ISO vs NSO: What’s the difference and why it matters for you

Stock options, sometimes referred to as equity options, give an employee the right to purchase stock in their company at a set price, often called a “strike price.” They allow employees to have ownership in their company, directly tying its success to their financial futures. 

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Ideally, an employee purchases shares at a certain price and is able to sell them later for much more, after the company has evolved and matured.

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Proponents say stock options boost employees’ morale and incentivize them to work harder. Most stock options come with a vesting period that stretches over a couple of years, encouraging employees to remain with the company for longer. If they leave before the period is over, they might not receive their full award.

The key differences between these two types of stock options come down to taxes and who can qualify.

Incentive stock options (ISOs)

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ISOs are tax-advantaged stock options reserved solely for employees. This means other people associated with the company, such as advisors, contractors, or board members, are not eligible.

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ISOs offer better tax treatment, as long as the employee meets the holding period rules. For an ISO sale to qualify, it must be made at least two years after the grant date and one year after the options were exercised, or purchased. If the employee makes a qualifying sale, they’ll report only the capital gain. If the sale doesn’t meet the holding period requirements, the employee will have to report any difference between the share price and exercise price as earned income.

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To take advantage of this option, employees generally must hold on to their stock for a longer time period. And some high earners who cash in on ISOs might be at risk of owing the alternative minimum tax (AMT). Strategic tax planning around startup equity — including AMT and QSBS —  is one factor employees often weigh with a tax professional.

ISOs can offer favorable tax treatment, but only if the option and the employee’s actions meet specific requirements. If those requirements are not met, the option does not simply disappear. In many cases, it is treated more like an NSO for tax purposes.

This can happen in several common situations.

Disqualifying dispositions and ordinary income

A disqualifying disposition happens when an employee sells ISO shares before meeting the required holding periods. The employee may still benefit from owning equity, but the sale may not receive the full ISO tax benefit.

When this happens, part of the gain may be treated as ordinary income. Any remaining gain may be treated as capital gain, depending on the sale price and holding period.

This matters because ordinary income is often taxed at a higher rate than long-term capital gains. The employee may have expected ISO treatment, but selling too early can change the final tax outcome.

When an ISO becomes an NSO for tax purposes

An ISO can also lose its status if it fails to meet ISO rules. One common example is the post-termination exercise window. Another is the rule limiting how much ISO value can first become exercisable in a calendar year.

When that happens, the option may still be valid under the company’s equity plan, but it may no longer qualify for special tax treatment. Instead, it may be taxed like an NSO.

For employees comparing ISOs or NSOs, this is one of the most important practical points: the label on the grant is not the only thing that matters. The timing of exercise, the timing of sale, and the employee’s status with the company can all affect the final tax result.

Non-qualified stock options (NSOs)

‍NSOs offer a more flexible stock option for a broader group of people: employees, advisors, or board members are all eligible. The stockholder will pay ordinary income tax on the “spread,” or the difference between the market and strike prices, whenever they exercise their shares.

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Unlike with ISOs, anyone exercising an NSO won’t have to worry about the AMT. But paying income tax on the spread can be less favorable than having to report only capital gains, and might result in a higher tax burden.

The key differences between ISOs and NSOs become much clearer when you put numbers behind them. The tax implications usually depend on four things: the strike price, the fair market value of the shares, the number of options exercised, and whether the employee later meets the ISO holding requirements.

Here is a simple example.

In both cases, the employee exercises 10,000 options at $1 per share when the shares are worth $5 per share. The spread is $40,000.

With an NSO, that $40,000 is generally taxable at the time of exercise. NSOs are taxed differently from ISOs because the bargain element is usually treated as compensation income. That means the employee may have income tax due at exercise, even if they do not sell the shares right away.

With an ISO, the employee may not owe regular income tax upon exercise. However, the same $40,000 difference may be included for AMT purposes. This is where ISOs can create an unexpected tax issue: the employee might owe tax before receiving cash from selling shares.

Capital gain treatment after selling shares

The tax outcome can also change when the employee sells the shares.

If the employee exercises ISOs and later sells after meeting the required ISO holding periods, the gain may qualify for special tax treatment. In that case, the gain may be taxed at a long-term capital gains rate instead of the employee’s ordinary income tax rate.

For ISOs, the employee generally must hold the shares for at least:

If those rules are met, the employee may receive favorable tax treatment on a qualifying sale. If the employee sells too soon, some of the gain may be subject to ordinary income instead of long-term capital gains tax.

