Basis Not Reported to IRS: Reconstruction and Form 8949 Reporting

When a Form 1099-B shows that the cost basis was not reported to the IRS, your broker told the agency how much you received from the sale but did not tell it what you originally paid. The IRS has no way to compute your profit from that information alone, and its systems will treat the full sale amount as taxable gain unless you supply the missing purchase figure on your return. Filling in the correct basis is your job, and the difference between doing it right and leaving it at zero can be thousands of dollars in tax you don’t actually owe.

What the Notation Actually Means

Cost basis is what you paid for an investment, including commissions and fees at purchase.1Internal Revenue Service. Topic 703 – Basis of Assets The IRS taxes only the difference between your sale proceeds and that basis. Buy 200 shares at $50, sell them at $80, and you owe tax on $6,000 of gain, not on the $16,000 of proceeds. Leave the basis blank, and the IRS computer treats every dollar of proceeds as gain.

Basis rarely stays put. Reinvested dividends add to it because each reinvestment is a new purchase. Stock splits spread it across more shares. Return-of-capital distributions reduce it, because they hand back part of your original investment rather than paying earnings. The running total after these events is your adjusted basis, and that’s the number that belongs on your return.1Internal Revenue Service. Topic 703 – Basis of Assets

Why the Basis Is Missing

Brokers weren’t always required to track what you paid. Congress phased in mandatory basis reporting by asset type, and anything acquired before the applicable cutoff is a “non-covered security” your broker has no duty to report basis on.2Office of the Law Revision Counsel. 26 U.S. Code 6045 – Returns of Brokers The cutoffs:

  • Corporate stock purchased on or after January 1, 2011.
  • Mutual funds and dividend reinvestment plan shares purchased on or after January 1, 2012.
  • Bonds, options, and most other securities purchased on or after January 1, 2013.

Anything bought before the applicable date is non-covered, and the “basis not reported to IRS” box gets checked on the 1099-B. If you’ve held a mutual fund or a stock position for decades, this is the usual reason.

Cutoff dates aren’t the only cause. Securities received as a gift, acquired by inheritance, or transferred between accounts in ways that broke the broker’s record chain can also arrive without basis, even if the acquisition happened after the covered-security dates. Brokers report what they can document; when the paper trail is incomplete, reporting falls to you.

Inherited Shares

Inherited investments generally take a basis equal to the fair market value on the previous owner’s date of death, not what they originally paid.3Office of the Law Revision Counsel. 26 U.S. Code 1014 – Basis of Property Acquired From a Decedent This “step-up” often produces a much higher basis than the decedent’s original cost. Brokers almost never have the documentation to calculate it, because the value depends on the death date and sometimes an appraisal. Some large estates elect an alternate valuation date six months after death, in which case that later value governs.4Internal Revenue Service. Gifts and Inheritances FAQ Either way, you have to look up the price yourself and hold onto the estate paperwork.

Gifted Shares

Gifts use the donor’s adjusted basis when the investment was worth more than the donor’s cost at the time of the gift. You carry over whatever they paid, plus any adjustments. If the investment was worth less than the donor’s basis on the gift date, a dual-basis rule kicks in: use the donor’s basis to figure a gain, and the fair market value on the gift date to figure a loss. If your sale price lands between the two figures, you report neither a gain nor a loss.5Internal Revenue Service. Publication 551 – Basis of Assets You need both numbers to file correctly.

How to Reconstruct the Missing Number

Start with the purchase itself. Old brokerage statements, trade confirmation slips, and online account history are the primary sources. Many firms keep records 10 to 15 years and will retrieve older ones on request. If the original broker no longer exists, the acquiring firm usually inherited the files. For very old holdings, historical stock price databases can identify the price on a known purchase date, and issuing companies’ investor relations departments often have historical pricing and corporate action data.

