Cost Basis for Equity Compensation: How to Avoid Double Taxation on Your Tax Return

Executive Summary

Every year, thousands of employees with equity compensation overpay their taxes by tens of thousands of dollars—not because the tax code is unfair, but because their broker's 1099-B reports incorrect cost basis. The IRS receives the same wrong number, and if you don't correct it on Form 8949, you'll pay ordinary income tax on your W-2 and capital gains tax on the same dollars.

The bottom line: Your cost basis for equity compensation is almost never what your broker reports. You must add the compensation income component (already taxed via W-2) to arrive at the correct basis. Failure to do so results in double taxation that can cost you 20–37% of your equity compensation value in overpaid taxes.¹

Critical Warning: The IRS matching system compares your Form 8949 against the 1099-B your broker files. If your reported proceeds and basis don't match the 1099-B, you'll receive a CP2000 notice proposing additional tax—even if your adjusted basis is correct. Always use the proper adjustment code in Column (f) of Form 8949 to explain the discrepancy.²

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Related Guides: For the complete RSU taxation framework, see our RSU tax guide. For ISO vs NSO differences, see our ISO vs NSO guide. For ESPP-specific strategies, see our ESPP taxation guide. For understanding Forms 3921 and 3922, see our equity compensation reporting forms guide.

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Why Your 1099-B Is Almost Always Wrong

The Structural Problem

When you sell shares acquired through equity compensation, your broker issues Form 1099-B reporting the sale proceeds and cost basis. However, brokers face a fundamental information gap: they know what you paid for shares but not what was already taxed as compensation income on your W-2.¹

This creates a predictable pattern of incorrect basis reporting:

Covered vs. Non-Covered Shares

The distinction between covered and non-covered shares determines your broker's reporting obligations and the Form 8949 box you'll use:²

For covered shares, the broker reports basis to the IRS, but it may still be incorrect for equity compensation. You must still adjust basis on Form 8949 using adjustment code B (basis reported to IRS is incorrect).

For non-covered shares, the broker does not report basis to the IRS at all. You must calculate and report the entire basis yourself, typically using adjustment code B in Column (f).

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RSU Cost Basis: The Most Common Error

The Correct Calculation

RSU cost basis is straightforward in principle but frequently botched in practice. When RSUs vest, the full fair market value is reported as ordinary income on your W-2. That same FMV becomes your cost basis.¹

RSU Cost Basis = Number of Shares × FMV on Vesting Date

Working Example: RSU Sale

Facts: Maria receives 500 RSUs that vest on March 15, 2025, when her company's stock is $120 per share. Her employer withholds shares for taxes and delivers 340 net shares. She sells all 340 shares on September 20, 2025, at $135 per share.

What the 1099-B likely shows:

Without adjustment, Maria would pay capital gains tax on $45,900 instead of $5,100—overpaying by $40,800 worth of taxable income that was already taxed as ordinary income on her W-2.

Form 8949 Adjustment for RSUs

On Form 8949, Maria reports:

Column (f) code B tells the IRS: "The basis reported on my 1099-B is incorrect; I'm adjusting it in Column (g)."

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NSO Cost Basis: Strike Price Plus the Spread

The Correct Calculation

When you exercise non-qualified stock options, the spread (FMV at exercise minus strike price) is taxed as ordinary income and reported on your W-2. Your cost basis equals the strike price you paid plus the spread already taxed:¹

NSO Cost Basis = Strike Price + (FMV at Exercise − Strike Price) = FMV at Exercise

In effect, the cost basis for NSO shares equals the FMV on the exercise date—because the spread has already been taxed as compensation.

Working Example: NSO Sale

Facts: James exercises 1,000 NSOs with a strike price of $15 per share when the stock is at $55 per share. He holds the shares and sells 8 months later at $70 per share.

What the 1099-B likely shows:

Without adjustment, James pays tax on $55,000 of gain instead of $15,000—double-taxing the entire $40,000 spread already on his W-2.

