Why RSUs get double-taxed
When your RSUs vested, their full value was added to your W-2 and you paid ordinary income tax on it. That vesting value is your cost basis — the amount you are treated as having already paid for the shares. When you later sell, only the change in price since vesting should be a capital gain. The double-tax happens when that basis is reported as $0, so the IRS treats the entire sale as a gain and taxes the vesting value a second time.
Why brokers report $0
This is not a mistake you can blame on one broker — it is systemic. For shares acquired through equity compensation, brokers are often only required to report the amount you paid out of pocket, which for RSUs is $0 (you did not buy them). The value already taxed on your W-2 is real basis, but it does not appear on the 1099-B. If you do not correct it, you volunteer to be taxed twice.
How to fix it on Form 8949
The correction happens on Form 8949, which feeds Schedule D. For each RSU sale: report the proceeds and the broker's basis as shown, then use column (f) adjustment code B and column (g) to enter a negative adjustment that raises your basis to the correct vesting-date value. The result is that only your true gain — the change since vesting — is taxed. Most tax software has a specific flow for "the cost basis on my 1099-B is wrong / missing."
Finding your correct basis
Your correct per-share basis is the fair market value on the vesting date — the same price used to compute the income added to your W-2. Sources: your employer's stock-plan portal (E*Trade, Fidelity, Schwab, etc.) usually shows a "supplemental" or "adjusted cost basis" statement, your year-end pay stub or W-2 shows the RSU income, and the vesting confirmation shows the price. Multiply shares vested by the vesting-date price to get total basis.
How much it costs if you skip it
The overpayment equals your capital-gains rate times the vesting value. If $40,000 of RSUs vested and you sell soon after, filing a $0 basis could tax that entire $40,000 again — roughly $6,000 at a 15% long-term rate, or about $12,800 at a 32% short-term rate. On larger vests the mistake reaches five figures. It is one of the most expensive single line-items to get right on a return.
A quick example
200 shares vested at $50 = $10,000 of income (already on your W-2). You sell at $55 for $11,000. Your real gain is only $1,000. But if the 1099-B shows a $0 basis, it looks like an $11,000 gain. The Form 8949 adjustment restores the $10,000 basis so you are taxed on $1,000, not $11,000 — saving the tax on $10,000 you already paid income tax on.