For NSOs, the amount taxed at exercise becomes part of the employee’s cost basis. Any later increase in value after exercise may be taxed as a capital gain when the employee sells. Whether that gain is short-term or long-term depends on how long the employee holds the shares after exercise.

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Startups will often include stock options as part of their compensation packages. Before signing a deal that includes ISOs or NSOs, prospective employees should assess the company’s potential for growth — and, in turn, the potential for their shares to appreciate in value. That includes taking a closer look at the company’s 409A valuation, which directly influences your strike price and tax treatment.

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Deciding when to sell startup shares depends on an individual's own financial goals, tax situation, liquidity needs, and risk tolerance — this is a personal decision best made with a qualified financial or tax professional, not a one-size-fits-all rule. If your startup is still private when you want to sell, platforms like Augment may be able to help connect you with a prospective buyer, subject to eligibility and platform terms.

Employees should also keep in mind the potential tax consequences that come with each option. The best way to anticipate these implications is to work with a tax professional.

Early exercise lets an employee exercise stock options before they vest. Not every company allows it, but when it is available, it can materially change the tax implications of equity compensation.

Stock options are typically exercised after they vest. With early exercise, the employee exercises earlier, often when the company’s valuation is still low. This can matter because the taxable amount is usually tied to the difference between the exercise price and the value of the shares at the time of exercise.

For example, imagine an employee receives options with a $1 strike price. If the fair market value is also $1 when the employee exercises, there may be little or no taxable spread at that moment. If the company grows and the shares are later worth $10, the employee may have started the capital gain holding period earlier and reduced the amount exposed to compensation-style taxation.

Early exercise and the 83(b) election

Early exercise is often discussed with an 83(b) election. An 83(b) election tells the IRS that the employee wants to be taxed based on the value of the shares when they are received, rather than as they vest over time.

This can be useful when the shares have a very low value at the time of exercise. But it also comes with risk. If the employee pays to exercise and the company’s value later falls, or if the employee leaves before the shares vest, the employee may not recover the money spent or the tax already paid.

Early exercise can also matter for qualified small business stock planning. In some cases, the QSBS holding period may begin when shares are acquired, not when options are granted. That can make the exercise date important for employees thinking about long-term capital gains planning.

Because early exercise can affect regular income tax, AMT, liquidity, and future sale timing, employees should review their option agreement and speak with a tax advisor before exercising.

One of the most important ISO rules comes up when an employee leaves the company. ISOs are employee-only options, and they generally need to be exercised within a limited period after employment ends to keep their ISO status.

This is often called the ISO 90-day rule. More precisely, the employee generally has three months after termination to exercise and preserve ISO tax treatment. If the employee exercises after that window, the option may still be exercisable if the company plan allows it, but it will usually no longer be treated as an ISO for tax purposes.

That means an option that started as an ISO can effectively become an NSO if it is exercised too late after termination.

Why the 90-day rule matters for income tax

The timing can create a difficult decision. After leaving a startup, an employee may have vested options but limited cash. Exercising may require paying the strike price and potentially facing tax exposure before the shares are liquid.

If the employee exercises within the ISO window, they may preserve the potential for preferential ISO treatment. If they wait too long, the option may be taxed as an NSO, which can create ordinary income at exercise based on the value of the shares at that time.

This is one reason employees should check their option agreement before leaving a company or immediately after departure. The company’s post-termination exercise window may be short, and the tax treatment may change even if the option itself has not expired.

ISO vs. NSO treatment after leaving

The important point is that the tax status and the expiration deadline are not always the same thing. A company may allow a longer exercise period, but the ISO tax benefit can still be lost if the employee misses the required ISO timing rules.

ISOs aren’t “better” than NSOs, and vice versa. The best path for you will depend first and foremost on what options your startup offers, and then on your risk tolerance and the stage of your company. Structuring your equity with liquidity in mind can also help ensure you're set up for stronger outcomes—especially if your situation changes before a traditional exit.

Planning around ISOs or NSOs before selling shares

The two main types of stock options can create very different tax outcomes. ISOs may offer preferential treatment, but they come with strict rules. NSOs are often more flexible, but they usually create taxable compensation when exercised.

Before an employee exercises, leaves a company, or sells shares, it is worth checking whether the options are ISOs or NSOs, what the current FMV is, whether exercise could trigger AMT, whether the employee has met the holding requirements, and whether there is a post-termination deadline.

Equity can be valuable, but the tax rules can change the real outcome. A little planning before exercise or sale can help employees avoid surprises and make better decisions about their startup shares.