ISO and NSO Stock Options — Fixing the Double-Counted Cost Basis on Your 1099-B

You exercised your stock options, sold the shares, and now your 1099-B is telling you that you made a much bigger profit than you actually did. The gain looks enormous — as if you paid almost nothing for the stock — even though you know you already had income taxed on these options through your paycheck. Something is being counted twice, and it's about to cost you.

This is the single most expensive tax mistake employees with equity compensation make, and it happens because of a broken cost basis on the 1099-B. When you exercise NSOs or make a disqualifying sale of ISOs, part of your gain is already taxed as ordinary income on your W-2 — but the broker reports your cost basis as only the strike price you paid, ignoring that income. The result: the same dollars get taxed twice, once as wages and once as capital gain. It's the identical trap that hits RSU cost basis and ESPP cost basis, and the fix is the same — an adjustment on Form 8949. This guide walks through both option types.

First, Know Your Two Cost Basis Numbers

Every option sale has two different "cost basis" figures, and confusing them is where the double-counting begins:

• What the broker reports (Box 1e): usually just the exercise cost — the strike price times the number of shares. That's the cash you actually handed over.
• Your true tax basis: the exercise cost PLUS any ordinary income already reported on your W-2 from the exercise.

Your real basis is higher than the broker's number whenever the exercise created W-2 income. The gap between them is exactly the amount being double-taxed, and it's exactly what you fix on Form 8949.

NSOs (Non-Qualified Stock Options): The Straightforward Case

With non-qualified stock options, exercising is a taxable event. The bargain element — the difference between the fair market value (FMV) at exercise and your strike price — is ordinary income, reported on your W-2 in the year you exercise. You pay income and payroll tax on it right away.

That means your true cost basis is:

Strike price + bargain element (the W-2 income) = FMV at exercise

Example: strike price $10, FMV at exercise $40, 1,000 shares.

• You paid $10,000 to exercise (strike).
• The bargain element is ($40 − $10) × 1,000 = $30,000 of W-2 income — already taxed.
• Your true cost basis is $40,000 ($10 + $30 per share).

But the broker's 1099-B likely shows basis of just $10,000. If you sell for $42,000 and use the broker's number, it looks like a $32,000 gain — when your real gain is only $2,000. You'd pay capital gains tax on $30,000 you already paid wage tax on.

ISOs (Incentive Stock Options): The Trickier Case

Incentive stock options have more favorable tax treatment but more moving parts, and the basis depends entirely on how and when you sell.

Qualifying disposition (you held the shares more than 2 years from grant AND more than 1 year from exercise):

• No ordinary income. The entire gain is long-term capital gain.
• Your cost basis is simply the exercise cost (strike price).
• The 1099-B basis is usually correct in this case — no adjustment needed for regular tax.

Disqualifying disposition (you sold before meeting the holding periods — the common case for same-day or short-term sales):

• Part of your gain becomes ordinary income on your W-2 (generally the bargain element at exercise, subject to limits).
• Your true cost basis rises to strike price + that ordinary income — just like an NSO.
• The broker still typically reports only the strike price, so you get the same double-counting and fix it the same way.

There's also the AMT wrinkle: exercising ISOs and holding creates an Alternative Minimum Tax adjustment, and ISO shares can carry a different basis for AMT than for regular tax. That's a separate calculation from the 1099-B fix, but it's why ISO holders should track both numbers carefully.

How to Fix It on Form 8949

Whether NSO or a disqualifying ISO sale, the correction is identical:

1. Report the sale on Form 8949 with the proceeds from the 1099-B (column d).
2. The broker's basis is reported to the IRS but wrong, so don't overwrite column e silently. Instead, keep the reported basis and enter adjustment code B in column f.
3. In column g, enter a negative adjustment equal to the W-2 income (the bargain element) to bring your gain down to the correct amount.

Alternatively, many people enter the correct full basis in column e and use code B to signal the correction — either approach lands on the right gain. Our Form 8949 adjustment codes reference shows the mechanics of code B step by step.

Where to Find Your True Basis

You need the ordinary income amount from the exercise. Look for:

• Form 3922 (ESPP) or your broker's supplemental information statement — brokers like Fidelity, Schwab, E*TRADE, and Morgan Stanley issue a "supplemental" 1099-B or stock plan statement that shows the adjusted (correct) cost basis alongside the reported one. This is the fastest source.
• Your W-2 — the ordinary income from NSO exercise or ISO disqualifying disposition is embedded in Box 1 wages (sometimes itemized in Box 14 or a separate stock-plan report).
• Exercise confirmations — showing FMV at exercise and shares.

The supplemental statement is the key document. If your broker provides it, it usually hands you the corrected basis directly — you just have to use it instead of the number on the main 1099-B.

FAQ

Why is my stock option cost basis wrong on the 1099-B?

Brokers report only the exercise cost (strike price) as basis. For NSOs and disqualifying ISO sales, part of your gain was already taxed as W-2 income, so your true basis is higher. The difference is double-taxed if you don't adjust.

What is the correct cost basis for NSO shares?

The strike price plus the bargain element (FMV at exercise minus strike) that was reported as ordinary income on your W-2 — which equals the fair market value at exercise.

Do ISO shares have the same problem?

Only on a disqualifying disposition, where part of the gain becomes W-2 income and the broker still reports just the strike price. A qualifying ISO disposition usually has correct basis and no adjustment.

Where do I find my adjusted cost basis?

On your broker's supplemental information statement (a stock-plan or "adjusted basis" 1099-B). It shows the corrected basis next to the reported one. Your W-2 also contains the ordinary income figure.

How do I fix it on my tax return?

On Form 8949, keep the reported proceeds, and use adjustment code B with a negative column-g adjustment equal to the W-2 income (or enter the correct full basis in column e). This lowers your gain to the right amount.

What about AMT on ISOs?

Exercising and holding ISOs creates an AMT adjustment and a separate AMT cost basis. That's a distinct calculation from the 1099-B fix but important to track if you held ISO shares across a year-end.

Bottom Line

Stock options come with a built-in trap: the income you already paid tax on through your paycheck is invisible to the broker, so the 1099-B reports a cost basis that's too low and a gain that's too high. For NSOs and disqualifying ISO sales, your real basis is the strike price plus the W-2 bargain element — and failing to adjust means paying capital gains tax on your own wages.

Grab your broker's supplemental statement, confirm the ordinary income already on your W-2, and correct the basis on Form 8949 with code B. The "huge gain" almost always collapses to the small real number it should have been.

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Sold option shares across several exercises and lots? Convert your 1099-B free — we extract every transaction with proceeds and dates intact, so you can layer in the adjusted basis from your stock-plan statement and build an accurate Form 8949 without hand-typing a thing.