How to Calculate Stock Option Cost Basis: NSOs and ISOs

To calculate the cost basis of stock option shares, start with what you paid to exercise and add anything the IRS already taxed as ordinary income on the spread. For non-qualified stock options, that means exercise price plus the spread reported on your W-2, which usually equals the fair market value on the exercise date. For incentive stock options, the regular-tax basis is just the exercise price, but a separate AMT basis includes the spread. Getting these numbers right, and correcting your broker’s 1099-B when it reports the wrong one, is the difference between paying capital gains tax once and paying it twice on the same dollars.

The Three Numbers You Need From the Exercise Date

Every basis calculation for option shares rests on three figures fixed at the moment of exercise:

Exercise price, sometimes called the strike price: the per-share amount your option contract lets you pay.
Fair market value (FMV) on the exercise date: for publicly traded stock, the closing price that day or the average of the day’s high and low.
The spread: FMV minus exercise price. An exercise price of $10 against a $35 stock price is a $25 spread.

The spread is the economic benefit of buying below market. Whether it’s taxed at exercise, and whether it becomes part of your basis, depends on which type of option you hold.

Basis for Non-Qualified Stock Options

When you exercise NSOs, the spread is taxed immediately as ordinary compensation income. Your employer reports it on your W-2, withholds payroll taxes on it, and sends the numbers to the IRS. Because that income has already been taxed as wages, it counts as part of your investment in the stock.

The formula is exercise price plus spread at exercise, which is the same as the exercise-date FMV. You paid the exercise price in cash and effectively paid the spread through the income tax on it.

Say you exercise 1,000 NSO shares at $10 when the stock is trading at $35. You pay $10,000 in cash. The $25 spread produces $25,000 of ordinary income on your W-2. Your basis in those 1,000 shares is $35,000.¹

Sell those shares later for $40,000 and your capital gain is $5,000. If you used $10,000 as your basis instead, you’d report a $30,000 gain and pay capital gains tax on $25,000 that was already taxed as wages. That is the most expensive mistake people make with option shares, and it happens routinely because of how brokers report basis.

Cashless Exercise and Sell-to-Cover

If you don’t have cash to fund the exercise, a cashless exercise sells some or all of the shares immediately to cover the exercise price and withholding. The basis math doesn’t change. Each share still has a basis equal to exercise-date FMV, regardless of how many shares were sold to fund the transaction.

In a full cashless exercise where every share is sold the same day, basis equals FMV and the capital gain is essentially zero after price movement and fees. The tax hit is the ordinary income on the spread, withheld from proceeds. In a sell-to-cover, the shares you keep carry the same per-share basis equal to exercise-date FMV.

Basis for Incentive Stock Options

ISOs run on two parallel basis tracks: one for regular tax, one for the alternative minimum tax. You need both numbers, and which one governs your eventual gain depends on how long you hold the shares.²

Regular Tax Basis

Exercising ISOs doesn’t produce ordinary income for regular federal tax purposes. Nothing lands on your W-2 from the spread. Because the spread wasn’t taxed at exercise, it isn’t added to basis. Your regular tax basis is the exercise price you paid.

What happens at sale depends on two holding periods: at least two years from the grant date, and at least one year from the exercise date.³

Meet both, and the entire gain from exercise price to sale price is long-term capital gain. Basis stays at exercise price. Exercise at $20 and sell at $60, and the full $40 per share is long-term capital gain.

Sell before either holding period is satisfied and you have a disqualifying disposition. You owe ordinary income tax on the lesser of the spread at exercise or the actual gain on sale. That ordinary income amount is then added to basis, exactly like an NSO.²

The lesser-of rule matters when the stock drops between exercise and sale. Exercise at $20 with a $60 FMV (a $40 spread), then sell at $45, and the ordinary income is $25 per share, not $40. Adjusted basis becomes $45, and there is no additional capital gain. If the stock falls below your exercise price and you sell at $15, there’s no ordinary income at all; you have a $5 per-share capital loss against a $20 basis.

AMT Basis

The ISO spread that isn’t taxed for regular tax purposes is an adjustment item for the alternative minimum tax. Exercise ISOs and hold the shares past year-end, and the spread has to be added to your AMT income for that year.²

That creates a separate, higher AMT basis: exercise price plus spread at exercise. Exercise 500 shares at $20 when the stock is worth $60, and your regular basis is $10,000 while your AMT basis is $30,000. Both numbers travel with the shares until you sell.

If the AMT calculation actually produces tax in the year you exercise, that tax generates a credit you can recover in later years by filing Form 8801. The credit carries forward until fully used.⁴ For 2026, the AMT exemption is $90,100 for single filers (phasing out at $500,000) and $140,200 for married filing jointly (phasing out at $1,000,000).⁵

When ISOs Become NSOs: The $100,000 Vesting Limit

ISOs that first become exercisable in any calendar year get ISO treatment only up to $100,000 in aggregate fair market value, measured at grant. Anything above that is automatically treated as NSO for tax purposes.⁶

If 5,000 shares granted at $30 vest in one year, that’s $150,000 of FMV. The first 3,333 shares stay ISO; the rest are NSO. Basis for the two batches follows different rules. The limit runs across all your ISO grants combined, not per grant.

