You changed jobs partway through the year, and RSUs are vesting at both the old employer and the new one in the same calendar year. Each employer’s withholding looks right on its own pay stub. Combined, they can leave a balance due when you file.
Each employer withholds on its own RSUs as if that were your only income for the year. It isn’t. Combine both W-2s, and part of your income can land in a higher bracket than either employer accounted for. A big enough gap can also trigger an underpayment penalty on top of the tax.
The short answer: both RSU grants are likely under-withheld
Most likely, yes, at both jobs. Each vest is taxed as ordinary income at the shares’ fair market value on the vest date and added to that employer’s W-2. Employers commonly withhold federal income tax on that income at the 22% flat supplemental rate in IRS Publication 15. Some use the aggregate method instead, which adds the vest to a regular paycheck and withholds as though that pay recurred all year.
That 22% is a withholding method, not your tax rate. For a single filer in 2026, the 32% bracket starts at $201,775 of taxable income and the 35% bracket at $256,225. So a gap can exist with a single employer, before a job change adds a second one.
Two employers’ worth of RSU income in the same year is what pushes part of your combined income into a bracket neither payroll system saw. Only the income above each threshold is taxed at the higher rate, not your entire paycheck, but the gap grows quickly once two W-2s are involved.
Why two RSU streams create a withholding gap
Neither payroll made a mistake. Each withholds only on the wages it pays.
Each employer withholds on its pay alone
Your old employer withholds only on what it paid you. Your new employer withholds only on what it pays you. Neither sees the other’s payroll, and neither sees a spouse’s income. Regular pay follows your W-4 and the graduated withholding tables. RSU income is usually withheld separately at the flat 22%, regardless of your other income.
Here is an illustration for a single filer with no other income who takes the 2026 standard deduction of $16,100:
| 2026 wages | Old employer | New employer | Combined |
|---|---|---|---|
| Salary | $90,000 | $110,000 | $200,000 |
| RSU vests | $60,000 | $90,000 | $150,000 |
| W-2 wages | $150,000 | $200,000 | $350,000 |
| RSU withholding (22%) | $13,200 | $19,800 | $33,000 |
Taxable income comes to $333,900. The salary fills the brackets through 24%, and the $150,000 of RSU income stacks on top of it: $17,875 at 24%, $54,450 at 32%, and $77,675 at 35%. That is about $48,900 of federal income tax on the RSUs, against $33,000 withheld. Even if salary withholding was exactly right, you are about $15,900 short.
A new job resets your withholding and Social Security
Your new employer’s payroll starts from zero. It calculates withholding on each paycheck as though that pay rate ran all year, and it has no record of your old W-2. Salary withholding can come out close to right, but nothing in either system accounts for the RSU income stacked on top.
Social Security resets too. Each employer withholds Social Security tax up to the annual wage base, $184,500 in 2026, independently. In the example, the old employer withholds 6.2% of $150,000, or $9,300, and the new employer withholds 6.2% of $184,500, or $11,439. That is $9,300 more than one person owes. The excess comes back as a credit when you file. In the example it offsets $9,300 of the $15,900 income tax shortfall, and you get none of it until then.
One more wrinkle: the 0.9% Additional Medicare Tax applies once your combined wages cross a threshold tied to your filing status ($200,000 for single filers, $250,000 for joint filers). Each employer, though, only starts withholding it once its own payments to you pass $200,000. In the example, neither employer paid more than $200,000, so neither withheld it, and you still owe 0.9% of $150,000, or $1,350.
How to model your combined tax bracket before the next vest
Start with a projection that covers both jobs.
Add up every income source across both jobs for the year
Get one number for the whole year. From the old job: wages and RSUs vested to date. From the new job: base salary and expected RSU vests through year-end. Then add a spouse’s income, bonuses, and other equity or investment income. What counts as taxable income walks through the pieces.
Compare your marginal rate to the 22% withheld
Your marginal rate is the rate on your next dollar of income, not your average rate across the year. For combined incomes in this range, it is usually 32% to 37%. Each vest’s withholding gap runs roughly your marginal rate minus the 22% withheld.
A $100,000 vest taxed at a 35% marginal rate but withheld at 22% leaves about $13,000 short. Multiply that by the number of vests at both employers. Run your own numbers with Nino’s RSU tax calculator, and see why RSUs are taxed so high for how the withholding breaks down.
The withholding moves to make now
Once you know the size of the gap, you have two ways to close it.
Add extra withholding on your W-4
Ask your new employer to withhold an extra dollar amount from each paycheck, using line 4(c) on Form W-4. The IRS treats withholding as paid evenly across the year, so an amount added in October also counts toward the April, June, and September installments.
Or make a quarterly estimated payment
Pay the IRS directly with Form 1040-ES by the quarterly due date after each vest: April 15, June 15, September 15, or January 15. You generally avoid the underpayment penalty if withholding and estimated payments cover 90% of this year’s tax or 100% of last year’s, rising to 110% of last year’s if your prior-year AGI was above $150,000. You can still owe a balance at filing without a penalty if you meet that safe harbor.
Time the fix to your job change and each vest
Set the W-4 amount when you start the new job, and recheck it after each vest rather than in December. That spreads the extra tax across the paychecks left instead of one January payment. RSU tax withholding and sell-to-cover explains how each vest is funded.
What Nino does when RSUs vest at two employers
After a job change, who keeps your RSU plan current across both employers? Your old payroll stopped at its own paycheck. Your new payroll started from zero. Your CPA typically sees both W-2s only after January 31, once the year has closed. A financial advisor sees your portfolio, not necessarily your payroll.
No one in that setup sizes the W-4 change or the estimated payment while there is still time to make it.
Nino’s Advisor plans give you a human CPA and CFP, and you enter both employers’ RSU grants in Nino’s grant form. Plans start at $300/mo, billed annually, and include federal and state tax filing: the CPA prepares the return, and nothing is filed until you approve it. See current plans, or how Nino compares to other options.
Ask your current CPA or advisor who is sizing your W-4 change or estimated payment before your next vest. If nobody is, book a free consultation.