ISO Spread at Exercise and the AMT Preference Item

When an employee exercises an incentive stock option (ISO), the difference between the stock’s fair value and the strike price (the “spread”) is not taxed as ordinary income at exercise. Instead, it becomes a preference item under the alternative minimum tax (AMT), increasing the employee’s AMT taxable income. For high-income earners exercising large grants, this can trigger a real tax bill in the exercise year, even before selling a single share.

The tax advantage and the catch

ISOs are specially designed for employee equity. When an employee exercises an ISO, no ordinary income tax is due at that moment. Compare this to a non-qualified stock option (NSO), where exercising triggers immediate ordinary income tax on the spread. This deferral is the entire point of the ISO structure: the employee defers taxation until she sells the shares, at which time any gain between the exercise price and the sale price is taxed as a long-term capital gain.

But the IRS does not let this deferral go unmonitored. The spread is treated as a preference item for alternative minimum tax (AMT) purposes. This means the spread—potentially millions of dollars on a late-stage startup employee’s large grant—is added to her AMT income in the exercise year.

How the alternative minimum tax works

The AMT is a parallel tax system designed to ensure high-income taxpayers pay at least a minimum effective tax rate. It works by adding back certain preference items and deductions into a separate AMT income base, then applying a flat 26% or 28% AMT rate to any AMT income in excess of an annual exemption.

For 2024 (with typical indexation), the federal AMT exemption is approximately $96,300 for married filers and $63,000 for single filers. If a taxpayer’s AMT income exceeds the exemption, she owes AMT equal to the rate (26% or 28%) times the excess. In most cases, she then pays the greater of her regular tax or her AMT.

The ISO spread is added directly to AMT income. If an employee exercises options and incurs a $1 million spread, her AMT taxable income increases by $1 million. If her other income is already $200,000, her total AMT income is $1.2 million. Subtracting the exemption of ~$96,000 leaves $1.104 million in AMT taxable income. At 26% federal AMT, she owes approximately $287,000 in AMT tax.

A concrete example

Scenario: A mid-level engineer at a late-stage startup holds 100,000 vested ISOs with a $5 strike price. The current fair value is $55 per share. She exercises all 100,000 options.

• Spread per share: $55 − $5 = $50
• Total spread: $50 × 100,000 = $5,000,000
• Ordinary income tax at exercise: $0

However, for AMT:

• AMT preference item: $5,000,000
• Assume her W-2 income is $300,000
• Total AMT taxable income: $5,300,000
• Less exemption (married filing jointly): ~$96,000
• Net AMT income: $5,204,000
• AMT tax at 26%: approximately $1,353,000

If her regular tax on $300,000 of W-2 income plus capital gains is $100,000, she will owe the AMT—the difference is payable in the exercise year, even though she has not yet sold the shares.

The cash-out-of-pocket trap

This creates a painful scenario: the employee has exercised options to own shares, but has not liquidated them to raise cash. Yet the tax bill is due when the personal tax return is filed (typically April 15 of the following year). The employee must find $1.353 million in after-tax income to pay the AMT liability, even if the shares are still illiquid or if the stock price has fallen since exercise.

In the case of a startup that is acquired or has an IPO between exercise and the tax filing date, the employee can often sell shares to cover the tax bill. But if the stock falls, the employee has locked in a loss: she paid AMT tax on a $5 million spread, but the shares are now worth less than $55 per share, so the actual economic gain is lower.

AMT credit and future relief

The AMT system includes a mechanism for relief called the “minimum tax credit.” In any year in which an employee pays AMT, she accrues a credit equal to the excess of AMT paid over regular tax. This credit can be carried forward indefinitely and used to reduce regular income tax liability in future years. In other words, the excess AMT payment in the exercise year is eventually recovered when the employee has higher regular income and lower (or zero) AMT exposure.

However, the timing of this recovery is uncertain. If an employee exercises before an IPO and then works at the public company for many years earning high salaries, the credit recovery might take a decade or more. The credit also does not offset the real cost of the early cash payment.

Minimizing or managing AMT exposure

Employees with large ISO exercises can structure their activity to moderate AMT impact:

1. Stagger exercises: Instead of exercising all options at once, exercising in tranches over multiple tax years spreads the preference items and can keep some years below the AMT threshold.

2. Time the exercise: Exercising in a year with lower W-2 income (e.g., a sabbatical year) reduces total AMT income. Conversely, exercising in a year with high bonuses or capital gains worsens AMT.

3. Disposition strategy: Some financial advisors recommend selling shares shortly after exercise in years when AMT is expected, locking in long-term capital gain treatment while the stock price is high and using proceeds to pay the tax bill.

4. ESPP vs ISO timing: If an employee receives both ESPP purchases and ISO exercises in the same year, she should monitor the total spread to see if it pushes her into AMT.

See also