Paycheck Taxes for RSU Holders: The Supplemental Withholding Gap

When your employer withholds taxes on an RSU vest, they do it at the IRS-mandated supplemental wage rate: 22% on the first $1 million of supplemental wages per calendar year, 37% on everything above $1M. That flat rate is a convenience designed for the average employee. If you sit in the 32% or 35% federal bracket, the 22% withholding leaves you under-withheld by 10-13 percentage points on every RSU vest. By April, that gap is a five-figure tax bill.

This is the supplemental withholding gap. It is not an accounting error, not a bug in your employer’s payroll system, and not something HR can fix for you. It is the IRS’s default rule, codified in Treas. Reg. §31.3402(g)-1. For tech employees earning $250k-$900k of total comp, the gap arrives every April, and the cash-flow surprise plus potential underpayment penalty under IRC §6654 is a recurring problem.

This guide covers exactly how the gap arises, the math at different income levels, the three ways to close it before penalties apply, and the payroll settings to review.

How supplemental wages get withheld

Under Treas. Reg. §31.3402(g)-1, supplemental wages (bonuses, commissions, RSU vesting income, non-qualified option exercise spread, severance) are subject to one of two withholding methods at the employer’s option:

The “flat rate” method: 22% on supplemental wages up to $1M YTD, 37% above $1M YTD. This is what almost all large tech employers use.
The “aggregate method”: add the supplemental wages to the most recent regular payroll and withhold on the combined amount using the employee’s W-4. This is used by some smaller employers and is closer to the employee’s actual marginal rate.

The 22% flat rate works out correctly for employees whose marginal bracket is 22% (taxable income $48,475-$103,350 single, $96,950-$206,700 joint for 2025). For higher earners, the flat rate is below the marginal bracket, which creates the gap.

For employees earning over $1M of supplemental wages in a calendar year, the 37% rate on the excess matches the top marginal bracket almost exactly, so very high earners with large IPO or acquisition events are roughly correctly withheld on the portion above $1M. The problem concentrates in the $250k-$900k supplemental-wages range.

The gap by income level

For a single filer in 2025:

Total taxable income	Marginal bracket	Gap on RSU vest
$100k-$192k	24%	2 points
$192k-$244k	32%	10 points
$244k-$609k	35%	13 points
Above $609k	37%	15 points
Above $1M YTD supplemental	37%	0 (matched)

For a senior engineer earning $200k base and $200k of RSU vests ($400k total taxable income, single filer, before deductions), marginal bracket on the RSU dollars is roughly 32-35%. Withholding at 22% on $200k of RSU is $44,000. True tax on that $200k at 32-35% is $64,000-$70,000. Gap: $20,000-$26,000 owed in April.

Add state tax for a California resident: state withholding on supplemental wages is 10.23% in California (a flat rate), which is close to the top California bracket of 13.3%. The California gap is smaller but still real. Net California tax liability on the $200k of RSU: $20,460 at 10.23% rate, versus $25,000+ at the 12.3-13.3% marginal rate. Gap: $5,000-$10,000.

Combined federal-plus-state gap: $25,000-$35,000 owed in April for this profile. Predictable every year.

The three ways to close the gap

Option 1: Elect additional withholding via W-4.

On your W-4, Step 4(c) allows you to specify an additional dollar amount to withhold from each paycheck. Calculate the annual gap (e.g., $25k), divide by pay periods (e.g., 26 biweekly), and set Step 4(c) to $962. Submit the updated W-4 to payroll.

This smooths the tax payment across the year automatically, avoids underpayment penalties, and requires no further action beyond monitoring whether the amount remains accurate as income changes.

Advantage: simplest option. Disadvantage: imprecise if RSU vests are lumpy or income is variable. For employees with one large December vest, the additional withholding might be over-withholding for 11 months then right-sized in month 12.

Option 2: Make quarterly estimated tax payments (Form 1040-ES).

Calculate expected gap per quarter and pay via Form 1040-ES or through EFTPS (the IRS electronic payment system). Due dates: April 15, June 15, September 15, January 15 of the following year.

The safe harbor under IRC §6654 says no underpayment penalty if you pay the smaller of:

90% of the current year’s total tax liability, or
110% of the prior year’s total tax liability (for taxpayers with AGI > $150k).

For employees with stable income, paying quarterly estimates that total 110% of last year’s tax liability is the easiest way to hit safe harbor. Calculate last year’s total tax (from Form 1040 line 24), multiply by 110%, divide by 4, subtract expected W-2 withholding per quarter, pay the difference.

