Employee Stock Options (ESOP) Explained: Strike Price, Vesting, Exercise and Taxes

What employee stock options actually are

An employee stock option is a contract that gives someone the right, but not the obligation, to buy a fixed number of company shares at a fixed price called the strike price. It is not stock itself. Until you exercise the option and pay the strike price, you own nothing, and if the company never becomes worth more than the strike price, the option is worth nothing.

An employee stock option plan, usually shortened to ESOP, is the framework a company uses to hand these options out. The plan reserves a pool of shares, sets the rules for grants, and defines what happens when someone leaves. Startups use options because they cannot match big-company salaries in cash, so they offer a share of the upside instead.

Anatomy of a grant: number, strike price and vesting

Every grant has four numbers worth reading in the offer letter: how many options you receive, the strike price, the vesting schedule and the expiry date. A grant of 20,000 options with a strike price of $1.00 means you may buy 20,000 shares for $20,000 in total, once they have vested, at any point before the grant expires. Most grants expire ten years after the grant date.

Vesting decides when you earn the right to exercise. The common structure is four years with a one-year cliff: nothing vests in the first 12 months, 25% vests at the cliff, and the rest vests monthly over the remaining three years. Our guide to vesting schedules and cliffs covers the mechanics, and the same logic applies to employees as to co-founders.

The strike price is normally set at the fair market value of the common stock on the grant date. In the United States that value comes from an independent 409A valuation, which is why early grants tend to carry a low strike price and later grants a higher one. This is the main reason joining early is cheaper: the strike price rises as the company grows.

Stock options vs restricted stock vs RSUs

Founders usually hold restricted stock, which is real stock that vests over time. Employees at early-stage startups usually receive options, while late-stage and public companies lean towards restricted stock units, or RSUs, which deliver shares at vesting without any strike price to pay. The three instruments split the risk and the tax bill differently.

The table below summarises the differences at a high level. Details depend on the country, the plan documents and the tax advisor's reading, so treat it as a map and not as advice.

ISO vs NSO: the two US option types

In the United States, options come in two flavours. Incentive stock options, or ISOs, can only go to employees and may receive favourable tax treatment if holding-period rules are met, with a $100,000 annual limit on the value that becomes exercisable for the first time. Non-qualified stock options, or NSOs, can go to employees, advisors and contractors, and the spread at exercise is generally taxed as ordinary income.

The practical difference is when the tax bill arrives. With NSOs you typically owe income tax on the gap between the share's value and your strike price at exercise, even though you cannot sell the shares yet. With ISOs the regular tax can be deferred, although the alternative minimum tax may apply. This is exactly the kind of case where a tax advisor earns their fee.

Outside the United States the rules differ widely. Several countries tax the benefit at exercise, others at sale, and some have special regimes for startups. If you work in Turkey or any non-US jurisdiction, ask a local lawyer how the plan is documented and taxed before you assume the US vocabulary applies one to one.

Exercising options, and what happens when you leave

Exercising means paying the strike price to turn vested options into actual shares. Doing it before the company is sold means paying cash for illiquid shares and, depending on the country, possibly owing tax on paper gains. Many employees wait for a liquidity event, an acquisition or an IPO, and then exercise and sell in the same step.

The catch is the post-termination exercise window. Under many plans you have only 90 days after leaving to exercise your vested options, or they expire. For someone with 40,000 vested options at a $1.00 strike, that is $40,000 in cash plus possible tax, due within three months of walking out the door. Some companies now offer extended windows of several years, and that is a fair question to ask during hiring.

Unvested options are different: they normally disappear the day you leave. That is why the cliff matters. Resigning at month eleven of a standard schedule means walking away from the entire grant.

The founder's side: option pools and dilution

For founders, an ESOP is a budget of ownership. The pool is typically reserved as 10% to 15% of fully diluted shares and is usually created or topped up before a priced round, which means the dilution lands on existing holders instead of the new investor. The mechanics are covered in our guide to the option pool shuffle.

A short example: a company with 8,000,000 shares outstanding creates a pool of 1,000,000 option shares. After that, a founder who owned 50% of the 8,000,000 shares holds 4,000,000 of 9,000,000 fully diluted shares, or about 44.4%. The pool is reserved on paper long before anyone exercises, which is why investors price it in at term sheet time.

Granting well is a discipline. Give meaningful grants to the first ten hires, document the vesting and exercise terms in writing, and keep the cap table in sync with every grant. Sloppy grants surface later in due diligence and cost real time.

How to evaluate an offer that includes options

Do not look at the number of options in isolation. Ask for the total number of fully diluted shares so you can compute your percentage: 20,000 options out of 10,000,000 fully diluted shares is 0.2%. Then ask about the latest preferred price, the strike price, the liquidation preference stack and the exercise window after you leave.

Next, run a rough scenario. If the company sells for $100 million and preferred holders take their preference first, common holders may receive far less than a naive 0.2% of $100 million. Options are a lottery ticket with unusually good information attached: treat the expected value as a bonus, not as salary, and keep the cash component of the offer fair on its own.

A founder reading this from the other side should also think about runway. Every equity grant is cheaper than cash today, but a hiring plan still raises burn, so look at how a new hire affects monthly burn and cash alongside the equity you give away.

Practice equity and hiring trade-offs in Founder Runway

In Founder Runway you make founder decisions across a run of 20 turns, with four options on every decision. You choose a B2B, B2C or B2G business model and start at Pre-Seed, Seed, Pre-Series A or Series A, then watch how team, cash and runway choices play out.

A run ends as a failure, a promising company, EBITDA positive or high-value exit potential. The game does not model employee option plans or tax; what it trains is the trade-off underneath them: how much cash and control you spend to build the team that gets you to a good outcome.