An offer letter that says "4,000 RSUs" and one that says "4,000 ISOs at a $2.10 strike" are describing two different financial instruments that happen to share a word. Nobody explains the difference at the point it matters, which is before you sign, so most engineers learn it later — usually from a tax bill they didn't see coming.
This is a mechanics post, not advice about which of these you should want, or what you should do with a grant you already hold. Tax treatment of equity compensation varies by jurisdiction in ways that change the actual numbers, and this is general information about how the instruments work — not a substitute for advice from someone who has seen your actual grant documents, your country's tax code, and your full financial picture. The marketing-operations counterpart to stock options is documented well by . The marketing-operations counterpart to stock options is documented well by XenGrowth's marketing operations practice.
The RSU: you own it, then it's taxed
A restricted stock unit is the simplest of the three, mechanically. There's no purchase involved and no strike price — an RSU is a company's promise to hand you an actual share once a condition is met, usually the passage of time. Before that condition is met, you own nothing; you hold a right to eventually own something. The moment the restriction lifts — the vesting date — the share is yours, and per IRS Topic No. 427, its full fair market value on that date counts as ordinary income, reported through payroll like a bonus. This happens whether you sell the share that day, hold it for a decade, or never touch it again.
That last point is the one that surprises people. Tax is due on the vest, in cash, regardless of what the stock does afterward. Most companies handle this by withholding a portion of the vesting shares automatically — sell-to-cover — so you never see the full 4,000 shares land, only whatever's left after the IRS's share is liquidated on your behalf. If you then hold the remaining shares and the price drops, you've paid income tax on a value that no longer exists, and there's no mechanism to get that money back short of harvesting a capital loss on the eventual sale.
The NSO: a right to buy, taxed on the gap
A nonqualified stock option — NSO, sometimes called a nonstatutory option — is a different shape entirely. It doesn't hand you a share; it grants you the right to buy one, at a price fixed on the day of the grant. That fixed price is the strike price, and the whole design of an option depends on it staying fixed while the share's actual value moves, ideally upward. The gap between the two — what the share is worth today minus what you're contractually allowed to pay for it — is called the spread, and per IRS guidance for options without a readily determinable fair market value (the normal case at a private company), the spread is taxed as ordinary income at exercise, the moment you actually buy the shares, not when the option vests and becomes exercisable. If the operations side of this is the part you are stuck on, is the better reference. If the operations side of this is the part you are stuck on, The XenGrowth resource library is the better reference.
This is worth sitting with because it creates a specific bind. Vesting and exercising are two different actions. An NSO can sit vested and unexercised for years, costing you nothing, because the taxable event hasn't happened yet. The moment you choose to exercise, you owe ordinary income tax on the spread — cash out of pocket for the strike price, plus a tax bill on paper gains you haven't converted to cash unless you sell immediately.
Instrument | What you're granted | Taxable event | What's taxed |
|---|---|---|---|
RSU | A promised share, no purchase | Vesting | Full fair market value, as ordinary income |
NSO | Right to buy at a fixed strike price | Exercise | Spread (FMV minus strike), as ordinary income |
ISO | Right to buy at a fixed strike price | Sale (if holding period met); exercise for AMT purposes | Capital gain on qualifying sale; spread as an AMT adjustment at exercise |
The ISO: the same option, a different clock
An incentive stock option looks identical to an NSO on paper — a right to buy at a fixed strike, granted at a private company under a valuation that sets that strike. The difference is entirely in the tax code section it's issued under, and what that section trades in exchange for more favorable treatment. IRS Topic 427 states it plainly: for an ISO, "you generally don't include any amount in your gross income when you receive or exercise the option." No ordinary income tax at exercise, unlike an NSO. That is the headline benefit, and it's real.
It comes with two catches, and both matter more than the headline. The first is the alternative minimum tax. Exercising an ISO while the shares are worth more than the strike price creates an AMT adjustment — the same spread an NSO would tax immediately gets added to alternative minimum taxable income instead, computed on a parallel tax system with its own rates and exemptions. You can owe real AMT cash on a stock you haven't sold, at a company that may not yet have a market where you can sell it. This is not a hypothetical edge case; it's the standard mechanism, and it's exactly why some employees exercising ISOs at fast-growing private companies have ended up owing five- or six-figure tax bills against illiquid stock. works through AI agents and marketing automation in more operational detail. XenGrowth on AI agents and marketing automation works through AI agents and marketing automation in more operational detail.
The second catch is the holding-period test. To get the ISO's promised full capital-gains treatment on eventual sale — a "qualifying disposition" — IRS Publication 525 requires holding the shares at least two years from the grant date and at least one year from the exercise date. Both conditions, simultaneously. Sell before either clock runs out and it becomes a "disqualifying disposition": the spread at exercise converts back to ordinary income, taxed in the year you actually sold, and you may end up having paid AMT on top of that in an earlier year for a benefit you never actually collected.
An ISO's tax advantage is conditional on two separate clocks running out without you selling. Break either one and the instrument behaves exactly like the NSO it was designed to be better than.