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ISO Cost Basis: Two Parallel Calculations

Qualifying Disposition (Regular Tax)

If you meet both ISO holding period requirements (2 years from grant date AND 1 year from exercise date), the entire gain above the strike price is taxed as long-term capital gains:³

ISO Basis (Qualifying Disposition) = Strike Price
Capital Gain = Sale Price − Strike Price (all long-term)

Disqualifying Disposition (Regular Tax)

If you sell before meeting both holding periods, the spread at exercise becomes ordinary income (reported on W-2), and your basis adjusts upward:

ISO Basis (Disqualifying Disposition) = Strike Price + Spread at Exercise = FMV at Exercise

AMT Basis (Both Dispositions)

For Alternative Minimum Tax purposes, the spread at exercise was already included as an AMT preference item. Therefore, your AMT basis is always:

ISO AMT Basis = FMV at Exercise Date

Working Example: ISO With Both Tax Calculations

Facts: Lisa exercises 500 ISOs with a $20 strike price when FMV is $80. She sells 14 months later (qualifying disposition) at $100.

Lisa pays regular tax on the $40,000 gain, but gets AMT credit for the difference between regular tax and AMT from the exercise year. Tracking both bases is essential for claiming the AMT credit carryforward correctly.

If Lisa had made a disqualifying disposition (sold within 14 months of exercise):

The 1099-B would show $10,000 basis (strike price only), creating a $40,000 apparent gain instead of the correct $10,000.

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ESPP Cost Basis: The Discount Trap

The Correct Calculation

ESPP cost basis depends on whether you make a qualifying disposition (held shares 2+ years from offering date AND 1+ year from purchase date) or a disqualifying disposition:³

Disqualifying Disposition:

ESPP Basis = Purchase Price + Ordinary Income (full discount amount on W-2)
           = FMV on Purchase Date

Qualifying Disposition:

ESPP Basis = Purchase Price + Ordinary Income (lesser of discount or actual gain)

Working Example: ESPP Sale

Facts: Kevin participates in his company's ESPP. The offering date FMV is $50, the purchase date FMV is $80, and the 15% discount applies to the lower price. He buys 200 shares at $42.50 per share. He sells 6 months later (disqualifying disposition) at $90.

What the 1099-B shows:

Without adjustment, Kevin pays tax on $9,500 instead of $2,000—double-taxing the $7,500 ESPP discount already on his W-2. For ESPP-specific strategies, see our ESPP taxation guide.

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Form 8949: The Correction Mechanism

Adjustment Code Reference

Form 8949 Column (f) uses single-letter codes to explain why your reported amounts differ from the 1099-B. For equity compensation, these are the relevant codes:²

Step-by-Step Form 8949 Process

Step 1: Gather your documents:

• Form 1099-B from your broker (for each sale)
• Form W-2 (showing equity compensation income in Box 1)
• Form 3921 (for ISO exercises) or Form 3922 (for ESPP purchases)
• Your employer's supplemental stock plan statement

Step 2: Determine the correct basis for each lot using the formulas above.

Step 3: Complete Form 8949:

• Report the 1099-B proceeds and basis as shown by your broker in Columns (d) and (e)
• Enter adjustment code B in Column (f)
• Enter the basis adjustment amount in Column (g)—this is the difference between correct basis and reported basis
• Calculate the correct gain or loss in Column (h)

Step 4: Transfer totals to Schedule D.

Reconciling W-2 Income With 1099-B

The key reconciliation principle: every dollar of equity compensation income on your W-2 should increase your cost basis by the same dollar amount. If it doesn't, you're being double-taxed.

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How to Fix Already-Filed Returns

Filing Form 1040-X (Amended Return)

If you've already filed a return with incorrect cost basis, you can recover overpaid taxes by filing Form 1040-X (Amended U.S. Individual Income Tax Return).⁴

Amendment Checklist

Statute of limitations: You must file the amended return within 3 years of the original filing date or 2 years from the date you paid the tax, whichever is later. Do not delay—every tax year that passes beyond the statute of limitations is money you cannot recover.

Expected timeline: The IRS typically processes amended returns in 8–16 weeks. E-filed 1040-X (available for tax years 2021 and later) may process faster than paper-filed amendments.

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Common Mistakes and How to Avoid Them

Mistake 1: Trusting the 1099-B Without Verification

The problem: Many employees (and some tax preparers) enter 1099-B data directly into tax software without adjusting cost basis for equity compensation.

The fix: Always cross-reference your 1099-B cost basis against your W-2 equity compensation income and Forms 3921/3922. If your 1099-B shows $0 basis for RSU shares, it is definitively wrong.

