What’s Your Cost Basis When You Exercise Stock Options? (w/Examples) + FAQs

This article reflects federal rules as of June 2026 and covers tax year 2025 (returns filed in 2026). State rules are addressed in their own section. Tax law changes — confirm current figures before you file.

Quick Answer

Your cost basis is the exercise price you paid plus any income already taxed on the spread. For non-qualified options (NSOs), basis equals exercise price plus the bargain element added to your W-2. For incentive stock options (ISOs) held through year-end, regular basis is just the exercise price, but AMT basis adds the spread.

When you exercise a stock option, you buy company shares at a fixed price. Your cost basis is the total you have “invested” in those shares for tax purposes, and it decides how much capital gain or loss you report when you sell. Getting it wrong is the single most expensive stock-option mistake people make, because brokers often report a basis that is too low — and that makes you pay tax twice on the same money.

The stakes are real and time-sensitive. The IRS reported tens of millions of returns with capital-gain transactions each year, and stock-comp returns are among the most frequently misfiled. If you exercised options in 2025 and you are filing now, the basis number on your Form 1099-B may be wrong — and only you can fix it on your return.

Here is what you will learn:

💵 How to calculate cost basis for NSOs and ISOs, step by step, with real dollar math.
⚠️ Why your 1099-B basis is often too low and how that triggers double taxation.
🧾 The exact Form 8949 fix (Code “B”) that lowers your taxable gain.
🧮 How ISO exercises create a second cost basis just for the Alternative Minimum Tax.
📅 The holding-period deadlines that decide whether you owe ordinary income or capital gains.

What “Cost Basis” Actually Means

Cost basis is the amount you have invested in an asset for tax purposes. When you sell, your capital gain or loss equals the sale price minus your cost basis. A higher basis means a smaller gain and less tax, so an accurate basis directly protects your money.

With stock options, basis is not simply the price you paid to exercise. It also includes any portion of the option’s value that was already taxed as ordinary income — usually as wages on your W-2. The rule exists to prevent double taxation: if you paid income tax on part of the stock’s value at exercise, that amount must increase your basis so you are not taxed on it again at sale.

The consequence of ignoring this is concrete. Morgan Stanley explains that cost basis “impacts the value” of your award, and a basis reported too low can cost you hundreds or thousands of dollars in overpaid tax. The fix is always available to you on your own return, but the IRS will not correct it for you.

The Two Option Types Drive Everything

Cost basis rules split sharply by option type. Before you calculate anything, identify which kind you hold — your grant agreement and your broker statements will say. The two federal categories are non-qualified stock options (NSOs), also called NQSOs, and incentive stock options (ISOs), also called statutory options.

Non-Qualified Stock Options (NSOs)

NSOs are the most common employee option and the simplest to tax. When you exercise, the difference between the fair market value (FMV) of the stock and your exercise price — the “bargain element” — is treated as ordinary wages. Schwab notes that your basis “is generally equal to the fair market value (FMV) at the time of the exercise.”

That happens because the spread is added to Box 1 of your W-2 and is taxed as income in the year you exercise. The consequence of forgetting this is double taxation: that spread is already taxed as wages, so it must also raise your basis. A common misconception is that basis equals only the cash you paid to exercise — it does not. What you should do is add the W-2 spread (often shown with Code “V” in Box 12) to your exercise price to get your true basis.

Incentive Stock Options (ISOs)

ISOs get preferential treatment but are far more complex. As FreeTaxUSA explains, a “qualifying ISO” generates no regular-tax income at exercise — you report income only when you sell, and if you meet the holding periods, the whole gain can be long-term capital gain.

The catch is the Alternative Minimum Tax. The spread at exercise is not income for regular tax, but it is an adjustment for AMT in the exercise year. The consequence is that ISOs create two different cost bases — one for regular tax (the exercise price) and a higher one for AMT (the FMV at exercise). A common misconception is that ISOs are “tax-free” at exercise; they are tax-free only for regular tax, not AMT. What you should do is track both bases from day one, because you will need both when you sell.

How Cost Basis Is Calculated: The Core Formulas

The universal rule, per tax guidance, is: cost basis equals exercise price paid, plus any compensation income already taxed, plus fees. How much “compensation income” gets added depends on the option type and on when you sell.

Situation	Your Correct Cost Basis
NSO exercise (any sale timing)	Exercise price + bargain element added to W-2
ISO, qualifying disposition (held long enough)	Exercise price only (for regular tax); AMT basis = FMV at exercise
ISO, disqualifying disposition (sold too soon)	Exercise price + ordinary income reported at sale

For ISOs, the holding period is decisive. A qualifying disposition requires holding the shares more than two years from grant and more than one year from exercise, as Schwab describes. Miss either deadline and you have a disqualifying disposition, which converts part of your gain into ordinary income — but also raises your regular-tax basis by that same ordinary-income amount, per RSM.

