Most employees who hold Incentive Stock Options (ISOs) in a private company understand the basic story: exercise, hold for the required period, pay long-term capital gains rather than ordinary income — yet many overlook employee option expiry and its effect on secondary timing. In a private market marketplace, however, what many employees overlook is the Alternative Minimum Tax — a parallel tax system that ignores most of the rules you thought you knew about ISOs and can produce a real cash tax bill in the year you exercise, even if your shares are locked up and worth nothing liquid yet. If you're still building foundational context on how pre-IPO investing, equity compensation, and secondary liquidity interact, review the complete guide to pre-IPO investing before making ISO exercise decisions.
This is not theoretical. It has happened to thousands of employees at high-valuation private companies across multiple market cycles. Understanding the mechanics — before you decide whether and when to exercise — is the minimum preparation required.
ISOs and the regular tax system: a quick baseline
Under the regular income tax system, exercising an ISO is not a taxable event. You acquire shares at your strike price, no income is recognized, and no tax is due until you sell. If you hold the shares for at least two years from the option grant date and one year from the exercise date — the 'ISO qualifying disposition' holding period — any gain on sale is taxed as long-term capital gain. That is the favorable treatment ISOs are designed to provide.
The Alternative Minimum Tax runs on a separate set of rules. Under AMT, the 'spread' at exercise — the fair market value of the shares on the date you exercise minus your strike price — is an AMT preference item. It is added back into your income for AMT calculation purposes even though it is invisible to the regular tax system.
How the AMT liability is calculated at exercise
The company's most recent 409A valuation establishes the fair market value of common stock for AMT purposes. If your strike price is $2.00 per share and the 409A is $42.00 per share when you exercise 50,000 options, your AMT preference item for the year is $2,000,000 ($40.00 spread × 50,000 shares).
That $2,000,000 is added to your other income in the AMT calculation. The current federal AMT rate on amounts above the exemption threshold is 26% up to approximately $232,600 of AMTI and 28% above that (thresholds adjust annually). After applying your exemption — which phases out rapidly at higher income — a $2,000,000 AMT preference item can produce an incremental AMT liability well into six figures.
The cruel part: this tax is due in April of the year after you exercise. The shares are still private. You cannot sell them to pay the bill unless a secondary market exists, a tender offer runs, or you decide to make a disqualifying disposition — which eliminates the long-term capital gains treatment you were trying to achieve.
The 409A valuation: the number that drives everything
Private companies are required to obtain a 409A valuation at least annually, and any time there is a material financing event. The 409A reflects the appraised value of common stock, which typically carries a discount to the preferred share price in a financing round — because preferred shares have liquidation preferences, anti-dilution rights, and other protections that common stock lacks.
In early-stage companies, this discount can be substantial — common stock 409A values are sometimes 20–40% of the preferred price. In late-stage companies approaching an IPO, the discount narrows because liquidation preferences matter less when the company is worth far more than the aggregate liquidation stack. A company like one of the mature issuers trading on secondary markets today may have a 409A value for common that is 70–90% of the last-round preferred price. That compression is what makes late-stage ISO exercises expensive from an AMT standpoint.
Timing matters. If a company just completed a major funding round, the new 409A will likely rise to reflect it. Employees who exercise before the new 409A is finalized may lock in a lower spread. This requires knowing when valuation updates typically occur — which most employees do not — and the window can be narrow.
Three strategies that reduce or manage AMT exposure
1. Exercise in tranches across multiple tax years
Because the AMT exemption applies annually, spreading exercises across two or more calendar years can keep each year's AMT preference item below — or closer to — the exemption threshold. This requires options that are vested and exercisable over a multi-year window, and a company that is not about to change its 409A dramatically mid-period. A tax advisor can model the optimal tranche size for your specific situation.
2. Exercise earlier in the company's life cycle, when the 409A is lower
The spread is smallest when the company is youngest. Early exercising — including the Section 83(b) election for unvested shares — locks in a low AMT preference item or eliminates it entirely if the 409A is at or near the strike price. For employees at companies that are already mature private issuers, this window has closed. But for employees with newly granted options, early exercise deserves serious consideration even before vesting begins.
3. Use a secondary sale to fund the tax bill
If a secondary market is active on the issuer at the time of exercise, it may be possible to sell a portion of the exercised shares — or a different tranche of previously exercised shares — to generate liquidity for the AMT payment. A sale in the year of exercise that does not meet the ISO qualifying disposition holding period creates a 'disqualifying disposition,' converting the gain on those shares to ordinary income. That sounds bad, but for shares sold to cover a tax bill it can be tax-neutral or even favorable compared to paying AMT on shares you then cannot liquidate.
Secondary markets are not available for every issuer at every price point. But for the 28 issuers covered on Limen Markets, buyer interest exists across a range of share classes, and sellers can often find a bid that funds the AMT liability without selling their entire position.
The AMT credit: recovery, not refund
AMT paid in an exercise year is not simply lost. It generates an AMT credit — a dollar-for-dollar reduction in regular federal income tax in future years when your regular tax liability exceeds your AMT liability. If you exercise today, pay $150,000 in AMT, and then sell the shares in a year when you have high ordinary income and low AMT exposure, that $150,000 credit reduces your regular tax bill.
The problem is timing. The credit is only usable when regular tax exceeds AMT in a future year. In a scenario where you exercise, the company fails or the IPO never comes, and you take a capital loss — you have paid AMT and the credit may take years to utilize. This is the scenario employees at companies that went through a down-round post-exercise know painfully well.
What to do before you exercise
- Get the current 409A valuation from your company's equity portal or legal team. Calculate your spread on the shares you plan to exercise.
- Run a preliminary AMT calculation with a tax advisor using your expected gross income for the year, including the AMT preference item from the planned exercise.
- If you plan to exercise a large block, model multi-year tranching and compare the cumulative AMT cost to the cost of a single-year exercise.
- Check whether the issuer has an active secondary market. Knowing what your shares could clear at — if you needed to sell a portion — changes the risk calculus significantly.
- Review your option grant agreement for expiration dates and any post-termination exercise windows before deciding on timing.
None of this is a substitute for advice from a CPA or tax attorney who knows your full financial picture. AMT calculations interact with state taxes, other preference items, and capital gain rates in ways that vary substantially by individual. The goal of this article is to ensure you arrive at that conversation knowing what questions to ask.
If you hold exercised shares in one of our covered issuers and want to understand what secondary liquidity looks like today, visit /marketplace for current indications. If you are weighing a full or partial secondary exit as part of managing your tax position, the seller-playbook guide walks through the transaction process from start to close. For a complete reference on accredited-investor tax treatment across the deal lifecycle, see our accredited investor tax guide. And if your original grant and holding period qualify, the QSBS exclusion can wipe out federal capital gains tax on up to $10M of gain — worth confirming with your CPA before any large secondary sale.