What Happens If Your 1099-B Shows the Wrong Cost Basis? (w/Examples) + FAQs

This article reflects federal IRS rules and general state guidance as of June 2026 and covers tax year 2025 (the 2026 filing season). Tax law changes — confirm current figures and forms before you file. This guide is educational and is not a substitute for advice from a licensed CPA, enrolled agent, or tax attorney for your specific situation.

Quick Answer

Nothing automatic happens — but you must fix it yourself before you file. For tax year 2025, if your 1099-B shows the wrong cost basis, you report the broker’s figure, then correct it on Form 8949 using column (g) and an adjustment code. A wrong basis left alone usually means you overpay tax.

A wrong cost basis is one of the most common errors on a 1099-B, and it almost always works against you. When your basis is reported too low — or as $0 — the IRS thinks you made a bigger profit than you really did, so you get taxed on money you never earned. This happens most often with restricted stock units (RSUs), employee stock purchase plan (ESPP) shares, inherited stock, and shares moved between brokers.

The fix is not hard, but the timing matters. You correct the number on Form 8949 before you file, or you amend with Form 1040-X after you file. According to the IRS, the agency sends millions of automated CP2000 underreporter notices each year, many triggered by 1099 mismatches — so an uncorrected basis can also invite a letter you do not want.

💸 How a wrong (or $0) basis quietly makes you overpay — and how to stop it
🧾 The exact Form 8949 columns, codes, and math to correct it, line by line
🏷️ Why RSUs, ESPPs, inherited stock, and transferred shares cause this most
📨 What to do if you already filed and the IRS sends a CP2000 notice
🗺️ Whether your state return needs the same fix, plus deadlines and costs

What “Cost Basis” Means and Why the 1099-B Gets It Wrong

Cost basis is what you paid for an investment, used to figure your gain or loss when you sell. The simple version: sale price minus cost basis equals your taxable gain. If your basis is wrong, your gain is wrong, and your tax bill is wrong. The IRS explains basis fundamentals in Publication 551, Basis of Assets.

A Form 1099-B is the tax form your broker sends (by mid-February) listing each sale, the proceeds, and — sometimes — the cost basis. The “sometimes” is the heart of the problem. Brokers are only required to track and report basis for what the IRS calls covered securities, meaning shares bought after the rules took effect (2011 for stocks, 2012 for mutual funds and dividend-reinvestment shares, and 2014–2016 for certain bonds and options). For everything else — noncovered securities — the broker may report nothing, leave the box blank, or guess.

The consequence is direct: when basis is missing or low, the broker reports a higher gain to the IRS than you actually have, and the IRS’s computers expect tax on that inflated number. A real-world example: Maria’s broker reports a $9,000 stock sale with a blank basis, so the IRS treats the entire $9,000 as profit — even though Maria paid $7,000 for the shares and her true gain is only $2,000. A common misconception is that “the broker reports it, so it must be right.” It is not always right, and you sign the return — so the error is legally yours to fix. What you should do: compare every 1099-B line against your own records and your broker’s supplemental statement before you file.

Covered vs. Noncovered Securities

This single distinction explains most wrong-basis cases. For covered securities, the broker reports basis to the IRS, and the IRS sees both your proceeds and your basis. For noncovered securities, the broker reports proceeds but not basis, so the IRS sees only the sale amount — and assumes the worst if you do nothing. Vanguard’s plain-English guide breaks this down well.

The consequence differs by type. For a noncovered share with no basis reported, the IRS is not “wrong” — it simply has no basis on file, so you must supply the correct number yourself or be taxed on the full proceeds. The deadline is your filing deadline: April 15, 2026, for a 2025 return. What to do: find the share’s true basis (your purchase records, the date-of-death value for inherited shares, or your employer’s supplemental statement) and enter it on Form 8949.

Security Type	How the IRS Sees It
Covered (bought 2011+)	Broker reports both proceeds and basis; goes in box A or D on Form 8949
Noncovered (older or transferred)	Broker reports proceeds only; goes in box B or E; you supply basis

Which Situation Applies to You?

The right fix depends on why your basis is wrong. Find your situation below, then jump to the matching example and the Form 8949 walkthrough.

You have RSUs or ESPP shares from an employer — your basis is almost always reported as $0 or as the raw purchase price, leaving out income already taxed on your W-2. See the RSU and ESPP example.
You inherited stock — your basis should “step up” to the value on the date of death, but the broker may show the original owner’s old cost or nothing. See the inherited-stock example.
You transferred shares between brokers — the new broker often loses the basis and labels the shares “noncovered.” See the transferred-shares note.
You had a wash sale — a disallowed loss must be added to the basis of the replacement shares, and the 1099-B may not reflect it. See the wash-sale section.
You already filed with the wrong number — you cannot edit Form 8949 now; you must amend with Form 1040-X. See the “already filed” section.

