Company Share Option Plan: Vesting, Exercise, and Tax Rules

An employee stock option plan lets you buy company shares at a locked-in strike price once your options vest, and the tax you owe depends on the type of option, when you exercise, and how long you hold the shares afterward. Vesting usually runs over four years with a one-year cliff, meaning nothing is yours to exercise for the first 12 months. After that, tax treatment splits sharply: incentive stock options can qualify for capital gains rates if you meet strict holding rules, while non-qualified options are taxed as ordinary income on the full spread the moment you exercise. Getting the details wrong can cost thousands or, worse, cause you to forfeit options you thought you owned.

ISOs and NSOs Are Taxed Differently

Every grant is one of two types, and the label on your paperwork controls everything that follows.

Incentive stock options (ISOs) can only go to employees. They carry potential tax advantages at exercise but come with strict rules: a $100,000 annual cap on the value of options first becoming exercisable in any calendar year, a mandatory holding period to qualify for long-term capital gains treatment, and a requirement that you exercise within three months of leaving the company.¹

Non-qualified stock options (NSOs) can go to anyone providing services to the company: employees, consultants, advisors, and board members. There’s no annual dollar cap and fewer structural restrictions, but the spread at exercise is taxed as ordinary income with no way around it.²

If the value of ISOs first becoming exercisable in a single calendar year exceeds $100,000 (measured by fair market value at the grant date), the excess is automatically reclassified as NSOs and taxed accordingly.¹ Check your grant agreement before making any exercise decisions.

How Vesting Works

Vesting is the timeline that determines when you actually earn the right to exercise. Until an option vests, you can’t touch it. Leave before it vests, and it disappears.

The Standard Four-Year, One-Year Cliff Schedule

The most common structure is four-year vesting with a one-year cliff. Nothing vests during the first 12 months. Hit your one-year anniversary and 25% of your options vest at once. After the cliff, the remaining 75% vest in equal monthly or quarterly installments over the next three years. Walk out at month 11 and you get nothing. Some plans skip the monthly installments and vest 25% chunks on each anniversary instead. Your grant agreement spells out which.

Performance-Based Vesting

Some companies tie vesting to specific targets rather than time alone: reaching a revenue goal, closing a set number of customers, or launching a product. This is more common for executives than for rank-and-file employees. If the milestone isn’t met, those options never vest no matter how long you stay.

Exercising Your Options

Once options vest, you have the right to buy shares at the strike price. Exercising is actually doing it. You notify the company (usually through its equity management platform), specify how many vested shares you want, and pay. Hold 1,000 vested options at a $5.00 strike, and you owe the company $5,000.

Three ways to handle the payment are common:

Cash exercise: you pay the full strike price out of pocket and receive the shares. Most control, most cash required upfront.
Cashless exercise: a broker sells all the shares immediately at exercise, deducts the strike price and applicable taxes, and sends you the remainder. You never hold shares.
Sell-to-cover: the broker sells just enough shares to cover the strike price and taxes, and you keep the rest.

Cashless and sell-to-cover are only practical at public companies where the shares trade. At a private company, you’ll almost always need to write a check.

Every option has an expiration date. For ISOs, the statutory maximum is 10 years from the grant date, and most NSO plans adopt the same limit by convention.¹ Let them expire and they’re gone.

Tax Rules at Grant, Exercise, and Sale

Tax consequences show up at three points, and what you owe at each depends on the option type.

At Grant

Neither ISOs nor NSOs trigger tax when they’re granted, as long as the strike price is at or above fair market value on the grant date.² For public companies, that’s the trading price. For private companies, it comes from a 409A valuation. If a private company sets the strike price below fair market value, Section 409A includes the spread in your gross income immediately, adds a 20% additional tax, and charges interest at the federal underpayment rate plus one percentage point.³ Those penalties fall on you, not the company.

NSOs at Exercise

When you exercise an NSO, the spread between the strike price and current fair market value is taxed as ordinary income in the year of exercise. Strike of $5, stock worth $25, spread of $20 per share taxed as wages. Your employer withholds federal income tax and FICA on that amount. Plans handle the withholding mechanics differently: some ask for a separate check, some withhold from other wages, some reduce the number of shares delivered to cover the tax.

ISOs at Exercise and the AMT Trap

Exercise an ISO and you owe no regular federal income tax on the spread.¹ The spread does count as income for the Alternative Minimum Tax, though. For 2026, the AMT exemption is $90,100 for single filers and $140,200 for married couples filing jointly, phasing out once alternative minimum taxable income reaches $500,000 and $1,000,000 respectively.⁴ Exercise a large ISO grant in a year when the spread is substantial and the AMT can produce a serious bill. Run the numbers before you exercise.

At Sale: Capital Gains and Disqualifying Dispositions

When you sell your shares, any gain above the value you were already taxed on is a capital gain. Held for more than a year, it qualifies for long-term capital gains rates, which top out at 20% for high earners in 2026.² Held a year or less, it’s short-term and taxed at ordinary rates.

