How stock options, vesting, and cliffs actually work

A stock option is the right to buy a fixed number of shares at a fixed price for a set period of time. At a startup, that right is earned over four years with a one-year cliff: you vest nothing until your first anniversary, then 25 percent lands at once, and the rest drips in monthly over the next three years. The fixed price you pay to buy is the strike price, set by a 409A valuation. Everything else (ISOs vs NSOs, exercise windows, AMT, RSUs) is detail layered on top of those two facts.

This post is the plain-English map of how startup equity actually moves, with real numbers. None of it is tax or financial advice, and the math below is illustrative. If you are weighing a specific offer, pair this with how to evaluate a startup job offer and a one-hour call with a CPA who has seen startup equity before.

What does a 4-year vest with a 1-year cliff mean?

Vesting is the schedule on which you earn your options. The startup standard is four years, monthly, with a one-year cliff. Say you are granted 48,000 options. Here is how they unlock:

• Months 0 to 11: you have vested 0 options. If you leave before month 12, you walk away with nothing. That is the cliff.
• Month 12 (the cliff): 12,000 options vest in one chunk (25 percent).
• Months 13 to 48: the remaining 36,000 vest at 1,000 per month.
• Month 48: all 48,000 are vested.

Vesting only gives you the right to buy. It does not put shares in your hands or money in your pocket. To turn vested options into shares you have to exercise them, which means paying the strike price. To turn shares into cash you need a liquidity event (an acquisition or IPO). Those are three separate steps, and engineers routinely conflate them.

What is the strike price and the 409A?

The strike price is what you pay per share when you exercise. It is fixed on the day your grant is approved, and it is set by a 409A valuation: an independent appraisal of the company's common stock that the board commissions, usually once a year or after a new round. The 409A price is almost always well below the per-share price investors paid in the last round, because common stock (yours) has fewer rights than preferred stock (theirs).

A worked example (illustrative, not advice). You join after the Series A. The 409A sets common stock at $1.20 per share, so your strike is $1.20. Your 48,000 options would cost 48,000 times $1.20, which is $57,600 to exercise in full. If the company later sells at $9.00 per share, your spread is $7.80 per share, or $374,400 before tax on the full grant. The strike is the floor; the spread is the upside.

Vesting gives you the right to buy. Exercising is buying. Selling is getting paid. They are three different days, sometimes years apart.

ISOs vs NSOs: what is the difference?

Options come in two tax flavors. ISOs (Incentive Stock Options) can only go to employees and get more favorable tax treatment if you hold the shares long enough. NSOs (Non-qualified Stock Options, sometimes written NQSOs) can go to anyone (contractors, advisors, employees) and are taxed as ordinary income on the spread at exercise. Most early startup employee grants are ISOs, but companies often issue NSOs once an individual crosses the ISO limits or for non-employees.

The practical takeaway: ISOs are friendlier on paper, but the friendliness depends on holding the shares, and holding means exercising and paying real money. That is where the exercise window and AMT come in.

What is an exercise window, and why does it matter?

When you leave a company, you usually have a limited time to exercise your vested options before they expire. The legacy default is 90 days. Miss the window and your vested options vanish, no matter how many years you put in. This is the single most expensive surprise in startup equity, because exercising can cost tens of thousands of dollars in cash plus a tax bill, all due within three months of your last paycheck.

Some companies have moved to extended windows of 5 or 10 years (one reason to read the actual plan documents, not just the offer letter). When you are comparing offers, the post-termination exercise window belongs on your list of questions to ask in a startup interview. A great grant with a 90-day window can be worth far less than a smaller grant you can actually afford to keep.

What is AMT, and when does it bite?

The Alternative Minimum Tax (AMT) is a parallel tax system that can apply when you exercise ISOs. Here is the trap: when you exercise an ISO and hold the shares, you owe no regular income tax, but the spread (current value minus strike) counts as income for AMT purposes. So you can owe a tax bill on paper gains from shares you cannot sell and may never be able to sell.

A worked example (illustrative, not advice). You exercise all 48,000 vested ISOs at a $1.20 strike. By exercise day the 409A has risen to $3.20. Your paper spread is 48,000 times $2.00, which is $96,000. That $96,000 is invisible to regular tax but visible to AMT, and could trigger a five-figure AMT bill in April, on shares that are still completely illiquid. This is why people exercise early (when strike and 409A are close, so the spread is tiny) or exercise small batches each year to stay under the AMT line. A CPA can model your specific number before you click exercise.

RSUs vs options: how are they different?

Restricted Stock Units (RSUs) are not options. An RSU is a promise to give you actual shares when they vest, with nothing to buy and no strike price to pay. Big public companies and some late-stage startups grant RSUs; early-stage startups almost always grant options. The trade-off is real: RSUs have no exercise cost and no AMT, but they are taxed as ordinary income the moment they vest and become deliverable, and at an early startup that income can be hard to pay tax on without a market to sell into.

If you are comparing a startup option grant against a big-tech RSU package, the headline numbers are not the same currency. RSUs are close to cash; early options are a lottery ticket with a cash cost. We walk through how to value that gap in startup vs big tech for a software engineer and the stage-by-stage picture in how much equity a startup engineer gets by stage.

The whole journey, end to end

1. 01Grant. You are offered 48,000 options at a $1.20 strike, 4-year vest, 1-year cliff.
2. 02Cliff. At month 12, 12,000 options vest. You own the right to buy, nothing more.
3. 03Vesting continues. 1,000 more options vest each month through month 48.
4. 04Exercise. You pay the strike to convert vested options into shares. ISOs may trigger AMT here.
5. 05Holding period. For ISO capital-gains treatment, hold 1 year after exercise and 2 years after grant.
6. 06Liquidity event. An acquisition or IPO finally lets you sell. This is the only step that produces cash.
7. 07Sale and tax. You sell, and pay capital gains (or ordinary income, depending on the path).