Fixing the Basis on Your 1099-B

When you sell option shares, your broker sends Form 1099-B. The basis in Box 1e is often too low. For NSO shares, brokers frequently report only the exercise price, leaving out the ordinary income already taxed through your W-2. For ISO shares in a disqualifying disposition, the same happens: the broker may not know about the wage adjustment and reports a basis that doesn’t include it.⁷

File using that number and the IRS sees a much larger gain than you actually owe. The IRS doesn’t automatically cross-check your W-2 income against the 1099-B to notice the double-count. You have to fix it yourself.

Using Form 8949

The correction happens on Form 8949, which flows into Schedule D.⁸ The method depends on whether the shares are a covered or non-covered security.

For covered securities where the broker reported basis, put the broker’s number in column (e), enter Code B in column (f), and enter the adjustment as a negative number in column (g). If the broker reported $10,000 and your correct basis is $35,000, column (g) is negative $25,000. The gain in column (h) then reflects the right, lower number.⁹

For non-covered securities, where the broker checked Box 5 and left basis blank, enter the correct basis directly in column (e) and put zero in column (g).⁹

Form 3921 for ISO Exercises

Employers must file Form 3921 for each ISO exercise and send you a copy. It shows the exercise price per share in Box 3, the fair market value per share on the exercise date in Box 4, and shares transferred in Box 5.¹⁰ Those three boxes give you every input for both your regular tax basis and your AMT basis. If you didn’t get one, ask for it. Reconstructing exercise-date FMV years later is hard.

Wash Sales Can Add to Your Basis

A wash sale happens when you sell stock at a loss and buy substantially identical stock within 30 days before or after the sale. The loss deduction is disallowed, but the disallowed amount gets added to the basis of the replacement shares.¹¹

Exercising options on the same employer stock counts as acquiring substantially identical shares. Sell company stock at a loss and then exercise options on that same company within the 61-day window, and the loss is disallowed and folded into the basis of the new shares.¹²

Sell 500 shares at a $5,000 loss, then exercise 500 option shares two weeks later. The $5,000 loss is disallowed, and $10 per share is added to the new shares’ basis. The economic loss isn’t gone; it shifts into a higher basis you’ll recover when you eventually sell the replacement shares. Report the disallowed amount on Form 8949 as a positive adjustment in column (g) using Code W.⁹

State Rules May Differ for ISOs

Not every state follows the federal treatment of ISOs. Some states tax the spread on exercise as ordinary income, the same as an NSO, regardless of whether you meet the federal holding periods. That can produce a different state-level basis and a state tax bill in the year of exercise even when nothing is owed federally. Check your state’s treatment before assuming an ISO exercise is tax-free at the state level.

Records to Keep

Basis disputes are almost impossible to win without documentation, and the IRS expects you to keep records as long as they’re relevant to computing gain or loss when you sell.¹³ That means keeping the grant agreement, exercise confirmation, and either Form 3921 (ISOs) or the W-2 showing the spread income (NSOs) until at least three years after you file the return that reports the sale.

Underreport income by more than 25% of the gross income on your return and the IRS has six years to assess additional tax instead of three.¹³ Because option exercises can create large, confusing income items, seven years of retention after filing is safer. If you hold ISO shares for years before selling, the clock doesn’t start until the return reporting the sale, so grant documents from earlier years still matter.

The records that hold up in an audit are the ones tying exercise-date FMV to a specific dollar amount: brokerage exercise confirmations with the date and price, Form 3921 copies, W-2s showing supplemental income, and screenshots of the closing stock price on the exercise date. If the company was private when you exercised, keep the 409A valuation or board resolution that established FMV. Those are far harder to reconstruct later.

1
Office of the Law Revision Counsel. 26 USC 83 – Property Transferred in Connection With Performance of Services
2
Internal Revenue Service. Topic No. 427, Stock Options
3
Office of the Law Revision Counsel. 26 USC 422 – Incentive Stock Options
4
Internal Revenue Service. Instructions for Form 8801 – Credit for Prior Year Minimum Tax
5
Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026
6
eCFR. 26 CFR 1.422-4 – $100,000 Limitation for Incentive Stock Options
7
Internal Revenue Service. Instructions for Form 1099-B (2026)
8
Internal Revenue Service. About Form 8949, Sales and Other Dispositions of Capital Assets
9
Internal Revenue Service. Instructions for Form 8949 – Sales and Other Dispositions of Capital Assets
10
Internal Revenue Service. About Form 3921, Exercise of an Incentive Stock Option Under Section 422(b)
11
Office of the Law Revision Counsel. 26 USC 1091 – Loss From Wash Sales of Stock or Securities
12
Internal Revenue Service. Case Study 1 – Wash Sales
13
Internal Revenue Service. Topic No. 305, Recordkeeping