Worked Example 1: NSO Exercise and Sale

Maria holds NSOs with a $10 exercise price on 1,000 shares. She exercises in March 2025 when the FMV is $40. Her bargain element is ($40 − $10) × 1,000 = $30,000, which her employer adds to her W-2 wages and taxes as ordinary income that year.

Her cost basis is the exercise price plus the taxed spread: ($10 × 1,000) + $30,000 = $40,000, or $40 per share. In December 2025 she sells all 1,000 shares for $50,000. Her capital gain is $50,000 − $40,000 = $10,000, a short-term gain because she held under one year.

Here is the trap. Her broker’s 1099-B often lists basis as only the $10,000 she paid to exercise. If Maria uses that number, the IRS thinks her gain is $40,000 — taxing the $30,000 twice. Her correct gain is $10,000, and she must adjust the basis on her return to claim it.

Worked Example 2: ISO Held Long Enough (Qualifying)

David exercises ISOs in February 2025: 2,000 shares, $5 exercise price, FMV of $25 at exercise. For regular tax, he reports no income in 2025. But the spread of ($25 − $5) × 2,000 = $40,000 is an AMT adjustment he must report on Form 6251.

David holds the shares more than two years from grant and more than one year from exercise, then sells in 2027 for $80,000. His regular-tax basis is just the exercise price: $5 × 2,000 = $10,000. His long-term capital gain is $80,000 − $10,000 = $70,000, all taxed at favorable long-term rates.

For AMT, his basis is higher — the FMV at exercise, or $25 × 2,000 = $50,000 — so his AMT gain is only $30,000. This dual basis, described by GHJ advisors, is why David may also claim a minimum tax credit on Form 8801 in later years to recover AMT he paid in 2025.

Worked Example 3: ISO Sold Too Soon (Disqualifying)

Priya exercises ISOs in January 2025: 1,000 shares, $8 exercise price, FMV $21 at exercise. She sells in November 2025 for $30 per share — less than one year after exercise, so it is a disqualifying disposition.

Because she broke the holding period, the bargain element of ($21 − $8) × 1,000 = $13,000 becomes ordinary income on her 2025 return, per Schwab. Her corrected cost basis becomes the exercise price plus that ordinary income: $8,000 + $13,000 = $21,000.

She sold for $30,000, so her remaining capital gain is $30,000 − $21,000 = $9,000 (short-term). Her 1099-B will likely show basis of only $8,000, which would overstate her gain by $13,000 — the exact amount already taxed as wages. She must adjust her basis to avoid paying twice.

The 1099-B Double-Taxation Trap

This is the costliest stock-option error, and it hits almost everyone. Since 2014 rules, brokers report only your exercise price as basis in Box 1e of Form 1099-B for option shares — they are generally barred from including the compensation income added to your W-2.

The result is a basis that is too low, so the gain reported to the IRS is too high. Forbes describes this as one of the biggest mistakes on stock-comp returns. The consequence is direct overpayment: in Maria’s case above, accepting the broker number would tax an extra $30,000.

What you must do is correct the basis on your own return using Form 8949 — the IRS expects you to make this adjustment, and it is fully legal and routine. You are not changing the broker’s report; you are adding the income that was already taxed elsewhere.

How to Fix It on Form 8949 (Step by Step)

The correction lives on Form 8949 and flows to Schedule D. Follow the 2025 Form 8949 instructions exactly:

Enter the sale in the right section: short-term (held one year or less) or long-term (held more than one year).
In Column (e), enter the basis exactly as shown on your 1099-B — usually the too-low exercise price.
In Column (f), enter adjustment Code “B”, which signals the broker basis is incorrect, per the 1099-B fix guidance.
In Column (g), enter the missing income as a negative number — this raises your basis and lowers your gain.
Column (h) recalculates your correct gain or loss automatically.

The negative adjustment in Column (g) is the dollar amount of compensation income already on your W-2. The consequence of skipping it is overpaid tax; the consequence of using the wrong sign is an IRS notice. Keep your Form 3922 or 3921, broker statements, and final pay stub to prove the number.

Which Situation Applies to You?

Your next move depends on your facts. Use this branch to find your path.

