Private Company Stock Options: Valuation and Tax

A private company sets your option strike price at the 409A fair market value on your grant date — and that single number, frozen for the life of the grant, determines whether exercising costs you almost nothing or triggers an alternative minimum tax bill before you have sold a single share. For an employee exercising incentive stock options on a spread of $200,000, the AMT preference alone can manufacture a five-figure federal tax liability on paper gains that are entirely illiquid. The stock cannot be sold. The tax is still due.

That mismatch — between a tax event and any cash to pay it — is the defining problem of private company equity. Public-company employees sell shares to cover withholding. Private-company employees often cannot, and the figures below show how wide the gap gets at a 37% federal marginal rate.

The numbers that define the cost

Source: IRS Revenue Procedure 2024-40 (2025 inflation adjustments); California Franchise Tax Board, 2025 brackets. Figures retrieved June 2026.

Three instruments, three tax timelines

Where ordinary income lands and when it lands is the entire game. The instrument you hold determines both, and the differences are not cosmetic.

RSUs are the simplest and the most expensive on a current-cash basis. There is no strike price and no exercise decision. At vest, the fair market value of the vested shares is ordinary income, reported on your W-2. For a private company that means a tax bill keyed to a 409A valuation, often with no public market to sell into. The IRS treats the full vest value as wages, so it carries the entire payroll-tax stack on top of income tax — a detail covered in depth in the RSU net yield breakdown.

NSOs defer the event to exercise. At that moment, the spread between your strike price and the current FMV becomes ordinary income, subject to W-2 withholding and FICA for employees. You control the timing of the exercise, which is the planning lever. You do not control the rate — it is your full marginal rate on the spread.

ISOs are the instrument with the asymmetric tax profile, and the one most distinctive to private companies. No regular tax is due at grant or exercise. If you meet the holding period — two years from grant and one year from exercise — the entire gain from strike to sale is long-term capital gain. Miss it, and you have a disqualifying disposition: the spread at exercise reverts to ordinary income. The catch sits between exercise and sale. The ISO spread at exercise is an AMT preference item, meaning it enters the parallel alternative minimum tax calculation even though it never touches your regular return. That comparison between ISOs and NSOs on total tax cost is worked through in the ISO and NSO tax cost comparison.

What the 409A valuation actually controls

Section 409A, enacted in 2004 after the option-backdating scandals, requires that private company options carry a strike price no lower than the fair market value of the common stock on the grant date. Public companies use their trading price; private companies need a formal valuation to determine fair market value and the strike price, which must be at or above that value. A qualified independent appraisal grants safe-harbor protection.

Two consequences follow for your tax bill. First, your strike price is locked at the grant-date 409A figure and never moves, so an early grant at a low valuation is a durable advantage — every dollar the company appreciates afterward is potential capital gain rather than ordinary spread. Second, the gap between common and preferred matters enormously. 409A valuations of common stock typically run 20% to 80% below the preferred share price set in a fundraising round, because common stock lacks the liquidation preferences and protections that venture investors negotiate. The headline “unicorn” valuation is the preferred price. Your option spread is measured against the lower common 409A figure — which works in your favor at exercise and against you at sale.

Funding rounds reset the clock. A Series B that doubles the company’s value typically triggers a new 409A and a higher common FMV, which raises the spread on any options you exercise afterward. The timing question — exercise before the next round at a lower valuation, or wait — is the core of the 83(b) election timing analysis for early exercisers.

The AMT trap, quantified

Consider an employee who exercises 10,000 ISOs at a $2.00 strike when the 409A FMV has climbed to $22.00. The spread is $200,000. For regular tax, nothing happens — no income is reported. For AMT, that $200,000 is added to alternative minimum taxable income as a preference item.

Run the parallel calculation. A single filer with $250,000 of regular wage income plus a $200,000 ISO preference reaches roughly $450,000 of AMTI before the exemption. The 2025 AMT exemption is $88,100 for singles, and the 28% rate applies to taxable AMTI above $239,100; below that, the rate is 26%. The exemption survives intact here because the 2025 phase-out does not begin until AMTI reaches $626,350 for single filers. Tentative minimum tax on roughly $362,000 of post-exemption AMTI lands near $98,000 — and if that exceeds the filer’s regular tax, the difference is owed as AMT.

The shares are still private. They cannot be sold to fund the bill. This is the structural cruelty of the ISO AMT preference: the tax is triggered by a paper spread on an asset you cannot liquidate. Who actually gets caught, and the income bands where it bites hardest, are mapped in the ISO AMT risk analysis. There is a partial offset — the AMT paid generates a minimum tax credit usable in future years when regular tax exceeds tentative minimum tax — but that recovery can take years and assumes a future tax profile that allows it.

Source: AMT rates and exemption per IRS Revenue Procedure 2024-40 (2025). Spread and incremental AMT are illustrative for a hypothetical $2 strike / $22 FMV grant; actual liability depends on the filer’s full AMTI. Retrieved June 2026.