Advantage: precise and adjustable. Disadvantage: requires active management and tracking.

Option 3: Use the W-4 safe harbor strategically.

Withholding counts as paid evenly throughout the year regardless of when it actually occurs (for underpayment penalty purposes). Estimated tax payments count as paid when actually made.

This creates a planning move: if you are underwithheld early in the year, dramatically increase W-4 withholding in Q4 to catch up. The year-end withholding counts retroactively across the year, hitting safe harbor even though the cash went out in December.

A late-year W-4 increase can be “withhold an extra $15,000 per pay period for the last two paychecks of the year,” which grosses the total withholding to the safe harbor level. Some employers cap at 100% of wages for a pay period, which limits this move for high earners, but most will accommodate substantial bumps.

The RSU-specific complication: sell-to-cover does not fix the gap

Many employees assume that the sell-to-cover mechanism on RSU vests handles the withholding correctly. It does not. Sell-to-cover mechanically withholds shares worth the statutory flat rate (22% federal plus state and FICA). The shares sold to cover produce the flat-rate withholding, not the marginal-rate withholding.

Example. 1,000 shares vest at $100. Total value: $100,000. Sell-to-cover at 22% federal plus 10.23% California plus 7.65% FICA = ~40% combined. About 400 shares are sold, netting $40,000 of withholding to the various tax authorities. The employee receives 600 shares of stock worth $60,000.

The underwithholding gap still exists. The employee’s actual federal marginal rate is 32-35%, and the 22% sell-to-cover leaves a 10-13 point gap on the federal side.

Some employers allow employees to elect a higher withholding rate on RSU vests (27%, 35%, 37%) to close the gap at source. Check your stock plan administrator’s settings. If the option is available, set it to match your marginal bracket.

When the gap becomes a penalty

IRC §6654 imposes an underpayment penalty when insufficient tax is paid in during the year. The penalty is calculated as an interest charge on the underpayment from each due date to the actual payment date, at the IRS short-term rate plus 3% (currently around 8%).

For a $25k underpayment spread across the year, the penalty is roughly $1,200-$2,000 depending on timing. Not catastrophic, but annoying, and avoidable with any of the three gap-closing moves.

The penalty does not apply if one of the safe harbors is met:

Total withholding plus estimates ≥ 90% of current year tax.
Total withholding plus estimates ≥ 100% of prior year tax (AGI ≤ $150k) or 110% of prior year tax (AGI > $150k).
Current year underpayment is less than $1,000.

The “110% of prior year” safe harbor is the easiest for most employees because it depends on last year’s number, not on current-year projections. Get last year’s total tax from Form 1040 line 24, multiply by 110%, and ensure your total withholding plus estimates hits that amount.

Frequently asked

Why does the IRS use a flat rate for supplemental wages? Administrative simplicity. Actual marginal rate depends on total annual income, which the employer cannot know at the time of a single paycheck. The flat rate is a compromise that works correctly for average earners and is close enough for most.

Can my employer withhold at a higher rate than 22%? Yes, at the employee’s election if the stock plan allows it. Some plans do; many don’t. If your plan only offers 22%, your options are W-4 additional withholding on regular pay or quarterly estimates.

What if my RSU vest is under $1M but my total comp puts me in the 37% bracket? Your marginal tax on the RSU dollars is 37%, but your employer withholds at 22% until you cross $1M of supplemental YTD. The gap is 15 points and the largest of any scenario for that year. Close it via W-4 or estimates.

Do I need to worry about state tax gaps too? Yes. Most states have their own supplemental withholding rates that are flat and often below the marginal rate. California’s 10.23% flat is below the top 13.3% bracket. New York’s 11.7% is below the top 10.9% bracket in most counts but close. Check your state’s specific rate and add any gap to your planning.

What if I owe more than $1,000 but hit safe harbor? You still owe the tax by April 15, but no underpayment penalty applies. Make the payment by April 15 to avoid failure-to-pay penalties and interest on the balance.

Next step

If you have RSU income above $100k per year and earn in the 32%+ bracket, verify your withholding position now, not in April. The RSU withholding calculator models your estimated gap based on expected vests, base salary, and state. Adjust W-4 or schedule estimated payments to close the gap before the next quarterly due date. The penalty and cash-flow cost of ignoring this is recurring and entirely avoidable.