RSU Cost Basis Correction Calculator
Your broker’s 1099-B often shows $0 cost basis on RSU shares — but you already paid ordinary-income tax on their value at vest. File off that $0 and you pay tax twice. See your correct basis and the Form 8949 fix below.
Tax year 2026 · Last updated May 29, 2026
What is going wrong
Your broker reported a $0 basis, so you would report a $6,000 gain. Your real gain is only $1,000 — the extra $5,000 was already taxed as wages on your W-2 at vest. Taxed again at 15% (plus NIIT and state), that is the overpayment above.
The Form 8949 fix
- Column (d) Proceeds: $6,000.00
- Column (e) Cost basis (as on 1099-B): $0.00
- Column (f) Code: B
- Column (g) Adjustment: -$5,000.00
- Column (h) Gain (result): $1,000.00
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How it works
- Enter shares sold and FMV at vest. How many shares you sold, and the fair-market value per share on the day they vested. That FMV is your real cost basis (IRC §83(a)).
- Enter the sale price and the basis your broker reported. The price per share you sold at, plus the cost basis shown in box 1e of your 1099-B — for RSUs this is usually $0.
- Pick holding period, filing status, and income. Held more than a year after vest is long-term; otherwise short-term. Your income sets the capital-gains rate and whether NIIT applies.
- Read your overpayment and the Form 8949 fix. See the tax you would overpay without correcting basis, and the exact column (g) adjustment with code B to enter.
Frequently asked questions
Why does my 1099-B show $0 cost basis for RSUs?
Under IRC §6045(g) brokers report only the basis from what you paid to acquire the shares. RSUs are acquired for $0 cash, so box 1e is frequently $0 — even though your real basis is the fair-market value at vest, which was already included in your W-2 wages (IRS Pub 550, Pub 551).
How does this double-tax me?
The FMV of the shares at vest is ordinary income on your W-2 under IRC §83(a) — you already paid wage tax on it. If you then report the sale with a $0 basis, that same value is taxed a second time as a capital gain. The fix prevents the second tax.
How do I actually fix it?
On Form 8949, report the proceeds and the basis your broker reported (box 1e), then enter adjustment code B in column (f) and a negative adjustment in column (g) that raises your basis to the FMV at vest. See the IRS Form 8949 Instructions. This calculator shows the exact column (g) figure.
What is my correct cost basis?
The fair-market value per share on the vest date multiplied by the number of shares (IRC §83(a); IRS Pub 525). You can usually find the per-share vest price on your vest confirmation, a pay stub, or your broker’s supplemental (not the official 1099-B) statement.
Does the 3.8% NIIT apply?
It can. The Net Investment Income Tax (IRC §1411) adds 3.8% on investment income — including capital gains — once modified AGI exceeds $200,000 (single) or $250,000 (married filing jointly). This calculator estimates that portion using your income as a MAGI proxy.
Can I fix prior years where I already overpaid?
Generally yes — you can amend an open year with Form 1040-X, usually within three years of filing. This is an educational estimate, not tax advice; confirm your specific situation with a CPA before amending.
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