NSO vs ISO at a Startup: The Decision Tree for Employers and Employees

A Series B founder finalized an equity plan in late 2024. Her lawyer sent over a draft that used ISOs for W-2 employees and NSOs for contractors and board members. Simple enough. Three months later, the CFO flagged an issue: half the senior engineering team had grants that would blow past the $100,000 annual vesting limit. The excess automatically converted to NSO. The plan administrator flagged 12 grants that were now partial-ISO, partial-NSO. The recruiter was already pitching candidates on ISO-only packages.

The founder asked a reasonable question: should the plan have been all-NSO from the start, or should it have been structured differently to maximize ISO coverage? The answer depends on the employer’s mix of employees, the company’s valuation trajectory, and the employees’ individual tax profiles. Understanding the trade-offs between ISO and NSO at the plan design stage avoids the post-grant confusion.

The core differences

ISO tax treatment (IRC §422)

No regular-tax income at exercise.
Bargain element is AMT preference.
Qualifying disposition (one year post-exercise, two years post-grant) produces long-term capital gain on full spread.
Disqualifying disposition produces ordinary income on spread.
Limited to employees (not contractors or directors).
Subject to $100,000 annual vesting cap.
10-year maximum grant life.
Strike must be at or above FMV at grant (§422(b)(4)).
5% shareholders limited to 110% FMV strike and 5-year life.

NSO tax treatment (IRC §83)

No tax at grant (assuming no readily ascertainable FMV).
At exercise, spread is ordinary compensation.
Withholding applies under §3402.
Social Security and Medicare apply under §3121.
No post-exercise tax benefit for qualifying holding.
Can be granted to any service provider: employees, contractors, directors, advisors.
No vesting cap.
Plan-specified grant life, typically 10 years.
Strike must be at or above FMV at grant to avoid §409A (not from §422, but similar effect).

The employer decision framework

Tax deduction differences

An ISO exercise produces no deduction for the company (unless the ISO becomes disqualified, in which case the company gets a deduction in the year of the disqualifying event, limited by §83(h)).

An NSO exercise produces a compensation deduction for the company equal to the employee’s ordinary income recognition. At a 21% corporate rate, a $1,000,000 NSO exercise produces a $210,000 deduction.

For profitable companies, NSOs are cheaper after-tax because of the deduction. For pre-revenue startups with no taxable income to offset, the deduction is a carryforward that may or may not be valuable depending on when profitability arrives.

Company stage considerations

Early-stage companies typically prefer ISOs because:

Employees are the dominant audience, and ISOs are only for employees anyway.
The tax deduction is not currently valuable given pre-revenue status.
Competitive pressure with other startups favors ISOs for candidate experience.

Late-stage private companies often prefer NSOs because:

The $100K cap makes ISOs harder to scale.
Senior hires have large grants that would be partially NSO anyway.
The eventual corporate tax deduction becomes valuable as profitability approaches.
Cross-border and multi-entity structures sometimes conflict with ISO requirements.

Public companies typically grant RSUs rather than either ISO or NSO. Where options still exist at public companies, NSO is the default.

The $100,000 annual vesting limit interaction

IRC §422(d) limits ISO vesting to $100,000 of aggregate grant-date FMV per employee per year. Above the limit, options convert to NSO automatically.

For a Series B company with a 409A of $3.50, the limit covers approximately 28,571 shares of first-time-vesting ISOs per year per employee. For a Series D company with a 409A of $12, only 8,333 shares fit inside the limit.

Multi-grant stacking

Multiple grants to the same employee combine for the limit. An initial grant vesting 25,000 shares per year with a $4 FMV uses $100,000 of limit. A refresh grant vesting 10,000 shares per year with a $6 FMV adds $60,000 of first-time vesting in the year the refresh starts vesting. Combined vesting: $160,000 FMV. $100,000 is ISO; $60,000 is NSO.

Companies need to track the §422(d) classification carefully. Plan administrators like Carta and Shareworks handle this automatically, but early-stage companies using spreadsheets often miss the conversion.