Finluxy Equity After-Tax Yield

The cleanest way to compare these instruments is to measure what fraction of pre-tax value actually survives the tax event. The Finluxy Equity After-Tax Yield expresses net after-tax proceeds as a percentage of the pre-tax fair market value at the event date: Yield = (FMV − total taxes paid) ÷ FMV × 100.

For an RSU vest, the calculation is direct because the full value is wage income. Take 1,000 shares vesting at $85 FMV — $85,000 of ordinary income for a California top-bracket employee. Federal at 37% is $31,450. California at 13.3% is $11,305. Medicare at 1.45% is $1,233 (wages above the Social Security cap, so no further OASDI). The additional 0.9% Medicare surtax adds $765 once total wages exceed $200,000. Total tax of roughly $44,753 leaves $40,247 net, for a yield of 47.3%. Push the same employee below the surtax threshold or out of California and the yield moves materially — the geography of the California equity tax burden is doing real work here.

Source: Finluxy calculation applying IRS 2025 federal rates (Rev. Proc. 2024-40), 3.8% NIIT, and California Franchise Tax Board 2025 top rate. ISO qualifying yield excludes any AMT timing cost at exercise, which can lower the realized yield in the exercise year. Figures are segment estimates for top-bracket filers; individual results vary with total income. Retrieved June 2026.

The yield spread between the ISO qualifying path (~63%) and every ordinary-income path (~47–48%) is roughly 15 percentage points of the pre-tax value. On $85,000 that is about $13,000. On a $850,000 equity event it is $130,000. That gap is the entire economic case for meeting the ISO holding period — and the entire risk embedded in the AMT bill you may have to front to get there.

The QSBS layer most coverage skips

Here is what most private-equity-comp analysis misses: for many private company employees, the capital gains rate on an eventual sale may not be 20% at all. It may be zero.

If the company qualifies as a qualified small business under Section 1202, gain on the sale of the stock can be excluded entirely. The One Big Beautiful Bill Act, enacted July 4, 2025, rebuilt this provision. For QSBS issued after July 4, 2025, a 50% exclusion applies at three years held, 75% at four years, and 100% at five years — a tiered structure replacing the old all-or-nothing five-year requirement. The per-issuer exclusion cap rose from $10 million to $15 million, and the company’s aggregate gross asset ceiling went from $50 million to $75 million, widening the universe of qualifying employers.

The interaction with options is precise and easy to get wrong. QSBS status attaches when stock is acquired, not when an option is granted — so for ISOs and NSOs, the clock and the qualification test run from exercise, the moment you actually hold shares. Exercising earlier therefore does double duty: it starts the capital gains clock and, if the company qualifies, the QSBS holding clock. There is a rate trap on the partial tiers, though. The non-excluded portion of gain on QSBS held three or four years is taxed at 28% rather than the standard 15% or 20% capital gains rate, so the 50% and 75% tiers are less generous than they appear. Stock acquired on or before July 4, 2025 stays under the prior rules — five-year hold, $10 million cap — which makes acquisition date a figure worth knowing precisely.

What the $150k+ household is actually deciding

At this income level the equity decision is rarely “should I participate.” It is a liquidity and timing problem layered on top of an already-high marginal rate. Three trade-offs dominate.

The first is the exercise-cost-versus-AMT-risk question for ISOs. Exercising early at a low 409A valuation minimizes the spread and therefore the AMT preference, and it starts both the capital gains and QSBS clocks. But it requires writing a check for the strike price on shares you cannot sell, plus potentially an AMT bill on top. A household earning $150k–$300k of base income has the marginal rate to make the ISO long-term-gain path worth roughly 15 points of yield, but may not have idle cash to absorb a six-figure exercise plus AMT on an illiquid position. The honest constraint is balance-sheet capacity, not tax theory.

The second is concentration. A successful private equity stake routinely becomes the largest asset a high-earning household owns, often exceeding the home and the retirement accounts combined, and entirely undiversified and illiquid. The after-tax yield figures above are the cost of converting that concentration into cash. A 47% RSU yield is not a reason to avoid selling — it is the price of diversification, and the more relevant comparison for many households is a near-50% haircut now against the documented risk of a single private company going to zero. Once the stock is public or a tender offer exists, the mechanics of selling into a fixed schedule run through a 10b5-1 trading plan structure, and the tax-timing math of selling versus holding at vest is the subject of the vest-and-sell decision math.

The third is the liquidity event itself. Most of these tax bills come due at an exit — acquisition, tender offer, or IPO — and the concentration of income into a single year can push an otherwise mid-bracket household into the 37% federal and 13.3% California top brackets simultaneously, while a large ISO exercise in the same year compounds AMT exposure. The full sequencing of an exit-year tax event is covered in the IPO equity tax cost analysis, and the foundational framework across all instruments sits in the equity compensation tax guide. The decision that pays off most is the one made before the liquidity event, when the strike, the 409A, and the QSBS acquisition date are still variables you can act on rather than figures you are reconciling on a return.