Option Tax Landmines

The AMT on ISOs: A Tax Bill Before a Single Share Is Sold

Mechanism.When you exercise an ISO (Incentive Stock Option), the regular income tax doesn't recognize the gain. But the AMT (Alternative Minimum Tax—a parallel calculation; you pay whichever of the two systems produces the larger bill) does pick up the bargain element:

You've sold nothing and received no cash, yet that number is already in the AMT base, taxed at 26% / 28%. The nastier part: it is fixed at the price on the day you exercised. If the stock later goes to zero, that year's AMT does not go away with it.

What to do.Three things:

• Find the crossover before you exercise. Each year, exercise only up to the share count that keeps you just under AMT (the "AMT crossover point"); tax software or a CPA can model it. Slicing it across years is far safer than exercising everything at once.
• Know what makes a disposition qualifying. Hold 2 years from grant and 1 year from exercise, and the entire gain is long-term capital gain. Miss either leg and it's a disqualifying disposition: the bargain element converts to ordinary income on your W-2.
• AMT isn't purely sunk. Extra AMT paid because of an ISO exercise creates a minimum tax credit, recoverable in later years when regular tax exceeds tentative minimum tax—but that can take many years. Don't count it as cash coming back soon.

There's also the "$100,000 rule": ISOs first exercisable in a single year, to the extent their grant-date value exceeds $100,000, are taxed as NSOs (Non-qualified Stock Options). AMT exemption amounts and phase-out thresholds are adjusted annually, and 2025 legislation tightened the phase-out—directionally, high earners get caught by AMT sooner. Use the figures the IRS publishes for the filing year in question.

83(b): A One-Way Ticket That Expires in 30 Days

Mechanism.The default rule under §83 is that stock subject to a substantial risk of forfeiture is taxed at each vesting date, at the fair market value on that date—so the more the company appreciates, the heavier your tax in later years. An 83(b) election is you volunteering: "tax me now, on the full value as of today." At an early-stage company today's FMV is often exactly what you paid, so the spread is near zero → near-zero tax today, and the long-term capital gains clock starts running immediately.

What to do.

• Applies to: restricted stock purchased by founders and early employees, and unvested shares you receive by early-exercising an option.
• Does not apply to: RSUs (Restricted Stock Units). An RSU is a promise, not stock—it isn't property for tax purposes, so there is nothing to elect on. This is the single most commonly mis-asked question here.
• 30 days is hard, running from the transfer date. No extension, no cure. The IRS provides a standard form (Form 15620); a self-drafted letter also works. Keep proof of mailing and a receipt either way.

State the cost plainly.An 83(b) moves the bet forward. If the company dies, or you leave before vesting and the shares are repurchased at cost, the tax you already paid does not come back, and the cash you fronted for the shares is most likely gone too. Whether it's worth it depends on how low the purchase price is and how confident you are that you'll stay.

Exercise Timing: This Is an Investment Decision, Not Collecting a Perk

Mechanism.Cash out of pocket = strike × shares + the tax triggered on the spot (an NSO creates ordinary income and withholding immediately; an ISO creates the AMT above). Private-company shares usually can't be sold, so what you're really doing is using after-tax cash to buy a highly concentrated, illiquid position that can go to zero. Three timing constraints are hard ones: (1) the smaller the spread, the smaller the tax, so exercising early is cheapest; (2) the post-termination exercise window is typically only 90 days (extending an ISO's exercise period beyond 90 days converts it into an NSO); (3) ISOs expire 10 years from grant by statute.

What to do (with a calibration).A common piece of advice is "don't put more than some multiple of your salary into a single stock." Any rule of thumb anchored to income systematically overstates capacity for high earners, people in high marginal brackets, and anyone paid mostly in equity—pretax income overstates what's actually spendable, and the rule quietly assumes you'll earn today's peak income for life. Use a harder ruler, anchored to spending:

Above 1, shrink the position. And the money should come from realized savings (= money you didn't spend and invested—not a checking balance), never from your emergency fund, consumer debt, or a loan secured by your own employer's stock. That last one is betting the same risk a third time.

IPO and Acquisition: The Tax Lands Before the Cash

Mechanism.IPO: private-company RSUs are usually double-trigger—they vest only when both the service condition and the liquidity condition are met. The result is several years' worth of shares recognized as ordinary income in one year. Meanwhile the default federal withholding on supplemental wages is just 22% (37% on amounts above $1 million). If your marginal rate is 32%–37%, that gap becomes a real bill at filing time the following year, possibly with underpayment penalties attached. And the lock-up typically runs 90–180 days: the tax is already fixed while the shares still can't be sold.

Acquisition: in a cash deal, options are usually cashed out at closing and taxed as ordinary income; cashing out an ISO is a disqualifying disposition, which voids whatever groundwork you'd laid for qualifying treatment. Part of the consideration may also sit in escrow or ride on an earnout and arrive a year or two later—the tax won't necessarily wait for it.

What to do.(1) Check your withholding gap: ordinary income recognized × (your marginal rate − 22%) is the order of magnitude you'll have to cover yourself; pay it via quarterly estimated tax or raise your W-4 withholding. (2) The safe harbor that avoids underpayment penalties: pay in 110% of last year's total tax (when last year's AGI, adjusted gross income, exceeded $150,000; otherwise 100%), or 90% of this year's actual tax. (3) Fix your selling rule before the lock-up expires—sell in scheduled tranches, or use a 10b5-1 plan to hand the decision to a discipline.