The employee perspective

Tax benefit of ISOs

For an employee in the 37% ordinary bracket planning to hold shares long-term, the ISO-to-NSO difference on a $500,000 spread is roughly:

Scenario	ISO (qualifying)	NSO
Exercise	No regular tax, AMT preference	$500K ordinary income at 37% = $185K
AMT on exercise	Up to $140K (recovered over time)	$0
Sale (1 year later at same price)	$500K LTCG at 23.8% = $119K	Sale at exercise FMV = ~$0 tax
Total federal tax	~$119K (AMT largely recovered)	~$185K
Tax saved with ISO	~$66K

The ISO saves about $66,000 on this exercise, plus the cash-flow benefit of deferring tax until sale and the AMT credit recovery mechanism.

For an employee who plans to exercise and sell immediately, the ISO advantage disappears because a same-year disqualifying disposition produces ordinary income equivalent to the NSO treatment.

Holding period flexibility

ISOs require a coordinated holding period to realize the tax benefit: one year from exercise and two years from grant. NSOs have no equivalent requirement; the spread is already taxed at exercise, and post-exercise appreciation is capital gain with standard one-year long-term threshold.

For employees who value flexibility to sell anytime, NSOs are simpler. For employees willing to commit to a hold, ISOs produce better after-tax outcomes.

Cash flow at exercise

NSOs require withholding to be funded at exercise. The withholding is typically satisfied via sell-to-cover, cashless same-day, or straight cash. ISOs have no withholding obligation at exercise (under current guidance), but AMT may be owed on the following April.

Same-day cashless NSO exercises are simpler from a cash-flow standpoint because the tax is withheld at execution. ISO exercise-and-hold requires the employee to fund AMT separately.

Mixed plans and eligibility edge cases

Ten-percent shareholders

IRC §422(b)(6) restricts ISO grants to 10%-or-greater shareholders. The strike must be at least 110% of FMV, and the grant life is limited to 5 years. Most founders of small startups are 10%+ shareholders, which effectively pushes founder grants toward NSO or restricts the ISO benefit.

Contractors and non-employee directors

ISOs are limited to employees. Contractors, board members, advisors, and consultants receive NSOs exclusively. A plan that contemplates a board or advisory pool must include NSOs for those recipients.

International employees

Employees of foreign subsidiaries may or may not qualify for ISOs depending on the plan structure and the local tax regime. Many international employees receive NSOs or local-equivalent grants (e.g., UK EMI options) rather than U.S. ISOs.

The decision tree

For a typical startup designing an equity plan, the decision reduces to:

Are we granting to employees only, or to a mixed population? If mixed, the plan must include NSOs for non-employees.
Are our employee grants likely to exceed $100K per year in aggregate first-time vesting FMV? If yes, expect NSO conversion on the excess.
Do we care about the eventual corporate tax deduction? If yes (profitable or soon-to-be-profitable), NSOs have a value the plan should capture.
Are our candidates sophisticated enough to understand ISO tax mechanics? If not, grant complexity may undermine the benefit.

The common result: an ISO-preferred plan with NSO grants for non-employees and for excess over the $100K limit. Some late-stage companies choose NSO-only for simplicity, accepting the employee-tax disadvantage.

Frequently asked

Can a company reclassify an existing ISO as NSO?

Not unilaterally without triggering tax consequences. A modification of the option terms can convert ISO to NSO but may also be treated as a new grant, with repricing and §409A implications. Reclassification is rare and typically done only in limited circumstances.

Does my grant document tell me if my options are ISO or NSO?

The grant agreement should specify. Look for “Incentive Stock Option” or “Non-Qualified Stock Option” in the header. Plan documents add additional terms. The §422(d) reclassification is not typically stated in the grant document but is tracked by the plan administrator.

Can the company grant both ISO and NSO to the same employee?

Yes. Many employees have both. A common pattern: initial grant is ISO up to the limit, with excess converting to NSO. Refresh grants in later years might be pure NSO or a mix.

What happens to my unvested ISOs if the plan is amended?

Amendments that substantively change ISO terms (extension, repricing, modified vesting) can trigger ISO disqualification. Plan amendments usually preserve existing grants unless specifically amended.

If I leave within 90 days, can the company convert my ISOs to NSO to give me more time?

Some plans offer post-termination extension that converts ISOs to NSOs under §422(a)(2). The extension preserves the exercise right but changes the tax treatment for exercises more than 90 days after termination.

Before you grant or accept, map the classifications using the NSO/ISO comparison calculator.