RSU Sell-to-Cover Tax Calculator

What is RSU Sell-to-Cover Mechanics?

When Restricted Stock Units (RSUs) vest, the IRS treats them exactly like a cash bonus. To pay the taxes, employers use a Sell-to-Cover transaction: the exact moment the shares hit your account, the brokerage instantly liquidates a portion of them to satisfy the Federal, State, and FICA tax withholdings, delivering only the Net Shares to your portfolio.

Mathematical Foundation

\text{Tax Withheld} = (\text{Shares} \times \text{Vest Price}) \times (t_{fed} + t_{state} + t_{fica})

\text{Tax Withheld}

= The total dollar amount owed to the government at the moment of vest.

\text{Vest Price}

= The official fair market closing price on the exact day of vest.

\text{Shares Sold} = \lceil \frac{\text{Tax Withheld}}{\text{Vest Price}} \rceil

\lceil x \rceil

= Math.ceil — always rounding UP to the nearest whole share.

Laws & Principles

The Fractional Refund: Brokerages cannot sell 10.4 shares to exactly cover your taxes. They must sell 11 whole shares. The extra 0.6 shares worth of cash will be refunded to you on your next standard paycheck.
The 22% Supplemental Trap: By default, the IRS mandates a flat 22% withholding rate on supplemental income under $1M. If your actual marginal tax bracket is 32%, you will owe massive unexpected taxes in April because your company was legally required to under-withhold at vest.
Capital Gains Zero-Point: Because the shares are taxed as income on the exact day they vest, you owe $0 in capital gains tax if you sell the rest immediately. Capital gains only apply to the growth that occurs after the vest date.

Step-by-Step Example Walkthrough

" 1,000 RSUs vest at a market price of $150. Total tax rate is 39% (22% Fed, 9.35% State, 7.65% FICA). "

Gross Value: 1,000 × $150 = $150,000
Tax Liability: $150,000 × 39% = $58,500
Shares to Sell: $58,500 / $150 = 390 exactly
Net Shares Kept: 1,000 - 390 = 610

Final Result: You will log into your brokerage account and see only 610 shares. 390 were instantly liquidated and sent to the IRS.

Quick Answer: How does the RSU Tax Calculator work?

This tool simulates the Sell-to-Cover transaction that occurs when your Restricted Stock Units vest. Enter the number of shares vesting, the fair market price on the vest date, and your Federal, State, and FICA withholding rates. The calculator computes the total tax liability, the exact number of shares your brokerage will liquidate (rounded up to the nearest whole share), the fractional cash refund, and the net shares deposited to your account — all instantly in your browser.

RSU Tax Withholding Formulas

Step 1 — Gross Vest Value

Gross Value = Shares Vesting × FMV on Vest Date

Step 2 — Total Tax Liability

Tax = Gross Value × (Federal + State + FICA)

Step 3 — Shares Sold (Rounded Up)

Shares Sold = ⌈ Tax Liability ÷ FMV ⌉

Step 4 — Fractional Cash Refund

Refund = (Shares Sold × FMV) − Tax Liability

FMV— Fair Market Value. The official closing stock price on the exact calendar day the RSUs vest. This is the "income" amount the IRS uses for tax withholding.
⌈ x ⌉— Ceiling function (Math.ceil). Brokerages cannot sell fractional shares for tax coverage, so they always round UP to the nearest whole share to fully satisfy the tax liability.
Refund— The excess cash raised by selling one extra whole share. This residual amount appears as a miscellaneous line item on your next paycheck (not as shares).

Real-World Scenarios

✓ Standard Vest — Texas (No State Tax)

Lower combined tax rate in a zero-income-tax state

Shares: 500 RSUs vesting at $200/share
Gross Value: 500 × $200 = $100,000
Taxes: 22% Fed + 0% State + 7.65% FICA = 29.65%
Tax Bill: $100,000 × 29.65% = $29,650
Shares Sold: ⌈$29,650 / $200⌉ = ⌈148.25⌉ = 149 shares
Net Kept: 500 − 149 = 351 shares

→ You keep 70.2% of your vested shares. The extra $150 from selling 149 shares instead of 148.25 gets refunded as cash on the next paycheck.