You exercised NSOs in 2025: your spread is on your W-2; add it to your exercise price and check your 1099-B for a too-low basis.
You exercised ISOs and still hold the shares at year-end: you owe no regular tax, but compute the AMT adjustment on Form 6251 for 2025.
You exercised ISOs and sold within the same year (or before the deadlines): you have a disqualifying disposition; expect ordinary income and a basis adjustment.
You exercised and sold in a cashless/same-day transaction: you almost certainly have a tiny gain or small loss, and the 1099-B basis is the classic too-low trap.
You exercised ISOs in a prior year and paid AMT: check whether you can claim the minimum tax credit on Form 8801 this year.

Cost Basis: ISO vs. NSO at a Glance

The difference between the two option types comes down to when income is taxed and which basis you track.

Feature	NSO	ISO
Income at exercise (regular tax)	Yes — bargain element on W-2	No — nothing for regular tax
AMT adjustment at exercise	No	Yes — spread is AMT income
Regular-tax basis	Exercise price + W-2 spread	Exercise price only
AMT basis	Same as regular basis	FMV at exercise (higher)
Holding period for best rate	One year to sell for long-term gain	Two years from grant + one year from exercise

Deadlines, Timing, and Costs

For 2025 exercises, the return is due April 15, 2026 (or October 15, 2026 with an extension). The ISO holding-period clocks — more than two years from grant and more than one year from exercise — decide your tax rate, and missing them turns capital gain into higher-taxed ordinary income.

Cost matters too. A DIY return using tax software handles the Form 8949 Code “B” adjustment for a modest fee, often under $100. A CPA who specializes in equity compensation typically charges several hundred to a few thousand dollars, but is worth it if you exercised a large ISO block, owe AMT, or face a multi-year minimum tax credit recovery.

If you exercised ISOs and the spread pushed you into AMT, the timing of any future sale also affects your AMT credit recovery. Run a midyear projection on Form 6251 before December so you are not surprised in April.

AMT and ISOs: The Numbers for 2025

For tax year 2025, the AMT exemption is $88,100 for single filers, $137,000 for married filing jointly, and $68,500 for married filing separately, per The Tax Adviser. The exemption phases out once AMT income exceeds $626,350 (single) or $1,252,700 (joint), as Carta confirms.

AMT income is taxed at 26% up to $239,100 and 28% above that for 2025. A large ISO spread can erase your exemption and trigger AMT in the exercise year, which is exactly why ISOs need a separate AMT basis. The good news, noted by a CPA discussion, is that AMT you pay on an ISO exercise often becomes a minimum tax credit you can recover in later years on Form 8801.

These thresholds are inflation-indexed and were made permanent by 2025 law, but Porte Brown warns that a 2026 phase-out reset to lower thresholds could pull more taxpayers into AMT. If you plan a big ISO exercise, the year you choose can change your tax bill.

Does Your State Follow These Rules?

Start with federal, then check your state — they do not always match. Most states with an income tax conform to the federal treatment of the bargain element as wages for NSOs, so your state basis usually mirrors the federal one. But AMT is different: only a handful of states have their own AMT, so an ISO exercise that triggers federal AMT may have no state AMT effect at all.

California is the major exception with its own AMT, so high-equity employees there often face state AMT on ISO exercises. The consequence of assuming your state copies federal rules is an over- or under-paid state return. What you should do is check your state tax agency’s guidance — for example, California’s FTB — and confirm whether your state recognizes the same dual basis.

Nine states have no broad income tax, including Texas, Florida, Washington, and Nevada, so residents there pay no state tax on option income and have no state basis to track. Even so, your federal basis rules are unchanged. If you moved between states during the vesting or exercise period, allocation rules can get complex enough to warrant a professional.

Mistakes to Avoid

Using the 1099-B basis as-is — it is usually too low and causes double taxation, costing you real tax dollars.
Forgetting the Code “B” adjustment on Form 8949 — without it, your reported gain is overstated.
Treating ISOs as “tax-free” at exercise — they are tax-free only for regular tax, not AMT.
Failing to track the separate AMT basis for ISOs — you will miscalculate gain and miss your minimum tax credit.
Missing the ISO holding periods — selling early turns favorable capital gain into ordinary income.
Ignoring Form 3921 (ISO) or 3922 — these IRS forms hold the FMV figures you need to compute basis.
Assuming your state copies federal AMT — most do not, but a few like California do, leading to wrong state returns.

Do’s and Don’ts

Do add the W-2 compensation income to your exercise price for NSO basis — it prevents being taxed twice.
Do keep Form 3921, 3922, broker statements, and pay stubs — you need them to prove your adjusted basis.
Do track two separate bases for ISOs — regular and AMT differ and both matter at sale.
Do run a Form 6251 projection before exercising a large ISO block — AMT can be a surprise bill.
Do consult a CPA for large or multi-year exercises — the minimum tax credit math is easy to get wrong.
Don’t trust the broker’s basis blindly — it omits compensation income by design.
Don’t sell ISO shares before the holding deadlines without knowing the tax cost — you may forfeit long-term rates.
Don’t report a positive number in Column (g) when you mean to lower your gain — use a negative.
Don’t assume no income tax means no reporting — you still file Form 8949 for the sale.
Don’t wait until April — projections and planning belong in the exercise year.