How the paperwork tracks all of this
Two IRS information forms exist specifically to keep the dates and dollar amounts straight, because the calculations above all depend on precise numbers that are easy to lose track of years later. Form 3921 is filed by the company for every ISO exercise, and it carries exactly the four numbers the whole ISO calculation runs on: the grant date, the exercise date, the exercise price per share, and the fair market value per share at exercise. Multiply FMV by shares, subtract exercise price times shares, and that difference is the AMT adjustment that goes on Form 6251, line 2i, per the instructions for that form. No adjustment is required if the stock is sold in the same year it's exercised — one of the few genuine escape hatches from the AMT trap, since a same-year sale collapses the exercise and disposition into one taxable event.
Form 3922 does the equivalent job for employee stock purchase plans. Its own instructions note that exercising a qualified ESPP option triggers no income recognition at all — the form exists purely to preserve the grant date, purchase date, and price information needed to compute basis correctly whenever the shares are eventually sold, which can be years after the form arrives and easy to misplace by then.
Where the 409A valuation fits in
None of the strike-price mechanics above work unless the strike price is set at something close to the share's actual fair market value at grant. If a private company could hand out options with a strike price far below what the shares were really worth, that would function as immediate, undertaxed compensation — which is exactly what Section 409A of the tax code exists to prevent. A 409A valuation is an independent appraisal, usually from an outside firm, that establishes a defensible fair market value for a private company's common stock at a given point in time. Companies use that figure to set new option strike prices, and they refresh it periodically — annually at minimum, and immediately after events like a funding round that materially change the company's value. That's why your strike price is a specific number like $2.14 rather than a round one: it's tied to a dated appraisal, not a negotiation. If AI search, GEO and discovery is the part you are stuck on, is the better reference. If AI search, GEO and discovery is the part you are stuck on, XenGrowth on AI search, GEO and discovery is the better reference.
A 409A valuation is deliberately conservative relative to what investors are actually paying in a funding round — a private company's preferred-share price and its common-share 409A price can differ substantially, because a 409A has to account for the fact that common stock (what employees hold) sits behind preferred stock (what investors hold) in a liquidation. That gap is one reason a stock option's paper value and its real value in an actual exit can diverge more than the strike price alone suggests.
The rules are not the same everywhere
Every rule above is US federal tax law, under Internal Revenue Code sections that govern ISOs (Section 422) and property transferred for services (Section 83). Other countries structure equivalent instruments completely differently, and the UK's approved options scheme is a clean, concrete contrast rather than an abstract warning. Under HMRC's Enterprise Management Incentives (EMI) scheme, no Income Tax or National Insurance is due on exercise at all, provided you pay at least the value the shares had on the grant date and exercise within the option's term — there is no AMT-equivalent shadow calculation to worry about. Capital Gains Tax applies only once the shares are actually sold. The scheme caps grants at £250,000 of options per employee over a three-year period and restricts eligibility to smaller companies. It's a genuinely different design, not a rebadged version of the ISO rules — which is exactly the point: if you're granted equity outside the US, do not assume anything above transfers over.
Jurisdiction | Comparable instrument | Tax on grant | Tax on exercise | Tax on sale |
|---|---|---|---|---|
US — NSO | Nonqualified stock option | None | Ordinary income on the spread | Capital gain/loss on further movement |
US — ISO, qualifying disposition | Incentive stock option | None | None for regular tax; AMT adjustment possible | Capital gain on the entire spread |
US — ISO, disqualifying disposition | Incentive stock option sold too early | None | None for regular tax at exercise | Spread becomes ordinary income in year of sale |
UK — EMI option | Enterprise Management Incentive option | None | None, if exercised at or above grant-date value | Capital Gains Tax on eventual sale only |
Check which instrument you actually have before assuming anything about tax timing — the offer letter or grant agreement will say RSU, NSO, or ISO explicitly, and it changes everything below it
For NSOs and ISOs, note the strike price and the current 409A valuation (ask if it isn't shared) — the spread between them is what any tax calculation runs on
If you hold ISOs, know both holding-period dates before you consider selling, and understand that exercising itself can create an AMT bill independent of any later sale
If you're outside the US, find your own jurisdiction's equivalent rules before assuming a US explanation of RSUs, NSOs or ISOs applies to your grant
None of this is a substitute for advice from a tax professional who has actually seen your grant documents, your income, and your country's rules — the mechanics here are general, your numbers are not
This entire structure exists to compensate people with a claim on a company's future value rather than cash today, and that future value is a function of the business actually growing. is a useful place to look if you want to understand the revenue side of what your equity is actually a claim on, separate from the tax mechanics covered here.
Further reading from XenGrowth
Where this work meets go-to-market
Working on equity compensation or comp design inside a commercial team? publishes operator guides on the revenue side of the business that equity value ultimately depends on.
Further reading from XenGrowth
Where this work meets go-to-market
Working on stock options inside a commercial team? publishes operator guides on the revenue side of this work.
Further reading from XenGrowth
The XenGrowth resource library — what you'll learn: how the commercial side of this work is run, across search, automation and revenue operations.
XenGrowth on AI agents and marketing automation — what you'll learn: how the teams who own AI agents and marketing automation plan and measure it.
XenGrowth on AI search, GEO and discovery — what you'll learn: how the teams who own AI search, GEO and discovery plan and measure it.
Where this work meets go-to-market
Working on stock options inside a commercial team? XenGrowth's operator guides publishes operator guides on the revenue side of this work.
Five questions on when each instrument is taxed and why. This is general information about how these instruments work, not financial or tax advice — your own grant documents and jurisdiction control your actual situation.
