Form 1099-B Proceeds vs Cost Basis Explained

The Form 1099-B is the IRS document that reports the gross proceeds from securities sales (the total sale price) and the adjusted cost basis (what you paid, plus adjustments). The difference between these two numbers is your realized gain or loss—the figure that determines your tax liability. Reconciling 1099-B data with your own records is critical because discrepancies between broker reports and your Schedule D can trigger IRS scrutiny.

The Two Core Numbers on Form 1099-B

When a broker sells a security on your behalf, it must report the transaction to the IRS using Form 1099-B. The form contains two critical figures:

Gross Proceeds (Box 1d): The total sale price of the security before any deductions. If you sold 100 shares at $50 per share, the gross proceeds are $5,000. This is the gross, not net—it includes any transaction costs paid by the broker on your behalf, though typically the proceeds shown are the amount you actually received or that was credited to your account.

Adjusted Cost Basis (Box 1e): The cost basis is what you originally paid for the security, adjusted for stock splits, return of capital, and other corporate actions. For example, if you bought 100 shares at $30 per share for $3,000 total, and the company later issued a 2-for-1 stock split, your cost basis would be adjusted to $1,500 (same total cost, now divided among 200 shares). The word “adjusted” is key: cost basis is not simply your purchase price.

How Gain or Loss Is Calculated

Your realized gain or loss is the difference:

Gain (or Loss) = Gross Proceeds − Adjusted Cost Basis

Using the earlier example:

Gross Proceeds: $5,000
Adjusted Cost Basis: $3,000
Realized Gain: $2,000

This $2,000 is what you owe tax on (subject to long-term capital gains or short-term capital gains rates, depending on how long you held the security).

If the proceeds were $2,500 and the basis was $3,000:

Gross Proceeds: $2,500
Adjusted Cost Basis: $3,000
Realized Loss: −$500

A $500 loss can offset other gains or, if you have no gains, up to $3,000 of ordinary income in the year of sale (with excess losses carried forward).

Cost Basis Methods: How It’s Determined

The adjusted cost basis depends on which basis method you used when you sold the shares. The most common methods are:

FIFO (First In, First Out): You sold the oldest shares first. If you bought 50 shares at $20 in 2020 and 50 shares at $40 in 2024, and sold 50 shares in 2025, FIFO assumes you sold the 2020 batch, so your basis is $20 per share, or $1,000 total. This method is the IRS default if you don’t specify another method.

LIFO (Last In, First Out): You sold the most recently purchased shares first. Using the same example, you’d have sold the 2024 batch, with a basis of $40 per share, or $2,000 total. LIFO often produces lower gains (and thus lower tax) in inflationary markets, though it’s not allowed for securities in most cases.

Specific Identification: You choose which specific lot to sell. This gives you the most control. You might sell the highest-cost-basis shares to minimize gain, or the lowest-cost shares to harvest losses. You must tell your broker which lot before the sale, and document it for the IRS.

Average Cost: You calculate the average price of all your shares. This is common for mutual funds and ETFs. If you bought shares at $20, $25, and $30, your average cost is $25 per share.

The broker reports the cost basis on Form 1099-B using the method you specified (or FIFO by default). If you didn’t specify, the broker will use FIFO, which may not be optimal for your tax situation.

Why Reconciliation Matters

Your broker’s Form 1099-B may not match your own records for several reasons:

Basis Adjustments: Corporate actions like stock splits, spin-offs, mergers, or dividend reinvestments adjust cost basis. The broker calculates these, but errors can occur. For example, a rights offering or return-of-capital dividend should reduce your basis, but the broker’s system might not always reflect it correctly.

Missing Transactions: If you’ve held a security for many years and moved it between brokers, the cost basis might be incomplete. An old lot from a prior brokerage might not carry complete adjustment history.

Wash Sales: If you sold a security at a loss and bought a substantially identical security within 30 days before or after, the loss is disallowed and added to the basis of the new purchase. The broker attempts to account for this, but errors in wash-sale calculations are common, especially across multiple accounts.

Rounding and Timing: Minor discrepancies from rounding or different dates for reporting (trade date vs. settlement date) can create small mismatches.

Form 8949 and Schedule D: Reconciling Discrepancies

You report gains and losses on your Schedule D (Capital Gains and Losses), and you must attach Form 8949 (Sales of Capital Assets) if there are discrepancies between your 1099-B and your reported amounts.

If your records show a cost basis of $3,000 for shares but the 1099-B shows $2,500, you have a reconciliation problem. You must:

1. Report the 1099-B figures on Form 8949
2. In the “Adjustment” column, add or subtract the difference
3. Attach a note explaining the discrepancy

The IRS matches your return against the 1099-B data it receives from the broker. If your Schedule D doesn’t match, you may receive a notice asking you to explain or amend.

Why Cost Basis Is Important (or Goes Missing)

If your broker fails to report cost basis—or reports it as zero—the IRS could deem the entire sale proceeds as gain, which is devastating. For example, selling a stock for $5,000 with unreported basis means the IRS initially treats it as a $5,000 gain.

This happened frequently in the past:

• Inherited securities: Basis is “stepped up” to fair market value at the date of death, but if you don’t document it properly, the broker may report zero basis.
• Mutual funds bought before brokers reported basis: If you bought a fund in the 1990s and transferred it to a new broker in the 2010s, the new broker might not have historical cost data.
• Options and cryptocurrency: Early brokers didn’t report cost basis for these assets (though rules have since tightened).

The Broker’s Obligation (and Your Burden)

Under current IRS rules (tightened in 2011 via the Dodd-Frank Act), brokers must report cost basis for equities, options, and mutual funds. For cryptocurrency and certain other assets, cost basis reporting is newer or incomplete.

But the burden is ultimately on you. The 1099-B is a starting point, not gospel. You must:

• Keep your own records of purchase price, purchase date, and any basis adjustments
• Verify the broker’s 1099-B against your records
• Report any discrepancies on Form 8949
• File Form 1099-B adjustments if the IRS challenges your return

Worked Example: Reconciling a Discrepancy

You sold 100 shares of XYZ Corp. for $5,000 (gross proceeds).

Your records:

• 50 shares bought at $20 in 2015 (cost basis $1,000)
• 50 shares bought at $30 in 2020 (cost basis $1,500)
• Sold using specific identification, selling the 2020 batch (basis $1,500)
• Realized gain: $5,000 − $1,500 = $3,500

The broker’s 1099-B shows:

• Gross proceeds: $5,000
• Cost basis: $2,000 (broker incorrectly used FIFO, reporting both lots)

On Form 8949, you report:

• Proceeds: $5,000
• Cost basis: $1,500 (your specific ID election)
• Adjustment: −$500 (to correct the broker’s overstatement of basis)
• Gain: $3,500

You attach a note explaining that you used specific identification to sell the 2020 shares only, and the broker’s FIFO calculation was overridden.

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