ISO vs NSO Stock Options: What Kestrel Bay Retirement Advisors Tells Clients Before They Exercise

The costly habit of treating both grants alike

Plenty of people exercise NSOs and then hold the shares for a year, expecting the spread to be taxed at a capital gains rate. It won't be. The full NSO spread was already taxed as W-2 wages on the exercise date, so holding the shares only changes the tax on growth after that date. If the stock doesn't move, waiting earned you nothing, and you carried the single-stock risk the whole time.

The mirror mistake is with ISOs. Some people exercise them and sell in the same year "to be safe," without checking how much AMT room they have. That gives up the chance to turn the spread into long-term gain. In the worked example below, that chance is worth $8,960 on just 2,000 shares.

Tax comes first in how we work. Before Kestrel Bay Retirement Advisors suggests exercising anything, it prices the exercise both ways. You see the regular tax, the AMT and the withholding gap for each grant type, then you decide.

How are ISOs and NSOs taxed when you exercise?

An NSO exercise adds the spread (market price minus strike, times shares) to your W-2 wages, so income tax, Medicare and possibly Social Security apply that year. An ISO exercise adds nothing to regular taxable income, but the spread counts for AMT if you still hold the shares on December 31. Everything else follows from that split.

In the table, look at two rows: withholding, and basis. NSO basis is the $60 market price, because that amount was already taxed. ISO regular-tax basis stays at the $20 strike, which is why a later sale produces a gain.

One more rule catches people. If the grant-date value of ISO shares that first become exercisable in one calendar year is above $100,000, the excess is treated as NSOs. Say 6,000 shares were granted at $20 and all vest in one year: that's $120,000 of grant value, so 1,000 of those shares are NSOs. You'll usually see this in the equity portal, which labels each lot. Form 3921 arrives for ISO exercises, and NSO income shows up in W-2 box 1.

A 2,000-share exercise, priced both ways

Hypothetical: Jonah and Mei are 46 and 44. He's a product director at a chip maker with ISOs and NSOs, she left a startup holding early-exercised shares, and they have two kids in middle school. Their options carry a spread of about $900,000 next to a $300,000 brokerage account. Jonah has 2,000 vested options with a $20 strike, and the stock trades at $60, a $40 spread.

The exercise costs 2,000 × $20 = $40,000, which Jonah pays out of their brokerage account. The spread is 2,000 × $40 = $80,000. As NSOs, that $80,000 is W-2 wages. At an assumed 35% federal rate it's $28,000 of income tax this year, plus Medicare tax, and his basis becomes $60 a share, or $120,000.

As ISOs, it goes differently, assuming the exercise fits under his AMT crossover and he makes a qualifying sale at $60. The $80,000 is long-term gain, taxed at an assumed 23.8%: $19,040. The difference is $28,000 − $19,040 = $8,960, for illustration only. If the ISO exercise does create AMT, that AMT usually comes back later as a credit, and our AMT page covers how.

Pros and cons of each grant type, side by side

Neither grant wins everywhere. ISOs can save tax but ask for patience and AMT homework, while NSOs are plainer and costlier. The lists below are what we see matter most in reviews.

• ISO pros: no regular tax at exercise; the spread can become long-term gain; no FICA on the spread even in a disqualifying sale.
• ISO cons: AMT exposure if held past December 31; no withholding, so estimated payments may be needed; a strict two-year and one-year holding test; status is lost if you don't exercise within about three months of leaving.
• NSO pros: simple, since tax is settled at exercise through payroll; no AMT; no holding test; post-employment windows are often longer (check the plan).
• NSO cons: the whole spread is ordinary income, up to the 37% top rate; payroll withholding often runs below a high bracket; holding after exercise adds stock risk with no extra tax benefit.

How Kestrel Bay Retirement Advisors orders an exercise of both grants

The order matters because one grant can pay for the other. Here's the sequence our advisors walk through with clients who hold both.

• Step 1: list every grant from the equity portal. Record type, strike, vest dates, expiration date and which ISO lots fall over the $100,000 line.
• Step 2: price any NSO exercise. Multiply the spread by your marginal rate and compare it with what payroll will withhold. Write the gap down as a set-aside or an estimated payment.
• Step 3: run the ISO exercise through tax software to find the AMT crossover. Our separate AMT article explains the method, so we won't repeat it here.
• Step 4: decide the funding. We would usually exercise and sell NSOs and use the after-tax cash to pay the ISO exercise cost, because the NSO tax is due either way and the sale lowers the share of net worth in one stock.

Myths about ISO and NSO taxes that cost real money

Myth: holding NSO shares a year makes the spread a capital gain. In fact, only growth above the exercise-date price can become long-term gain. The spread itself is wages, full stop.

Myth: ISOs are tax-free. The spread is an AMT item if you hold past year-end, and a sale before the holding test is met turns it into ordinary income. Yes, that means some people owe tax on shares they haven't sold.

Myth: the company withheld, so I'm covered. Nothing is withheld on an ISO exercise or a disqualifying sale. NSO withholding uses the flat federal supplemental rate (look up this year's figure on IRS.gov). On an $80,000 spread taxed in a 35% bracket, that withholding can fall thousands short.

Myth: the broker's 1099-B basis is right. For NSO lots it often shows only the $20 strike. If you report it as is, the $80,000 spread is taxed twice, costing about $19,040 more at an assumed 23.8%. That's the most expensive paperwork error we see, and it's fixed on Form 8949 with an adjustment.

Does the better choice change with age or account size?

Yes, both change it. At 38 with no children, a long holding period is easy to wait out. At 46 with two kids heading to college in five to seven years, Jonah and Mei need part of the gain liquid, which favors selling NSOs and holding fewer ISOs.

Size matters too. The $40,000 exercise cost is about 13% of their $300,000 brokerage account. Someone with a $50,000 account may have to sell other shares just to exercise, which removes the ISO advantage. Larger households can pay AMT from cash and still hold, while smaller ones should exercise only what fits under the crossover.

Near retirement, concentration matters more than the rate difference. A 15% drop on a $120,000 ISO position is $18,000, more than double the $8,960 saved in the example. Only ISOs have a tax reason to be held after exercise. Holding NSO shares is purely an investment bet and should pass the same concentration test as any other holding of your employer's stock. And if you'd be uncomfortable owning more of the stock, holding ISOs for the rate isn't worth it.

What to do this week, and what to ask an advisor

Start with paperwork, not trades. Don't exercise until both routes are priced. The lists below cover what to gather and what to ask.

Kestrel Bay Retirement Advisors reviews grant summaries for clients across the US, meeting over video or phone. We work with households that have $400K or more in investable assets. If that's you, the contact form on this site is the way to reach us. Our guides on equity compensation planning and concentrated stock positions go deeper, and the RSU article covers vesting tax.

• This week: download the grant summary and the expiration dates.
• This week: check which lots are ISOs and which are NSOs.
• This week: pull last year's total tax for the estimated-payment safe harbor.
• Ask: Which of my ISO lots are over the $100,000 line?
• Ask: How much ISO spread fits this year before AMT?
• Ask: Should NSO sales fund the ISO exercise?
• Ask: How will you correct the basis on my 1099-B?
• Ask: What share of our net worth will be employer stock afterward?