Why your RSU tax bill seems too high — the 22% withholding gap, explained

If you got an RSU vest this year and a tax bill that made you blink twice in April, you are not alone. The cause is almost always the same: your employer withheld federal income tax at a flat 22% under IRS supplemental-wage rules, but your actual marginal rate is much higher. The gap between those two numbers is the shortfall — and for a typical tech worker in the 32% or 35% bracket, it routinely lands between $2,000 and $30,000 on a single vest.

How RSU withholding works

When RSUs vest, the IRS treats their fair-market value as ordinary wages. But because vests do not happen on a regular pay cycle, employers default to a flat "supplemental wage" withholding rate published by the IRS in Publication 15-T:

• 22% on the first $1,000,000 of supplemental wages paid to you in the calendar year.
• 37% on every dollar of supplemental wages above $1,000,000.

Your employer applies these rates regardless of your actual tax bracket.

The 22% number is not based on any specific worker's tax situation — it is a one-size-fits-all default set by Treasury under Treas. Reg. 3402(g)-1.

The gap between this flat rate and your real marginal rate is what creates the April surprise.

Where the 22% rule came from

The supplemental-wage withholding rule traces back to the 1986 Tax Reform Act and has been adjusted only modestly since.

Treasury set 22% as an approximation of the average filer's marginal rate on incremental wages — historically reasonable when most filers landed in the 25% bracket.

After the 2017 Tax Cuts and Jobs Act lowered the brackets to 22%, the flat rate happened to match the new third bracket — but for anyone in the 32%, 35%, or 37% brackets (most senior tech workers), the rate now under-withholds materially.

There is no mechanism in the rule for the employer to adjust based on your actual W-2 wages. The flat 22% applies whether you earn $80,000 or $800,000.

Worked example — single filer, $200k base, $50k vest

You are single, earn $200,000 in regular W-2 wages, and receive a $50,000 RSU vest in March. Federal piece only (ignoring state, Medicare, Social Security for clarity):

• Employer withholds federal supplemental at 22% × $50,000 = $11,000.
• Your total taxable income for the year (with the vest) is $250,000. At 2026 single brackets, the marginal rate at $250k is 35%.
• Actual federal tax owed on the $50,000 vest at the margin: $50,000 × 35% = $17,500.
• Shortfall: $17,500 − $11,000 = $6,500 owed at filing time.

Add California state shortfall (~$1,000–$2,000 on top, since CA supplemental is also a flat 10.23% vs ~12.3% marginal) and Additional Medicare reconciliation, and the all-in April balance owed often crosses the IRS $1,000 safe-harbor threshold under IRC §6654 — triggering an underpayment penalty in addition to the tax itself.

State withholding makes it worse in a few states

Some states apply a flat supplemental rate to RSU vests, mirroring the federal rule:

• California — 10.23% supplemental, but top marginal is 13.3% (or 14.3% above $1M with the Mental Health Services Tax surcharge under Cal. Code Regs. tit. 18, §17041). Real marginal gap: 2–4 percentage points.
• New York — 11.7% supplemental for high earners, top marginal 10.9% state + up to 3.876% NYC. The gap can be small at state level but compounds with NYC local tax for residents.
• Texas, Florida, Washington, Tennessee, Nevada, South Dakota, Wyoming — 0% state, no gap.

For a CA tech worker, the combined federal + state shortfall on a $50,000 vest with the same $250k total income works out to roughly $8,500 — close to a fifth of the net vest value, owed back to tax authorities by April.

When 22% is actually enough (the under-the-radar case)

If your taxable income (including the vest) keeps you in the 22% federal bracket, then 22% supplemental withholding correctly covers the federal piece — no shortfall. This happens for:

• Single filers with total taxable income under ~$48,000 in 2026 (the top of the 22% bracket starts at $48,475 for singles).
• Married-filing-jointly with combined taxable income under ~$96,950.
• Smaller vests on lower base salaries — e.g., a $5,000 vest for someone earning $50,000 base stays inside the 22% bracket.

For most tech workers with senior salaries plus equity, this safe zone does not apply. But it is worth knowing: 22% is not under-withholding for everyone, just for high earners.

How to fix it before December 31

Two approved methods to cover the shortfall before year-end:

1. 1Form W-4 line 4(c) — "Extra withholding". Open your payroll system (Workday, ADP, Gusto), find the W-4 form, enter a flat dollar amount on Line 4(c) labeled "Additional withholding per pay period." Federal withholding is treated as paid evenly across the year per IRC §6654(g)(1) — so a Q4 top-up retroactively cures earlier under-withholding without triggering the per-quarter underpayment math. This is the preferred fix.
2. 2Quarterly estimated tax payment via IRS Direct Pay. Pay the shortfall as a "Form 1040 estimated tax" payment at irs.gov/payments. Only counts from the payment date — does not retroactively fix earlier-year under-withholding. Use this if you missed the W-4 window before December 31.

Long-term fix: aggregate-method withholding

Treas. Reg.

3402(g)-1(a)(2) authorizes employers to use the "aggregate method" instead of the 22% flat — adding the vest to your regular paycheck wages and withholding at your effective rate from the IRS withholding tables.

For high earners this typically produces 28–32% federal withholding, much closer to the actual marginal rate.

Most companies default to the flat supplemental method because it is operationally simpler. A minority (some financial-services firms, some legacy industrials) use aggregate. Both are legal.

You usually cannot pick — but it is worth asking HR or reading your equity plan document to find out which method applies, so you know whether to expect a shortfall or not.

The takeaway

The 22% federal supplemental withholding rate is the IRS-mandated default for RSU vests, set by Treasury under Treas. Reg. 3402(g)-1.

For most tech workers in the 32%, 35%, or 37% bracket, that flat rate under-withholds the actual federal tax by 10-15 percentage points. The gap is the April surprise.

Fix it before December 31 with Form W-4 line 4(c) — the most powerful single tool because §6654(g) treats withholding as paid evenly throughout the year, retroactively curing earlier under-withholding.

The math is not magic; the IRS rule was just designed for a 1986 payroll system, not for modern equity comp.