✗ High-Tax State Vest — California

Severe combined withholding in the highest state tax bracket

Shares: 500 RSUs vesting at $200/share
Gross Value: 500 × $200 = $100,000
Taxes: 22% Fed + 13.3% CA + 7.65% FICA = 42.95%
Tax Bill: $100,000 × 42.95% = $42,950
Shares Sold: ⌈$42,950 / $200⌉ = ⌈214.75⌉ = 215 shares
Net Kept: 500 − 215 = 285 shares

→ You keep only 57% of your vested shares. California's 13.3% state tax rate consumes 66 additional shares compared to the Texas scenario.

Federal Supplemental Withholding Rates — Quick Reference

Supplemental Income Level	Federal Withholding	Applies To	Common Trap
Under $1,000,000	22% flat	Most tech employee RSU vests	Under-withheld if bracket > 22%
Over $1,000,000	37% flat	Senior executive mega-vests	Often matches marginal bracket

*FICA (Social Security 6.2% + Medicare 1.45% = 7.65%) is withheld on top of this. Social Security stops at $168,600 (2024 wage base).

Pro Tips & RSU Tax Planning

Do This

✓Check if your withholding covers your actual bracket. If your marginal tax rate is 32% or 35% but only 22% was withheld at vest, you will owe the remaining 10-13% tax shortfall in April. Set aside cash or make quarterly estimated tax payments to avoid an underpayment penalty.
✓Track your cost basis for remaining shares. The FMV on vest date becomes the cost basis for the net shares you keep. If the stock rises to $300 from a $150 vest price, you owe capital gains tax on the $150 per-share appreciation. This is separate from the income tax already paid at vest.

Avoid This

✗Do not assume taxes were fully covered at vest. The IRS mandates withholding at 22% flat, regardless of your actual tax bracket. If you earned $250k+ this year and your marginal bracket is 35%, the 13% under-withholding on a $100,000 vest is a $13,000 surprise tax bill in April.
✗Do not double-count the income on your tax return. RSU vest income appears on your W-2 as ordinary compensation income. It is already included in Box 1 wages. Do not also report it separately as investment income — this is the most common RSU filing error and results in paying tax twice on the same income.

Frequently Asked Questions

Why does my brokerage sell more shares than the exact tax calculation?

Brokerages cannot execute fractional share sell-to-cover transactions. If the tax liability requires selling 148.25 shares, they must round up to 149 whole shares to fully cover the obligation. The excess cash ($150 in this case) is refunded to you as a miscellaneous cash credit on your next regular paycheck — it is not returned as stock.

Do I owe capital gains tax if I sell my remaining RSU shares immediately?

No. The FMV on the vest date establishes your cost basis. If you sell your net shares immediately at the same price, there is zero capital gain (sale price = cost basis). Capital gains tax only applies to price appreciation that occurs after the vest date. If the stock drops after vesting, you can actually harvest a capital loss for tax purposes.

Does FICA stop being withheld after a certain income level?

Yes, partially. The Social Security portion (6.2%) stops being collected after your YTD wages exceed the taxable wage base ($168,600 in 2024). However, the Medicare portion (1.45%) has no cap and continues on all wages. Additionally, an extra 0.9% Medicare surtax applies to wages above $200,000 for single filers ($250,000 for married filing jointly). If your RSU vest hits late in the year after you have already exceeded the Social Security cap, the FICA withholding will only be the 1.45-2.35% Medicare portion.

How do I avoid double-counting RSU income on my tax return?

RSU vest income is automatically included in your W-2 Box 1 wages. When your brokerage issues a 1099-B for the sell-to-cover shares, it will often show $0 cost basis (or no cost basis), making it appear as if the entire sale proceeds are taxable gain. You must manually adjust the cost basis on your Schedule D to the vest-date FMV to avoid paying income tax twice on the same amount. This is the single most common RSU tax filing error.