Then adjust that starting figure for everything that happened while you owned the position:

  • Reinvested dividends. Every reinvestment was a separate purchase at a specific price. Add all of them to your basis.
  • Stock splits. A 2-for-1 split doubles your share count and halves the per-share basis; the total is unchanged.
  • Return of capital. These distributions reduce basis. Once cumulative returns of capital exceed your original basis, the excess becomes taxable gain.
  • Wash sales. A loss disallowed under the wash sale rule gets added to the basis of the replacement shares.6Internal Revenue Service. Income – Capital Gain or Loss Workout

Choosing Which Lot You Sold

When you sell only part of a position, the specific shares you sold determine your basis. If you can identify the lot, the specific identification method lets you pick the one that produces the best tax result. Without that identification, the IRS treats the oldest shares as sold first (FIFO).7Internal Revenue Service. Stocks (Options, Splits, Traders) 3 Mutual funds also allow an average basis method. The choice can matter a lot when you accumulated shares over years at very different prices.

When the Records Are Truly Gone

If a genuine search turns up nothing, you aren’t automatically stuck reporting a zero basis. Longstanding tax practice allows a reasonable estimate when primary records don’t exist, so long as the estimate rests on some factual foundation. Historical price data for the approximate purchase date, a pattern of regular purchases on bank statements, or credible testimony about how and when you acquired the shares can support a number. The estimate has to be defensible, and the burden is on you to show it. Avoidable record loss gets less benefit of the doubt than genuinely unrecoverable history.

Reporting the Sale on Form 8949

A sale where basis wasn’t reported goes on Form 8949, which flows to Schedule D.8Internal Revenue Service. About Form 8949 – Sales and Other Dispositions of Capital Assets The form sorts transactions by whether basis was reported, and the correct checkbox matters. Choose the wrong one and you can trigger an automatic mismatch notice.

For sales where you received a 1099-B but the basis was not reported:

Box C and Box F are for sales where you didn’t get a 1099-B at all. Because brokers still report gross proceeds on non-covered securities and only skip the basis, most missing-basis situations belong in Box B or Box E. This is a common filing error.

Put the sale proceeds from the 1099-B in column (d) and your calculated adjusted basis in column (e). The gain or loss goes in column (h).10Internal Revenue Service. Form 8949 – Sales and Other Dispositions of Capital Assets When basis was simply missing and you’re supplying it for the first time, enter it in column (e) with $0 in column (g), and leave column (f) blank unless you have a separate adjustment such as a wash sale.11Internal Revenue Service. 2025 Instructions for Form 8949 Code B in column (f) is for a different situation: correcting a basis your broker did report but reported incorrectly.

What Happens If You Get It Wrong

Leaving basis blank or reporting zero overstates your tax. The IRS won’t penalize you for overpaying, but you’re handing over money you don’t owe. The real risk runs the other way: claiming a basis you can’t support and understating tax as a result.

When the IRS finds an understatement caused by an inflated basis, and the underpayment exceeds the greater of $5,000 or 10% of the tax that should have been on the return, a 20% accuracy-related penalty applies to the underpaid amount.12Office of the Law Revision Counsel. 26 U.S. Code 6662 – Imposition of Accuracy-Related Penalty on Underpayments On a $50,000 underpayment, that’s $10,000 on top of the tax and interest.

You can avoid the penalty by showing reasonable cause and good faith. The IRS weighs the effort you made to get the number right: whether you gathered available records, consulted a tax professional, or used a defensible estimation method.13eCFR. 26 CFR 1.6664-4 – Reasonable Cause and Good Faith Exception A round number pulled from thin air fares much worse than one built from historical price data with a documented method.

How Long to Keep the Proof

The IRS generally has three years from your filing date to assess additional tax. If income was understated by 25% or more, the window stretches to six years. There’s no statute of limitations if you never filed or filed fraudulently.14Internal Revenue Service. Time IRS Can Assess Tax

For any sale where basis wasn’t reported, hold every document that supports the number you claimed for at least seven years after filing the return that reports the sale. That covers the six-year window with room to spare. Keep original trade confirmations, statements showing reinvested dividends, corporate action notices for splits or mergers, and any worksheets or historical price printouts you used to rebuild a missing figure. For inherited assets, keep the death certificate and estate valuation documents. For gifts, keep whatever you have on the donor’s original cost and the fair market value on the date you received the shares. Once those papers are gone, nothing replaces them.