How to Correct the Wrong Basis on Form 8949

The IRS built a specific process for this, and the 2025 Instructions for Form 8949 spell it out. The rule is firm: do not overwrite the broker’s number when the basis was reported to the IRS. Instead, enter the broker’s basis as shown, then make an adjustment. Skipping this step and just typing your own number triggers a mismatch with the IRS computers and can generate a CP2000 notice.

Here is the column-by-column flow for a transaction that needs a basis fix. Each Form 8949 line has these key columns: (d) proceeds, (e) cost basis as reported, (f) adjustment code, (g) adjustment amount, and (h) gain or loss. Get these right and Schedule D and your 1040 follow automatically.

Column (e) — cost or other basis. If your sale is in box A or D (basis was reported to the IRS), enter the broker’s basis here exactly as shown, even though it is wrong. The IRS instructions are explicit about this.
Column (f) — code(s). Enter B to flag “the basis shown on the 1099-B is incorrect.” You can stack codes — for example, B plus W for a wash sale — separating them with spaces.
Column (g) — adjustment. Enter the dollar amount that fixes the gain. If the broker’s basis was too low, you increase basis, which lowers your gain, so column (g) is a negative number (a number in parentheses).
Column (h) — gain or loss. Compute it as proceeds (d) minus basis (e) plus adjustment (g). This is the number that actually flows to Schedule D.

Worked Numeric Example — RSU With $0 Basis

This is the single most common wrong-basis case, and the math is the article’s whole point, so follow it dollar by dollar. As this RSU tax guide explains, your true RSU basis is the fair market value on the vesting day — the amount already taxed as wages on your W-2.

Say David’s 100 RSU shares vested when the stock was $50, so $5,000 was added to his W-2 wages and taxed as ordinary income. He sells all 100 shares months later for $5,400. His broker’s 1099-B reports $5,400 proceeds and $0 basis (a covered sale, box A).

Step 1 — broker’s reported gain: $5,400 − $0 = $5,400 (wrong; this double-taxes the $5,000 already on his W-2).
Step 2 — true basis: 100 shares × $50 vesting price = $5,000.
Step 3 — true gain: $5,400 − $5,000 = $400.
Step 4 — on Form 8949, enter $0 in column (e), code B in (f), and ($5,000) in column (g).
Step 5 — column (h): $5,400 − $0 + (−$5,000) = $400.

David is now taxed on a $400 gain, not $5,400. At a 15% long-term capital gains rate, that is the difference between owing about $60 and owing about $810 — a $750 swing on one sale. The records to keep: his employer’s supplemental stock plan statement, which lists the per-share basis the broker left off.

Worked Numeric Example — Inherited Stock Step-Up

Inherited assets get a “stepped-up” basis equal to the fair market value on the decedent’s date of death, under Internal Revenue Code Section 1014. Brokers often miss this and show the original purchase price or a blank.

Lena inherits 200 shares her father bought decades ago for $4 each ($800 total). On his date of death, the shares were worth $60 each, so her stepped-up basis is $12,000. She sells them for $12,500. The broker — a noncovered sale, box E — reports $12,500 proceeds and no basis.

True gain with step-up: $12,500 − $12,000 = $500.
If Lena does nothing, the IRS treats the basis as $0 and taxes the full $12,500.
For a noncovered sale, she enters her correct $12,000 basis directly in column (e); no code B is needed because the broker did not report a basis to the IRS.
The records to keep: a date-of-death valuation (broker historical price or an appraisal).

The Wash-Sale Wrinkle

A wash sale happens when you sell at a loss and buy the same or substantially identical security within 30 days before or after. The IRS disallows the loss for now, but it is not gone — it is added to the basis of the replacement shares, per the wash-sale rule in IRC Section 1091. Brokers report wash sales on covered shares, but they do not track them across two different brokers or across your IRA, so the basis can come out wrong.

The consequence of ignoring it cuts both ways. If a disallowed loss is not added to your replacement shares’ basis, you lose the benefit later and overpay when you finally sell. A common misconception is that a wash sale erases your loss permanently — it only defers it. On Form 8949 you report the loss with code W and enter the disallowed amount as a positive number in column (g); when you later sell the replacement shares, you increase their basis by that same amount. What to do: reconcile trades across all your accounts, not just one broker’s 1099-B.

What If You Already Filed With the Wrong Basis?

