Your RSU vest withholding looked nothing like 22% — or payroll says they aggregate supplemental wages with your regular paycheck. You want to know whether that is allowed, how the stub math works, and why your rate might differ from the flat supplemental rate.
In plain terms
Gather before you start
- confirmation showing method if listed.
- Pay stub for the pay period with federal tax line detail.
- Prior pay stub from a salary-only period for comparison.
- Form W-4 on file with your employer.
- IRS Pub. 15 section for the tax year.
How the tax works
include income for most employees. IRS Publication 15 describes two main federal approaches: flat rate on the supplemental portion, or aggregate with regular wages in the same pay period. Flat supplemental uses the Pub. 15 rate for amounts under the annual dollar threshold — employers often quote this as a round percentage on confirmations.
Aggregate adds to salary wages on one pay stub and runs federal tables on the combined amount as if it were all regular pay subject to your Form W-4 elections. That can produce higher than flat supplemental when your salary rate is already high — or lower when W-4 settings produce light on the merged gross.
Some employers always use flat supplemental on payroll runs even when salary pays biweekly separately. Others time the to land on a salary paycheck and aggregate. Large vests on isolated off-cycle payroll often use flat supplemental because no salary wages share the period. Payroll software labels vary — AGGREGATE, REG, COMBINED, or no label when flat supplemental is used.
does not change which method applies; gross wages are still full . State may follow different rules than federal and may not mirror Pub. 15’s supplemental-versus-aggregate choice. Year-end Box 2 totals reflect whatever was withheld — the method itself is invisible on the .
Either method is prepayment toward filing, not your final bracket. Changing W-4 affects aggregate on future combined pay periods more directly than flat supplemental runs. If federal seems wildly off, verify aggregate versus flat on the stub and confirmation before assuming a payroll error.
What to check on your end
- confirmation or payroll FAQ for supplemental vs aggregate wording.
- Federal dollars divided by on the stub.
- Compare to the flat supplemental rate from Pub. 15 for the tax year.
- Whether the posted on a salary pay date or a separate off-cycle run.
- W-4 filing status and extra elections.
- State line using the same or a different method.
- YTD federal withheld after the vs an annual estimate.
Assuming every RSU vest uses the same flat supplemental percentage
What to pull from your files
- pay stub with gross wages and federal tax lines.
- confirmation PDF.
- Employer equity tax guide if published.
- Form W-4 on file.
- December YTD pay stub for cumulative .
Vest on biweekly salary check with aggregate withholding
Illustration only, not your tax situation.
Questions people ask
- What is aggregate withholding on RSU vests?
- Payroll combines wages with regular salary wages in the same pay period and withholds federal tax on the total using ordinary wage tables and your W-4. It is an alternative to flat supplemental under IRS Pub. 15.
- Is 22% required on RSU vests?
- No. Flat supplemental at the Pub. 15 rate is one allowed method. Aggregate on a combined paycheck can produce a different percentage.
- How do I know which method my employer used?
- Compare federal tax withheld to on the pay stub. Ask HR or read the equity payroll FAQ. Confirmations sometimes state supplemental flat vs combined payroll.
- Does aggregate withholding mean my taxes are done?
- No. It is still , not final tax. High earners may still owe more at filing when income stacks across the year.
- Off-cycle RSU payroll — aggregate or flat?
- Often flat supplemental when no regular wages share the run. Employer practice varies; check whether the landed on a salary pay date.
When a CPA is worth it
- Federal zero on a large aggregate paycheck.
- Payroll cannot explain the method used vs Pub. 15.
- Multi-state payroll on an aggregated check.
- far below any reasonable estimate on high income.
Sources and notes
Primary tax claims on this page are supported by the official and secondary sources below. Broker and software links describe reporting mechanics — confirm rules against IRS or state guidance.
Supplemental vs aggregate wage withholding methods under IRS Pub. 15.
- IRS Publication 15 (Circular E) — Supplemental wages
Internal Revenue Service · Official
Section 7 describes supplemental wage withholding, including the optional 22% flat rate and 37% rate above $1 million of supplemental wages in a calendar year.
- IRS Publication 525 — Taxable and Nontaxable Income
Internal Revenue Service · Official
Covers compensation income from stock-based pay, including restricted property under section 83.
- Equity Compensation — RSU taxation at vest and on sale
Charles Schwab (Workplace Financial Services) · Brokerage explainer
Plain-language explainer: RSU value at vest on W-2, FICA, withholding may not cover full tax, separate capital gains on sale.
Related calculators
Related pages
- Why Was My RSU Withholding Only 22%?
Employers commonly use flat supplemental rates on RSU vests. Your actual tax can be higher if you are in a higher bracket.
- Reading Your Pay Stub After an RSU Vest
Vest FMV adds to gross wages on the same pay stub as salary — supplemental withholding and FICA lines spike, but net pay alone does not prove your full-year tax is covered.
- RSU Pay Stub Line by Line
Each pay stub line on an RSU vest check maps to wage reporting — compare gross vest FMV, withholding, and FICA to your confirmation before calling payroll.
- RSU Withholding vs Actual Tax
Why flat supplemental withholding on RSU vests often differs from your actual tax when salary, bonus, and vests stack in one year.
- W-4 After RSU Vest
Updating Form W-4 to increase paycheck withholding is often simpler than quarterly estimated tax when RSU vest withholding fell short of your marginal rate.
For learning, not filing
VestingTax.com is not a CPA firm or tax preparer. Grants, employers, and states all differ. Use the cited IRS and state sources above, your own documents, and a qualified tax professional before you make decisions from this guide.