What this line means
Each row in Part I reports a single short-term capital asset transaction — an asset you held for one year or less before selling or disposing of it. For every transaction you list the description of the property, the date you acquired it, the date you sold it, the proceeds (column d), your cost or other basis (column e), any adjustment code (column f), the adjustment amount (column g), and the resulting gain or loss (column h).
Before entering any transactions you check one of three boxes at the top of Part I. Check Box A if your broker reported the cost basis to the IRS on your 1099-B. Check Box B if your broker did not report basis to the IRS. Check Box C if you did not receive a 1099-B at all. You file a separate copy of Form 8949 for each box type. 1
Does this apply to you?
- You sold stocks, ETFs, bonds, or mutual fund shares that you held for one year or less
- You received a 1099-B from a brokerage showing short-term sales
- You sold cryptocurrency that you held for one year or less
- You disposed of a capital asset through a taxable exchange, redemption, or worthlessness event within one year of acquiring it
- You need to report adjustments to basis or gain that differ from what your broker reported on the 1099-B
Easy to overlook
Separate copies of Form 8949 for each box type If your 1099-B has some transactions with basis reported (Box A) and others without basis reported (Box B), you need two separate copies of Form 8949 Part I — one for each box. Mixing box types on a single form causes the IRS matching system to flag every transaction on that page. 1 IRS Form 8949 instructions — Reporting short-term transactions
1099-B forms from brokers you forgot about Robo-advisors, old brokerage accounts, and employer stock plan platforms all send 1099-Bs. The IRS receives every one. Filers who skip even one 1099-B get a CP2000 notice months later showing unreported proceeds and proposing extra tax based on zero cost basis. 2 CP2000 pattern — unreported 1099-B proceeds
Wash sale adjustments that need manual entry If you sold a stock at a loss and repurchased substantially identical shares within 30 days before or after the sale, the loss is disallowed under wash sale rules. Your broker flags some wash sales on the 1099-B, but cross-account wash sales (selling at one broker and buying at another) are your responsibility to report using code W in column f. 1 IRS Form 8949 instructions — Reporting short-term transactions
Watch out for this
Leaving cost basis blank or entering zero when your broker did not report basis to the IRS. If you check Box B and leave column e empty, the IRS treats the entire sale proceeds as taxable gain. You still know what you paid for the asset — dig up trade confirmations, account statements, or dividend reinvestment records to establish your basis. Reporting zero basis when your actual basis is $5,000 turns a $200 gain into a $5,200 gain on your return.
Footnotes
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IRS Form 8949 Instructions, Sales and Other Dispositions of Capital Assets. https://www.irs.gov/instructions/i8949 ↩ ↩2 ↩3
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IRS CP2000 Notice, 1099-B Proceeds Matching. https://www.irs.gov/individuals/understanding-your-cp2000-notice ↩