Mistake 2: Confusing Gross Shares With Net Shares

The problem: Your employer may vest 500 RSUs but withhold 160 shares for taxes, delivering only 340 net shares. Some employees incorrectly use the gross share count when calculating basis.

The fix: Calculate basis per share using the vesting-date FMV, then multiply by the number of shares you actually received and subsequently sold.

Mistake 3: Using the Wrong Basis for ISO Qualifying Dispositions

The problem: For qualifying ISO dispositions, your regular tax basis is the strike price—not the FMV at exercise. Using the FMV at exercise (the AMT basis) for regular tax purposes will understate your gain and create an IRS discrepancy.

The fix: Maintain separate regular tax and AMT basis tracking for every ISO lot. Use the strike price for regular tax Form 8949 and the exercise-date FMV for AMT calculations on Form 6251.

Mistake 4: Ignoring the ESPP Ordinary Income Component

The problem: For ESPP qualifying dispositions, the ordinary income component (lesser of actual discount received or gain at sale) is reported on your W-2. Many employees forget to add this to their cost basis.

The fix: Obtain Form 3922 from your employer and use it to calculate the ordinary income portion. Add this to your purchase price to determine the correct basis. For detailed ESPP calculations, see our ESPP taxation guide.

Mistake 5: Not Requesting Corrected 1099-B Forms

The problem: Some brokers will issue a corrected 1099-B with the proper basis if you contact them with documentation of your compensation income.

The fix: Contact your broker's equity compensation department with your W-2 and supplemental stock plan statement. Even if they can't correct the 1099-B, they may provide a supplemental statement that simplifies your Form 8949 reporting.

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Frequently Asked Questions

My tax software imported my 1099-B automatically. Is the cost basis correct?

Almost certainly not for equity compensation shares. Tax software imports whatever the broker reported, which typically excludes the compensation income component. You must manually adjust the cost basis in your tax software using the methods described in this guide.

What if I can't find my vesting-date FMV for RSUs?

Check your brokerage account's transaction history, your employer's stock plan administrator portal (E*Trade, Schwab, Fidelity, Morgan Stanley), or your W-2 supplemental stock plan statement. Your employer's HR or payroll department can also provide historical vesting-date prices.

Does this apply to shares I received in an acquisition or merger?

Yes. If your employer was acquired and your equity was converted, the cost basis rules still apply—but the calculation may be more complex. The acquiring company's exchange ratio and any cash consideration affect your basis. See your merger documentation and Form 8937 (Report of Organizational Actions Affecting Basis of Securities) if issued.

Source: IRS Form 8949 Instructions

What if I sold shares over multiple years and never adjusted basis?

You can file Form 1040-X for any open tax year (generally the last 3 years). For older years beyond the statute of limitations, the overpaid taxes are unfortunately not recoverable. Going forward, correct your basis for all future sales.

Can my employer fix the 1099-B?

Your employer does not issue the 1099-B—your broker does. However, your employer provides the compensation data to the broker. Contact your employer's stock plan administrator to ensure they transmitted the correct compensation income information to the brokerage.

Source: IRS Publication 525

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Complete Cost Basis Quick Reference

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Footnotes

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Disclaimer: This guide discusses legal tax optimization strategies only. Tax evasion is illegal and is never recommended. This content is for educational purposes and does not constitute tax, legal, or financial advice. Tax laws vary by jurisdiction and change frequently. Always consult a qualified tax professional (CPA, tax attorney, enrolled agent) before making decisions based on this information. The authors accept no liability for actions taken based on this content.

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Last Updated: March 9, 2026
Research Team: VestingStrategy Editorial Team

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Primary Sources

Footnotes

1. IRS Publication 525 establishes that compensation income included in W-2 wages increases the employee's basis in shares received through equity compensation arrangements. ↩ ↩² ↩³ ↩⁴

2. IRS Form 8949 Instructions specify that Column (f) code B is used when the basis reported on Form 1099-B is incorrect or was not reported to the IRS. ↩ ↩² ↩³

3. IRS Publication 550 provides detailed rules for determining cost basis of stock acquired through employee stock purchase plans and incentive stock options. ↩ ↩²

4. IRS instructions for Form 1040-X specify the 3-year or 2-year statute of limitations for claiming refunds on amended returns. ↩