Pros and Cons of Exercising Stock Options

Pro: Exercising locks in a low basis and starts the clock for long-term capital gains, which are taxed at lower rates.
Pro: ISOs can avoid ordinary income tax entirely at exercise for regular-tax purposes, deferring tax to sale.
Pro: AMT paid on ISO exercises may return to you later as a minimum tax credit.
Pro: Early exercise of low-spread options minimizes both ordinary income and AMT exposure.
Pro: A correctly adjusted basis directly lowers your taxable gain at sale.
Con: NSO exercises create immediate ordinary income, even if you do not sell the shares.
Con: ISO exercises can trigger AMT in the exercise year, a cash-tax hit on paper-only gains.
Con: Exercising requires cash for the strike price and any tax, tying up money in one stock.
Con: The dual-basis tracking for ISOs is complex and error-prone without good records.
Con: A falling stock price after exercise can leave you taxed on value you never realized.

What to Do Next

Identify your option type from your grant agreement — NSO or ISO — because it sets every rule that follows.
Gather your records: Form 3921 (ISO exercise), Form 3922, the 1099-B, your W-2, and your final pay stub.
Calculate your correct basis: exercise price plus any compensation income already taxed.
Compare it to Box 1e on your 1099-B — if the broker number is lower, you must adjust.
Report the sale on Form 8949 with Code “B” in Column (f) and the negative adjustment in Column (g), flowing to Schedule D.
For ISOs held at year-end, complete Form 6251 to check AMT, and review Form 8801 for any credit.
If your exercise was large, spanned multiple states, or involves AMT recovery, call a CPA before the April 15, 2026 deadline.

This article is educational and is not a substitute for advice from a licensed tax professional for your specific situation.

Frequently Asked Questions

What is my cost basis when I exercise stock options? Your cost basis is the exercise price you paid plus any income already taxed on the spread. For NSOs, add the W-2 bargain element. For ISOs held long enough, regular basis is just the exercise price, while AMT basis adds the spread.

Is the cost basis on my 1099-B correct? No, usually not. Brokers report only your exercise price in Box 1e and omit the compensation income added to your W-2. You must raise the basis yourself on Form 8949, or you will be taxed twice on the same income.

How do I fix a wrong cost basis on my tax return? Use Form 8949 with adjustment Code “B.” Enter the broker basis in Column (e), Code “B” in Column (f), and the missing compensation income as a negative number in Column (g) to lower your taxable gain.

Do I pay tax twice on stock options? No, not if you adjust correctly. The spread taxed as wages must increase your basis so it is not taxed again at sale. Skipping the Form 8949 adjustment is what causes accidental double taxation for many filers.

What is the cost basis for an ISO? For regular tax, it is the exercise price you paid. For AMT, the basis is the fair market value at exercise, which is higher. ISOs therefore require tracking two separate cost bases until you sell.

What is a disqualifying disposition? It is selling ISO shares before the holding deadlines. You must hold more than two years from grant and one year from exercise. Selling early converts part of your gain into ordinary income for the 2025 tax year.

Does exercising NSOs create taxable income? Yes. The bargain element — FMV minus exercise price — is taxed as ordinary wages on your W-2 in the exercise year, even if you do not sell the shares. That same amount then increases your cost basis.

What are the AMT exemption amounts for 2025? $88,100 single, $137,000 married filing jointly, and $68,500 married filing separately for tax year 2025. The exemption phases out above $626,350 for single filers and $1,252,700 for joint filers.

Can I get back the AMT I paid on an ISO exercise? Yes, often. AMT paid on an ISO exercise typically becomes a minimum tax credit you can claim in future years on Form 8801, in years when your regular tax exceeds your AMT.

Which IRS form shows my ISO exercise details? Form 3921. Your employer issues it after you exercise ISOs, and it lists the exercise price and fair market value you need to compute both your regular and AMT cost basis.

Does my state tax stock-option income the same as the IRS? It depends on your state. Most income-tax states follow federal treatment of NSO wages, but only a few, like California, have their own AMT. Nine states have no income tax at all.

What deadline applies to my 2025 stock-option sale? April 15, 2026, or October 15, 2026 with an extension. Report the sale on Form 8949 and Schedule D, and complete Form 6251 if your ISO exercise triggered AMT for 2025.

Related reading