If you already filed and overpaid because of a low basis, you can amend with Form 1040-X, Amended U.S. Individual Income Tax Return, and attach a corrected Form 8949 and Schedule D. You generally have three years from the original filing date (or two years from when you paid the tax, whichever is later) to claim the refund — so a 2025 return filed in April 2026 can usually be amended through about April 2029.

The flip side is a notice. If you underpaid — say you used too high a basis — or the IRS sees a mismatch, it mails a CP2000 notice proposing extra tax, penalties, and interest. A CP2000 is not a bill and not an audit; it is a proposed change, and you have 30 days to respond. What to do: if the CP2000 is wrong because the basis was understated, respond by the deadline with your Form 8949 math and supplemental statements showing your true basis — often the proposed tax drops to zero. Missing the 30-day window lets the IRS finalize the assessment, after which fixing it is harder and may require Form 1040-X or Tax Court.

Three Common Scenarios and What Happens

These are the three situations readers hit most, built from real filer questions.

Wrong-Basis Situation	What Happens If You Don’t Fix It
RSU/ESPP sale shows $0 basis	You pay capital gains tax a second time on income already taxed on your W-2
Inherited stock shows the old cost or blank	You lose the date-of-death step-up and pay tax on decades of gain you never owed
Shares transferred to a new broker, now “noncovered”	The IRS sees full proceeds as gain; you owe tax on money you already paid for the shares

Federal vs. State: Does Your State Need the Same Fix?

Start with federal, then check your state — they are not always the same. Most states that have an income tax start from your federal taxable income or federal adjusted gross income, so when you correct the basis on your federal Form 8949, the corrected gain usually flows straight onto your state return with no separate state form. The federal fix is the engine; the state return rides along.

But the details vary, and guessing costs money. A handful of states have no income tax at all — including Florida, Texas, Washington (which taxes only certain high capital gains), Nevada, South Dakota, Wyoming, Alaska, and Tennessee — so a wrong 1099-B basis has no state tax effect there. Other states diverge sharply: states with their own basis or depreciation rules, or that did not conform to certain federal provisions, can require a separate adjustment.

The consequence of assuming conformity is a mismatched state return and a state notice of its own. If you amend federally with Form 1040-X, you almost always must file a matching state amended return too, on the state’s own form, by the state’s own deadline. What to do: after you fix the federal number, open your state’s department of revenue page, confirm it starts from federal income, and check whether any state-specific basis adjustment applies. When the dollars are large or you live in a state that does not conform, this is the point to bring in a CPA.

Mistakes to Avoid

Each of these turns a simple fix into a costly one.

Leaving a $0 or low basis alone. The outcome: you pay capital gains tax on money you never made, sometimes thousands of extra dollars.
Overwriting the broker’s basis in column (e) on a covered sale. The outcome: your numbers no longer match the broker’s, triggering a CP2000 mismatch notice.
Forgetting the adjustment code in column (f). The outcome: the IRS cannot tell why your gain differs and may reject or question the adjustment.
Using the original owner’s cost for inherited shares. The outcome: you skip the step-up under Section 1014 and overpay on gain that legally vanished at death.
Ignoring wash sales across multiple accounts. The outcome: a disallowed loss is never added to replacement-share basis, so you permanently lose a deduction.
Missing the supplemental statement for RSUs and ESPPs. The outcome: you file the $0 basis and double-pay tax on W-2 income.
Blowing the CP2000 30-day deadline. The outcome: the IRS finalizes the extra tax, plus penalties and interest, and your easy fix becomes an amended return or appeal.
Forgetting the matching state amended return. The outcome: your state still has the wrong gain on file and sends its own notice.

Do’s and Don’ts

Do compare every 1099-B line to your own records and broker supplemental statements — because the broker’s basis is wrong often enough to check every time.
Do enter the broker’s basis as shown on covered sales, then adjust in column (g) — because that is the exact method the IRS instructions require.
Do keep your vesting statements, purchase confirmations, and date-of-death valuations — because the burden of proving basis falls on you.
Do amend with Form 1040-X if you already overpaid — because you can usually still recover the refund within three years.
Do respond to a CP2000 by the deadline with your math — because a timely, documented reply often zeroes out the proposed tax.
Don’t assume “the broker reported it, so it’s correct” — because covered-vs-noncovered gaps make blank and $0 bases common.
Don’t type your own number straight into column (e) on a covered sale — because that creates the mismatch you are trying to avoid.
Don’t treat a wash sale as a lost deduction forever — because the loss is only deferred into the replacement shares’ basis.
Don’t ignore your state return after a federal fix — because most states piggyback on federal income and need the same correction.
Don’t wait past the statute of limitations to amend — because after roughly three years your refund is gone.

