The RSU 22% gap, in your numbers.
Payroll withholds a flat 22% when RSUs vest. Your real bracket is probably higher. This free calculator shows the difference, which is the size of your April surprise.
Base pay, bonus, and any other wage income for the year, before 401(k). Combine both spouses if filing jointly. Exclude the RSUs.
The fair market value of all shares vesting this calendar year. Your equity portal shows this per vest date.
Payroll deductions that reduce your federal taxable wages. The 2026 401(k) employee limit is $24,500.
Payroll withholds $33,000 on this vest, but at your income the IRS actually wants about $44,005, an effective 29.3% with your top dollars taxed at 35%. The difference lands as a balance due in April, plus a possible underpayment penalty unless you reach a safe harbor first.
Federal estimate for tax year 2026 using the standard deduction, the ordinary brackets in Rev. Proc. 2025-32, and flat supplemental withholding of 22% (37% above $1,000,000 of supplemental wages, Pub. 15). Assumes RSUs are your only supplemental wages and ignores state tax, the 0.9% Additional Medicare Tax, itemized deductions, credits, and other income. A planning estimate, not tax advice. Runs entirely in your browser; nothing is stored or sent.
Three numbers, no mystery.
The shortfall is not a payroll error. It is the predictable result of a flat withholding rate meeting a progressive tax system. The calculator reproduces the same arithmetic we run for clients before a vest.
Your real tax on the vest
RSU income stacks on top of your salary, so we compute 2026 federal tax on your income with and without the vest. The difference is the tax the vest actually creates, at your real marginal rates.
What payroll withholds
Employers withhold supplemental wages at a flat 22%, and at a mandatory 37% only on the portion of supplemental wages above $1 million for the year. That rate is applied no matter what bracket you are really in.
The gap
Real tax minus withholding. Positive means a balance due in April and possibly an underpayment penalty. Negative means payroll over-withheld and you get it back as a refund.
Asked at every vest.
Why does my employer only withhold 22% on RSUs?
When RSUs vest, the share value is taxed as supplemental wages, the same bucket as bonuses. IRS rules let employers use an optional flat 22% withholding rate on supplemental wages up to $1 million per year, and most payroll systems default to it because it is simple. The rate ignores your salary, your filing status, and your real bracket, which is why high earners come up short.
How accurate is this RSU tax calculator?
It is a planning estimate, not a tax return. It uses the official 2026 federal brackets and standard deduction, and assumes your RSUs stack on top of your other W-2 wages. It ignores itemized deductions, credits, self-employment or investment income, the 0.9% Additional Medicare Tax, and state tax. For most W-2 tech employees it lands close to the real gap, and a planning call can tighten it to your exact facts.
What should I do if the calculator shows I'm under-withheld?
You have two levers. You can make a quarterly estimated payment on Form 1040-ES for the quarter of the vest, or you can file a new Form W-4 with extra withholding on line 4c. The W-4 route is usually stronger because withholding is treated as paid evenly through the year, so it can cure an earlier quarter's shortfall. Aim for a safe harbor: 90% of this year's tax, or 100% of last year's (110% if your prior-year AGI was over $150,000).
Does this calculator include state taxes?
No, it is federal only. State withholding on RSUs has the same structural problem, though. Utah taxes the vest at its 4.5% flat rate, and California withholds supplemental wages at 10.23%, which can also miss your real California bracket. If you vested RSUs after moving between states, sourcing rules can put the same vest on two state returns.
Is my information stored anywhere?
No. The calculator runs entirely in your browser. Nothing you type is sent to a server, stored, or shared.
Close it before April does.
Thirty minutes is enough to pick the cheaper safe harbor, set the right W-4 line 4c amount or Q4 estimate, and keep the underpayment penalty off your return.