Employee Stock Option

An employee stock option is a call option your employer grants instead of cash: the right to buy company shares at a fixed strike (usually the share price on the grant date) for up to ten years. Stock triples → each option is worth the difference. Stock goes nowhere → the options expire as expensive-feeling paper.

Three features make ESOs unlike any listed option. They vest — typically over four years with a one-year "cliff", so leaving early forfeits the unvested portion; that's the retention machinery. They are non-transferable — you can't sell them, only exercise. And exercising usually requires cash and triggers tax, at moments not of your choosing.

For startup employees, options are frequently the majority of total compensation — which makes the vocabulary (strike, 409A valuation, dilution, preferences) financially load-bearing. A "1% stake" in option form can be worth millions, or nothing, and the difference often lies in terms the offer letter didn't emphasise.

What an unvested grant is worth

Companies expense ESOs at fair value — Black–Scholes or a lattice, with haircuts for expected early exercise and forfeiture:

The effective life \(T_{\text{eff}}\) is typically 4–6 years, not 10: employees exercise early — for liquidity, diversification or departure — systematically leaving option value on the table (a listed 10-year ATM call at 30% vol is worth ~45% of spot; exercising at year 4 the moment it's 20% in the money captures far less).

The startup-specific stack

• 409A strike: private-company strikes are set at an appraised fair value, usually far below the last VC round's preferred price — that gap is the built-in head start.
• Preferences ahead of you: VCs hold preferred stock with liquidation preferences. In a mediocre exit, preferences absorb the proceeds and common (your options) can get little — the headline valuation was never your valuation.
• Post-termination window: standard terms give 90 days after leaving to exercise or forfeit — often forcing a cash-and-tax outlay on illiquid shares. Extended windows (5–10 years) are a materially valuable, negotiable term.
• Dilution: each funding round adds shares; your percentage shrinks even as (hopefully) the pie grows.

Valuation refinements the textbooks skip

• Utility-based exercise: employees are undiversified — their human capital and options ride the same stock. Rational risk-averse holders exercise "early" by Black–Scholes standards; models (Hall–Murphy) put the subjective value of an ESO at 30–60% of its market cost to the firm. The gap is the deadweight cost of paying in options.
• ISO vs NSO tax asymmetry (US): ISOs defer ordinary tax to sale (capital gains if held) but the exercise spread hits AMT — the trap that bankrupted dot-com employees who exercised at the top, owed AMT on paper gains, and watched the shares collapse before the sale. NSOs tax the spread as ordinary income at exercise, brutally simple.
• Early-exercise provisions + 83(b): exercising unvested shares at grant (strike ≈ FMV, spread ≈ 0) and filing an 83(b) election starts the capital-gains clock with near-zero tax — the standard optimisation for founders and first employees, worthless once the 409A has run up.

The modern substitution: RSUs

Listed companies have largely switched to restricted stock units — shares delivered at vest, no strike, no exercise decision:

RSUs are worth something in every state of the world (lower risk, lower upside — a forward, not an option); options survive where volatility is the point: early-stage companies selling employees the right tail. Late-stage privates add double-trigger RSUs (vest requires both time and a liquidity event) to avoid taxing employees on unsellable shares.

The formulas above are standard textbook formulations, simplified for teaching. They explain the mechanism — they are not a valuation tool, and they will not reproduce a dealer’s price.