. Add extra federal withholding per paycheck on Form W-4 line 4c. The IRS explicitly points underwithheld taxpayers to this option and notes it can reduce or eliminate the need for estimated quarterly payments.⁷

The practical rule: if your plan offers rates close to your real marginal rate (22% and your marginal is 24%; 32% and your marginal is 32%), change the vest election. If the jumps are too coarse (plan offers 22% or 35% and your marginal is 32%), use W-4 line 4c instead. Line 4c lets you fine-tune by dollar amount per paycheck rather than by arbitrary 10-percentage-point steps.

The calculator

Enter your filing status, YTD wages and withholding, your next vest, and the rates your plan offers. The calculator projects your full-year federal tax, models Social Security and Medicare per vest date, checks your IRS safe-harbor status, and recommends an election based on the goal you pick (hit safe harbor, minimize April balance, or build a refund buffer). If even your highest available rate falls short, the W-4 line 4c alternative shows the per-paycheck dollar amount that closes the gap.

Safe harbor: why it matters even if you'll owe

The IRS charges an underpayment penalty when you have too little withheld during the year. You avoid that penalty if your total withholding meets the safe harbor, which is the smaller of:⁷

• 90% of your current-year total tax, or
• 100% of your prior-year total tax (110% if your prior-year AGI was above $150,000; $75,000 if married filing separately).

The prior-year rule is the useful one for tech workers. If your 2025 total federal tax was $45,000 and your AGI was above $150,000, you can write a check for any 2026 balance due as long as you withheld at least $49,500 (110% of $45,000) during 2026. That is true even if your 2026 tax comes in at $80,000. You will still owe the $30,500 at filing, but no underpayment penalty applies, which is a legitimately defensible cash-flow strategy for people whose RSU income spikes unpredictably.

When NOT to change your election

This section is the one that gets cut from most RSU content, which is why most RSU content reads like a financial-sales funnel. There are real cases where changing your election does not make sense:

• Your cash flow is comfortable and you prefer simplicity. If you are saving aggressively, have a large emergency fund, and routinely write a quarterly estimated payment, leaving the vest at 22% and paying the balance at filing is fine. The underpayment penalty is currently around 8% annualized on the underpaid amount, not crippling.
• You are near the end of the year. A vest on December 15 has almost no remaining paychecks to rebalance. If your plan does not offer a rate near your marginal, it is cleaner to make an estimated tax payment by January 15 than to rewrite your W-4 for two paychecks.
• You plan to harvest losses that will offset the vest. If your broker account has $40,000 of unrealized losses and you will sell the losers in December to offset RSU-related gains, your effective income for the year may drop enough that 22% is close to the right rate.
• Your total supplemental wages will exceed $1 million. At that point, every dollar above $1M is withheld at the mandatory 37% flat rate regardless of your election. Adjusting the rate on earlier vests may not be necessary if the above-$1M portion over-withholds enough to cover the gap.²

The counterpoint to this whole guide: the goal is not to land exactly on $0 balance due. It is to avoid the April surprise, stay inside the safe harbor, and not lend the IRS more cash than you need to. For most tech workers, an election in the 24%-32% range and a small W-4 top-up does that, and further optimization is noise.

Frequently asked questions

Is 22% always my RSU withholding rate?

No. 22% is the IRS-permitted flat rate for supplemental wages under $1 million per year, and it is the most common default. But employers can instead use the aggregate method (combining the vest with your regular paycheck and withholding as if it were all one wage), and many plans let you elect 24%, 32%, 35%, or 37% on the vest. The available menu is set by your employer and plan, not by the IRS.

What if my plan only offers 22%?

Use W-4 line 4c. Take the annual gap between your projected tax and your projected withholding, divide by your remaining paychecks, and enter that dollar amount on line 4c. The calculator above computes this number for you. A W-4 update usually takes effect in one to two pay cycles.

How do I actually change my W-4 to close the gap?

Log into your employer's payroll portal (Workday, ADP, Paylocity, etc.), find the tax withholding section, and submit a new Form W-4. Line 4c is titled "Extra withholding." Enter a dollar amount per paycheck. You can update it again mid-year if your plan changes.

Do I still need estimated quarterly payments?

Only if you cannot close the gap through employer withholding. The IRS explicitly prefers W-4 adjustments over estimated payments because withholding is considered paid evenly across the year, regardless of when it actually happens. An estimated payment in Q4 does not fix a Q1 underpayment the same way a higher W-4 does.⁷

What is the IRS safe harbor rule exactly?

You avoid the underpayment penalty if your total federal withholding plus timely estimated payments meets the smaller of 90% of your current year's tax, or 100% of last year's tax (110% if last year's AGI was over $150,000). This is a floor, not a ceiling. You can owe a large balance at filing and still be safe-harbor-compliant.

Does any of this apply to non-U.S. employees?

No. This guide is U.S. federal tax only. RSU taxation varies by country and often by employment-status category within a country (resident, non-resident, treaty-exempt, etc.). If you vested while working outside the U.S., consult a cross-border tax professional.

Related guides

RSU withholding sits inside a larger equity-compensation strategy:

• Sell Your RSUs at Vest is the portfolio-decision companion to this withholding guide: the cash-bonus test plus a side-by-side of sell-all, sell-to-cover, and deposit-cash. Withholding is the tax mechanic; that post resolves what to do with the shares.
• The RSU Bridge Strategy covers how to run household cash flow when salary is deliberately routed into tax-advantaged accounts and RSU vest proceeds become the primary liquidity source, with a paycheck-bridge calculator.
• RSU Tax Strategy covers the sell-vs-hold decision after the vest, bracket impact, and tax-loss harvesting when diversifying.
• Concentration Risk shows how to measure single-stock exposure with HHI and build a multi-year diversification plan.
• Human Capital Risk for Tech Workers explains why your employer is already your largest undiversified asset before you buy a single share of its stock.
• Tax-Loss Harvesting is the most common way to offset the capital-gains side of the RSU picture once you start selling.
• Roth vs. Traditional 401(k) interacts with your marginal rate and therefore with your optimal RSU withholding. High-income RSU recipients usually want to push more of their 401(k) contribution toward traditional, not Roth.

Sources

1. IRS. Topic no. 427, Stock options (including RSU guidance).
2. IRS. Publication 15 (Circular E), Employer's Tax Guide. Supplemental wages, 22% flat rate, $1M threshold, 37% above.
3. Fidelity Stock Plan Services. Managing your restricted stock tax withholding: a quick-start guide. Notes that method and rate options vary by employer and grant.
4. IRS. Tax inflation adjustments for tax year 2026 (Rev. Proc. 2025-32). Bracket thresholds and standard deduction.
5. Social Security Administration. Contribution and Benefit Base. 2026 wage base is $184,500.
6. IRS. Topic no. 560, Additional Medicare Tax. 0.9% rate, filing-status thresholds, and the $200,000 employer withholding trigger.
7. IRS. Underpayment of estimated tax by individuals penalty. Safe-harbor rules and W-4 guidance.