NSOs vs. ISOs: The Tax Difference and What It Means for Your Exercise Plan

Most people who hold stock options know the strike price and the vesting schedule and not much else. But the grant paperwork says NSO or ISO, and that one word decides how the option is taxed, when the tax is due, and what the exercise strategy should look like. The two types look identical right up until exercise. Then they go completely different directions.

Nonqualified stock options

An NSO is the default. At exercise, the difference between the stock's fair market value and the strike price (the spread) is ordinary compensation income. It lands on the W-2, it's subject to income tax withholding (at the flat supplemental rate, 22% up to $1 million of supplemental wages in a year and 37% above) and payroll taxes, and it's taxed whether or not you sell the shares. Your basis in the shares becomes the fair market value at exercise, and anything after that is capital gain or loss, long-term after a year.

The withholding is a trap. At 22%, it's well below the 32% to 37% bracket a large exercise puts most people in. The balance is due with the return, or through estimated payments, and a household that spends the proceeds before April can end up owing six figures. We see this more than we'd like.

Since the tax is the same whether you hold or sell, the usual NSO plan is to exercise and sell in one transaction, take the spread as cash, and redeploy it, unless there's a specific reason to hold. Holding after exercise means paying ordinary tax on the spread now and then taking single-stock risk on the after-tax proceeds. That's rarely worth it.

Incentive stock options

An ISO, granted under Section 422, gets different treatment in exchange for meeting some conditions. At exercise there's no regular income tax. Hold the shares at least one year after exercise and two years after grant, and the entire difference between the sale price and the strike price is long-term capital gain, taxed at 15% or 20% plus the 3.8% NIIT instead of at ordinary rates. No payroll tax, no withholding.

The price is the alternative minimum tax. The spread at exercise is an AMT preference item, so a large ISO exercise can trigger AMT in the exercise year even without a sale. The AMT you pay generates a credit that comes back in later years when regular tax exceeds AMT, so it's largely a prepayment. But it's real cash out the door. Our post on ISOs and the AMT covers the calculation and the crossover point where exercising more starts to cost you.

Sell before the holding periods are met and you've made a disqualifying disposition: the spread at exercise (or the actual gain, if less) becomes ordinary income, though without payroll tax or withholding, and the AMT preference goes away. An exercise-and-sell of ISOs is taxed almost like an NSO exercise.

The rules that convert ISOs to NSOs

The $100,000 limit. Only $100,000 of options, valued at the grant-date strike price, can first become exercisable in any calendar year as ISOs. Anything above that is an NSO, and grant paperwork often splits a large grant accordingly.

Ninety days after termination. ISOs have to be exercised within three months of leaving the company to keep ISO treatment. After that they're NSOs, if the plan allows exercise at all. This is the deadline that catches people who leave with a big in-the-money grant and no cash for the strike and the AMT.

Ten years. ISOs expire ten years from grant.

Employees only. ISOs can only go to employees. Contractors and directors get NSOs.

The exercise plan for each

NSOs. Exercise when the household wants to capture value, which usually means when the option is well in the money and diversification is the priority, or when expiration is getting close. Exercise and sell together. Set aside the tax beyond the withholding. Use the proceeds to fund the plan. Spreading exercises across years smooths the income, and a large exercise is generally better split across two tax years to stay under a bracket edge.

ISOs. Exercise early in the year, so that if the stock drops before December you can sell in a disqualifying disposition and erase the AMT preference. Size the annual exercise to the AMT crossover, the amount you can exercise before AMT exceeds regular tax, which we compute every year. Hold for the qualifying period if the concentration is tolerable and the outlook is reasonable; if not, sell and accept ordinary treatment. Watch the ninety-day clock at any job change. For private-company ISOs, consider early exercise with an 83(b) election when the spread is close to zero.

Cash

Both types need cash for the strike price, and NSOs need cash for the tax on top. Cashless exercises, where the broker sells enough shares to cover both, solve this for public companies. For private companies there's no market, so the cash comes from savings or from financing arrangements that carry their own costs and deserve a hard look. The startup equity checklist walks through the private-company sequence.

What we do with them

For each client with options, the plan lists every grant by type, strike, vesting, and expiration, models the tax under each exercise path, and sets an annual exercise target and a sale rule. The projection updates as the stock moves. It's a core part of equity compensation planning, and getting the type right is where it all starts.

Frequently asked questions

Joseph G. Valerio, CFP®, EAFounder and Lead Advisor at Valerio Wealth Management, a fee-only, fiduciary virtual family office in Miami, Florida. One advisor, credentialed in both financial planning and federal tax. About Joseph

This article is general education, not individualized advice, and it is not tax, legal, or accounting advice. Figures reflect federal law for the year stated and can change. Valerio Wealth Management does not prepare or file tax returns. Speak with a qualified professional about your own situation before acting.