For ISOs there’s an extra holding rule beyond the standard one year. To keep the favorable tax treatment, you must hold the shares at least two years from the grant date and at least one year from the exercise date.¹ Sell before meeting both and you trigger a disqualifying disposition: the spread at exercise gets reclassified as ordinary income, and the tax benefit of holding ISOs vanishes. This is where most people trip up. They exercise, watch the stock rise, sell quickly, and end up owing ordinary income tax on the entire spread they thought would be taxed as capital gains.

Early Exercise and the 83(b) Election

Some plans let you exercise options before they’ve vested. Doing so gives you restricted stock that stays subject to the original vesting schedule. Leave before it fully vests and the company can buy back the unvested portion, usually at the price you paid.

The tax reasoning behind early exercise is simple. Exercise when the spread between strike price and fair market value is small (often near zero right after a grant) and you lock in a minimal tax bill now. Under the default rule of Section 83, you’d otherwise owe ordinary income tax each time a batch of shares vests, calculated on the spread at that moment. If the stock has climbed by then, the tax grows with it.⁵

To freeze the tax liability at the exercise-date value, you must file a Section 83(b) election with the IRS within 30 days of the exercise. The deadline is absolute. There are no extensions, and courts have consistently rejected requests for relief from late filings.⁶ The election tells the IRS you want to be taxed on the current value of the shares now, not at each vesting date. If the stock rises after you file, all future appreciation shifts into the capital gains column.

Filing means completing IRS Form 15620 and sending it to the IRS office where you file your federal return; electronic filing is also available. Send a copy to your employer as well. The form asks for a description of the shares, the fair market value at transfer, the price you paid, and the amount you’re including in gross income.⁶

The catch: if the shares are later forfeited because you leave before vesting, you get no tax deduction for the forfeiture. You’ve paid tax on something you no longer own. Early exercise with an 83(b) is a bet that the stock will rise and that you’ll stay long enough to vest. For early-stage startup employees receiving options at a very low strike, the math usually works. For later-stage employees sitting on a large spread, the calculation looks different.

What Happens When You Leave

Your departure starts a countdown. Most option agreements give you 90 days after your last day of employment to exercise vested options. Miss that window and vested options you earned over years of work expire worthless. Some companies extend the window to six or twelve months, but 90 days is the default unless your agreement says otherwise.

For ISOs specifically, the tax code requires exercise within three months of leaving to keep ISO tax treatment. Exercise later and your ISOs convert to NSOs for tax purposes, meaning the spread at exercise becomes ordinary income.¹ If you’re disabled as defined by the tax code, that window extends to one year.

Unvested options are typically forfeited entirely at departure, regardless of the reason.

Acquisitions and IPO Lock-Ups

For private-company options, an IPO or acquisition is usually the moment shares finally become liquid. Getting from “the company was sold” to “money in your account” involves several steps.

Acceleration on Acquisition

When a company is acquired, your unvested options don’t automatically vest in full. What happens depends on your plan’s acceleration provisions. Single-trigger acceleration vests all or part of your unvested options immediately when the acquisition closes, regardless of whether you keep your job. Acquirers generally dislike this because it removes the incentive for key employees to stay. Double-trigger acceleration vests only if two things happen: the company is acquired and you are terminated without cause (or resign for good reason, such as a significant pay cut or forced relocation) within a set period after closing, typically 9 to 18 months. Double-trigger is the more common structure.

If your plan has no acceleration at all, the acquirer may assume your options, convert them into options in the acquiring company, or cash them out. The time to negotiate acceleration is when you accept the grant, not after a deal is announced.

Post-IPO Lock-Ups

After an IPO, you typically can’t sell your shares immediately. Companies and their underwriters enter lock-up agreements that prevent insiders and option holders from selling for a set period, most commonly 180 days after the offering.⁷ These lock-ups aren’t required by regulation; they’re contractual agreements designed to prevent a flood of insider selling right after the IPO. Check your specific agreement for the exact duration and any early release provisions.

Transfer Restrictions on Private Company Shares

Owning private company shares doesn’t mean you can sell them freely. Most plans include a right of first refusal, which requires you to offer your shares back to the company or existing shareholders before selling to any outside buyer. The company can match a third-party offer, controlling who joins the ownership group.

Beyond the right of first refusal, shares acquired through employee option plans are restricted securities under federal law. Selling them requires meeting a federal exemption such as Rule 144, which imposes conditions on holding period, volume, and manner of sale. Even with a federal exemption, state securities requirements may still apply.⁸

Clawback provisions add another layer. Many plans let the company reclaim shares or profits under certain conditions: termination for cause, violation of a non-compete or confidentiality agreement, financial restatements, or conduct that harms the company’s reputation. These provisions can survive your employment, meaning the company can reach back months or years after you’ve left.

1
Office of the Law Revision Counsel. 26 USC 422 – Incentive Stock Options
2
Internal Revenue Service. Topic No. 427, Stock Options
3
Office of the Law Revision Counsel. 26 USC 409A – Inclusion in Gross Income of Deferred Compensation Under Nonqualified Deferred Compensation Plans
4
Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026
5
Office of the Law Revision Counsel. 26 USC 83 – Property Transferred in Connection With Performance of Services
6
Internal Revenue Service. Form 15620, Section 83(b) Election
7
U.S. Securities and Exchange Commission. Initial Public Offerings, Lockup Agreements
8
U.S. Securities and Exchange Commission. What Is a Private Secondary Market