Pros and Cons of Fixing It Yourself

Pro — it’s free. DIY tax software handles the adjustment, so a self-fix costs nothing beyond the software you already bought.
Pro — you keep the refund. Correcting a $0 RSU basis can save hundreds or thousands in a single filing.
Pro — it builds a clean record. Proper Form 8949 entries make any future IRS question fast to answer.
Pro — it prevents double taxation. The adjustment stops you paying twice on W-2 income already taxed.
Pro — it’s faster than a notice. Fixing before filing avoids the months-long CP2000 back-and-forth.
Con — the codes confuse people. Mixing up codes B and W, or the sign in column (g), is easy and leads to wrong gains.
Con — multi-account wash sales are hard. Cross-broker reconciliation is genuinely tricky to do by hand.
Con — large estates or many lots get complex. Inherited portfolios with hundreds of lots can overwhelm a DIY filer.
Con — software limits. Some programs make it hard to enter code B in forms mode, frustrating users.
Con — mistakes have YMYL stakes. A wrong fix can replace one error with another and still draw a notice.

When the dollars are large, the lots are many, or wash sales cross accounts, hiring a CPA or enrolled agent — typically $200 to $600+ for a return with investment complexity — is usually money well spent.

What to Do Next

Take these steps in order before you file (or before your CP2000 deadline).

Pull every 1099-B and match each sale against your own records and any broker supplemental statement.
Flag any line showing $0, blank, or a basis that looks too low — especially RSUs, ESPPs, inherited, or transferred shares.
For covered sales, enter the broker’s basis in column (e), code B in (f), and the correction in column (g) on Form 8949; for noncovered sales, enter your correct basis directly.
Carry the totals to Schedule D and then to your Form 1040.
If you already filed, prepare Form 1040-X with a corrected Form 8949 and file a matching state amended return.
If you have a CP2000, respond within 30 days with your Form 8949 math and supporting statements.
For large or messy cases — many lots, cross-account wash sales, big estates — call a CPA or enrolled agent before the deadline.

FAQs

What does code B on Form 8949 mean? Code B means the basis on your 1099-B is incorrect. You enter the broker’s basis in column (e), put B in column (f), and the dollar correction in column (g). It tells the IRS exactly why your gain differs.

Will the IRS catch a wrong cost basis? Yes, often. IRS computers match your 1099-B proceeds against your return. If you report a gain that differs from what the broker reported without a code or adjustment, you may get a CP2000 notice proposing extra tax for tax year 2025.

Is a wrong cost basis the broker’s fault or mine? It’s legally yours to fix. Brokers only must track basis for covered securities, so blanks and errors are common. You sign the return, so the IRS holds you responsible for reporting the correct gain.

Why is my RSU cost basis $0? Because the broker often omits the vesting-day value. Your true RSU basis is the fair market value on the vesting date, already taxed on your W-2. Adjust it on Form 8949 so you are not taxed twice.

What is the cost basis for inherited stock? The fair market value on the date of death. Under IRC Section 1014, inherited shares get a stepped-up basis, erasing the prior owner’s gain. Use a date-of-death valuation, not the original purchase price.

Can I just change the basis number on my 1099-B? No, not on a covered sale. Enter the broker’s reported basis as shown, then correct it with an adjustment in column (g) and code B. Overwriting it directly causes an IRS mismatch.

What if I already filed with the wrong basis? File Form 1040-X to amend. Attach a corrected Form 8949 and Schedule D. For tax year 2025, you generally have three years to claim a refund from a basis that was reported too low.

How long do I have to respond to a CP2000? 30 days from the notice date. Respond in writing with your Form 8949 math and supplemental statements. A timely, documented reply often reduces the proposed tax to zero.

Does fixing my federal basis change my state return? Usually yes. Most income-tax states start from federal income, so the corrected gain flows to your state return. If you amend federally, file a matching state amended return too.

What’s the difference between covered and noncovered securities? Covered shares have basis reported to the IRS; noncovered do not. Covered sales go in box A or D and need code B to adjust; noncovered sales go in box B or E, where you enter the correct basis yourself.

Do crypto sales have this problem too? Yes, increasingly. Many exchanges report proceeds without accurate basis, and the new Form 1099-DA is phasing in digital-asset basis reporting. Until basis is fully reported, you may need to supply and adjust it yourself.

What records prove my correct cost basis? Purchase confirmations, vesting/supplemental statements, and date-of-death valuations. Keep brokerage statements and employer stock-plan documents. The burden of proving basis is on you if the IRS asks.

This article reflects federal IRS rules and general state guidance as of June 2026 and covers tax year 2025. Confirm current figures and forms before you file, and consult a licensed tax professional for your specific situation.

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