Equity
RSU Withholding Gap: How It Saves You $5,000-$25,000 in 2026
When RSUs vest, your employer withholds federal tax at the 22% supplemental wage rate, but if you're in the 35% or 37% bracket, you actually owe 35-37%. This creates a withholding gap of 13-15% on every vesting event. On $100K in RSU income, that's a $13K-$15K gap that becomes a surprise tax bill (plus potential underpayment penalties). The fix: increase your W-2 withholding or make quarterly estimated payments to cover the gap.
Who Qualifies
- Receive RSU compensation and are in the 32%+ tax bracket
- RSU income exceeds $1M/year (37% flat supplemental rate applies above $1M)
- Any tech employee with significant equity compensation
Who does NOT qualify
- Tax bracket matches or is below the 22% supplemental rate
- No equity compensation
How the Math Works
RSU withholding: $150K × 22% = $33K. Actual tax at 35% marginal rate: $150K × 35% = $52,500. Gap: $19,500.
Filing an updated W-4 or making $4,875/quarter in estimated payments covers the gap and avoids the underpayment penalty (~$1,200 at 8% IRS interest rate).
Legal Basis & IRC Citations
- IRC §3402(g): Supplemental wage withholding
- IRC §6654: Underpayment of estimated tax penalty
- Treas. Reg. §31.3402(g)-1: Flat rate method for supplemental wages
What to Tell Your CPA
“My RSU vestings this year total $[amount] and my employer withheld at the 22% supplemental rate. My actual marginal rate is [35%/37%]. Can you calculate the withholding gap and recommend either a W-4 adjustment or estimated payment schedule to avoid underpayment penalties?”
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Frequently Asked Questions
Why are RSUs only withheld at 22%?
IRS rules allow employers to withhold supplemental wages (bonuses, RSUs, commissions) at a flat 22% regardless of the employee's actual tax bracket. For employees in the 35-37% bracket, this creates a structural withholding shortfall.
How do I close the RSU withholding gap?
Option 1: File a new W-4 with additional withholding (Line 4c) to cover the gap across paychecks. Option 2: Make quarterly estimated tax payments (Form 1040-ES). Option 3: Some employers allow you to elect higher supplemental withholding, ask your payroll team.
What happens if I don't cover the gap?
You'll owe a large tax bill in April plus an underpayment penalty (currently ~8% annualized). The penalty is calculated quarterly on the shortfall amount. For a $20K gap, the penalty can be $1,000-$1,600.
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