Statutory & Regulatory Citations
This guide is grounded directly in the following federal and state tax statutes, Treasury regulations, and revenue rulings:
- 26 U.S.C. § 3402(g) — Overlapping Pay Periods & Supplemental Wages
- 26 C.F.R. § 31.3402(g)-1 — Supplemental Wage Payments & Mandatory Flat Rate
- IRS Publication 15 (Circular E), Section 7 — Supplemental Wages
- 26 U.S.C. § 83(a) — Property Transferred in Connection with Performance of Services
1. Why RSUs Are Taxed as Ordinary Supplemental Wages at Vest
Under Internal Revenue Code (IRC) § 83(a), Restricted Stock Units (RSUs) are not taxed when granted because they represent an unfunded, conditional promise to deliver shares in the future. Instead, the taxable event occurs upon share settlement (the "vest date"), when substantial risk of forfeiture lapses and shares are delivered to your brokerage account.
On the exact date your RSUs vest, the fair market value (FMV) of the vested shares—multiplied by the number of shares vesting—is recognized as ordinary W-2 compensation income. Unlike Incentive Stock Options (ISOs), RSUs do not qualify for long-term capital gains treatment upon vest, nor can standard single-trigger or double-trigger RSUs be early-exercised via an IRC § 83(b) election.
Because RSU vest distributions occur outside your regular biweekly or semi-monthly base salary payroll cycle, the IRS classifies them as "supplemental wages" alongside performance bonuses, sign-on bonuses, commissions, and severance payments.
2. Treasury Reg. § 31.3402(g)-1: The 22% vs. 37% Two-Tier Cliff
Under Treasury Regulation § 31.3402(g)-1 and IRS Publication 15 (Circular E), employers that withhold income tax from an employee’s regular base salary are permitted to use the "Optional Flat Rate Method" on supplemental wages, provided the employee has not received more than $1,000,000 in cumulative supplemental wages during the calendar year.
Virtually every major technology employer and corporate stock plan administrator (Charles Schwab, Fidelity NetBenefits, Morgan Stanley at Work, E*TRADE, and Carta) uses the Optional Flat Rate Method as its system-wide default because stock plan systems do not have real-time visibility into your spouse’s W-2 income, outside investment income, itemized deductions, or tax filing status.
- Tier 1 — Cumulative Supplemental Wages ≤ $1,000,000 (22.00% Flat Rate): For all cash bonuses, sign-on payments, and RSU vests that total $1,000,000 or less within a single calendar year, employers withhold a flat 22% federal income tax. Even if your Form W-4 claims Married Filing Jointly or single with high deductions, the brokerage ignores your W-4 exemptions when applying the flat 22% rate.
- Tier 2 — Cumulative Supplemental Wages > $1,000,000 (37.00% Mandatory Rate): Once your cumulative annual supplemental wages (annual bonus + YTD RSU vests at that employer) cross $1,000,000, Treasury Regulation § 31.3402(g)-1(a)(2) mandates that the employer withhold 37% on every dollar exceeding $1,000,000. However, the first $1,000,000 remains withheld at only 22%, leaving up to a $150,000 federal gap on that initial $1,000,000.
| 2026 Taxable Income Bracket | True Marginal Federal Rate | Brokerage Flat Withholding | Unpaid Federal Gap per $100k Vest |
|---|---|---|---|
| $50,400 – $107,300 | 22.00% | 22.00% | $0 (Fully Covered) |
| $107,300 – $204,700 | 24.00% | 22.00% | $2,000 Shortfall |
| $204,700 – $260,050 | 32.00% | 22.00% | $10,000 Shortfall |
| $260,050 – $650,600 | 35.00% | 22.00% | $13,000 Shortfall |
| Over $650,600 (Up to $1M Supp.) | 37.00% | 22.00% | $15,000 Shortfall |
3. Worked Numerical Case Study: Senior Engineer in California
Consider a Senior Software Engineer (Single filer, residing in California) earning a $210,000 base salary who vests 500 shares of stock at an FMV of $400/share ($200,000 gross RSU vest) during the 2026 tax year, bringing total W-2 compensation to $410,000.
Because the engineer’s $210,000 base salary already exceeds the 2026 Social Security wage base cap of $184,500, $0 of the $200,000 RSU vest is subject to the 6.2% Social Security tax. However, the entire $200,000 vest exceeds the $200,000 Additional Medicare Tax threshold, subjecting 100% of the vest to both regular Medicare (1.45%) and the Additional Medicare Surtax (0.90%), totaling 2.35% FICA.
| Tax Component | Statutory Brokerage Withheld | True Marginal Tax Liability | Net April Tax Shortfall |
|---|---|---|---|
| Federal Income Tax | $44,000 (22.00% flat) | $69,081 (~34.54% effective on vest) | +$25,081 Owed |
| California Income Tax | $20,460 (10.23% flat) | $21,218 (9.3%–11.3% progressive) | +$758 Owed |
| Social Security (6.2% up to $184.5k) | $0 (Capped by base salary) | $0 | $0 |
| Medicare + Add. Medicare (1.45% + 0.9%) | $4,700 (2.35%) | $4,700 (2.35%) | $0 |
| Total Tax on $200,000 Vest | $69,160 (34.58%) | $94,999 (47.50%) | +$25,839 Shortfall |
4. Three Ways to Eliminate the Supplemental Withholding Gap
Technology employees have three compliant mechanisms to bridge the gap between statutory sell-to-cover and their true marginal tax liability:
- Strategy 1: Increase Your Brokerage Supplemental Withholding Election (Where Supported): Employers such as Google, Meta, and Nvidia allow employees to log into Charles Schwab or E*TRADE and override their federal supplemental withholding rate from 22% up to 37% prior to the quarterly or monthly cutoff window. Because the higher tax is withheld directly via payroll, it automatically satisfies IRS quarterly timing rules.
- Strategy 2: Sell Incremental Shares Upon Vest & Remit Form 1040-ES Vouchers: If your employer locks supplemental withholding at 22%, log into your brokerage on the settlement date and manually sell the exact number of incremental shares required to cover your projected gap (Incremental Shares = ceil(Shortfall / FMV)). Place the cash proceeds into a high-yield Treasury money market fund and remit quarterly estimated payments via IRS Direct Pay.
- Strategy 3: Increase Regular Paycheck W-4 Step 4(c) Extra Withholding: Under IRC § 6654(g), all W-2 payroll tax withheld during the calendar year is treated by the IRS as if it were paid in four equal quarterly installments—even if you increase your W-4 Step 4(c) "Extra Withholding" late in Q3 or Q4.
Calculate Your Specific RSU Tax Gap
Want to run these numbers on your own grant? Use our RSU tax calculator to estimate your 2026 federal and state withholding gap, the $184,500 Social Security cap offset, and how many extra shares to sell at vest: