Equity Comp and the AMT Trap: What ISOs Do That RSUs Don’t

Incentive stock options get compared to RSUs so often that people assume the tax treatment is basically the same, just with an extra step. It isn't, and the difference is the kind that can cost real money if nobody catches it in time.

With RSUs, taxes get withheld automatically the moment they vest. With ISOs, nothing gets withheld when you exercise, because exercising an ISO isn't a taxable event for regular income tax purposes. That sounds like good news, and for regular tax it is. But there's a second, separate tax calculation running in the background called the alternative minimum tax, and exercising ISOs is one of the things that can trigger it. The bargain element, the difference between what you paid to exercise and what the shares were actually worth that day, counts as income under the AMT rules even though it doesn't count as income under the regular rules.

This is where people get caught. Because nothing was withheld and nothing showed up on a pay stub, exercising ISOs can feel like a non event. Then April comes, the AMT calculation kicks in, and there's a tax bill tied to paper gains on shares that may have since dropped in value, been held rather than sold, or in a worst case scenario, become illiquid or worthless if the company hits trouble before an exit. Owing tax on a gain you never actually realized in cash is the single hardest version of this problem, and it has genuinely happened to people who exercised early at a private company that didn't work out.

A few things reduce the risk. Running an AMT projection before you exercise, not after, tells you roughly what the exercise would cost you in the worst case, before you're committed. Exercising in smaller amounts across more than one calendar year, rather than all at once, can sometimes keep you under the AMT threshold in any single year. And if you do end up paying AMT, it's worth knowing that the excess you paid over your regular tax often comes back to you in later years as a credit, though getting that credit back can take longer than people expect and depends on your income in those later years.

None of this means ISOs are a bad form of compensation. They can be a meaningful part of a total package, especially early at a company with real upside. The mistake isn't holding ISOs, it's exercising them without knowing what the AMT math looks like first.

If ISOs are part of your compensation and you're weighing when to exercise, we're happy to run the numbers with you before you commit to anything.