ISO vs RSU: Certain Value or Better Tax?

A restricted stock unit is deferred pay: the company promises to hand you shares on a schedule, and when it does, you have received compensation, full stop. An incentive stock option is a purchase right: the chance to buy shares at today's price years from now. If the stock triples, the option captures the whole move on shares you never had to be given; if the stock stalls below your strike, the option is a souvenir.

The tax system follows the economics. Compensation delivered is compensation taxed, so RSUs hit your W-2 at vest. A purchase right exercised is, under the ISO statute, not income yet, so the tax waits for the sale and can arrive at capital-gains rates. Every practical difference between the two flows from those two sentences.

The day RSU shares vest and settle, their full market value becomes wage income: federal and state income tax, Social Security up to the wage base, and Medicare all apply, exactly as if the company had paid you a cash bonus and you had immediately bought stock with it. The amount lands in Box 1 of your W-2, and employers typically withhold by selling a slice of the shares, the sell-to-cover mechanics explained on our sell-to-cover guide.

Withholding is where RSU holders get hurt: the flat supplemental rate applied to vests sits well below the top brackets, so heavy vest years are routinely under-withheld. The rates, the math, and the estimated-payment fix are on the RSU tax withholding page.

After vest, you simply own stock with a cost basis equal to the vest-date value and a holding period that starts that day. Sell immediately and there is almost no additional tax; hold a year and further growth qualifies for long-term rates. The basis mechanics, including the 1099-B trap that double-taxes careless filers, are on the RSU cost basis guide.

One wrinkle worth knowing at private companies: double-trigger RSUs vest on time AND a liquidity event, deferring the tax until there is a market to sell into. That structure has its own timing traps, covered on the double-trigger RSU page.

ISOs are quiet at grant and quiet at vest; a vested ISO is just permission to buy. The tax story begins when you exercise. Exercising costs real cash (strike price times shares) and triggers no regular income tax, but the spread between the share value and your strike is added to your alternative minimum tax income. Small exercises usually slip under the AMT exemption ($90,100 single, $140,200 joint for 2026); large ones generate a real AMT bill on paper gains. The exercise-sizing math lives on the ISO AMT guide.

Then the clock: hold the exercised shares at least two years from grant and one year from exercise, and the sale is a qualifying disposition taxed entirely at long-term capital gains rates, 0%, 15%, or 20% for 2026 plus the 3.8% net investment income tax at higher incomes. Sell before the clock runs and the spread at exercise converts back to ordinary W-2-style income (a disqualifying disposition), which is the standard defensive play when a stock drops in the exercise year.

For the full statutory comparison against nonqualified options, including the $100,000 annual ISO limit that silently converts big grants into a mix, see ISO vs NSO.

The last row is the quiet one that matters most for planning. RSU taxation happens to you on the vesting schedule; ISO taxation is largely scheduled by you. Control over timing is worth real money to someone with fluctuating income, a sabbatical year, a move between states, or a charitable plan, and it is worth nothing to someone who never exercises.

This is the honest frame for offer negotiations: RSUs are a floor, options are a call option, and the ratio between them prices the risk. The broader mechanics of that trade, beyond ISOs specifically, are on RSU vs stock options.

Early-stage startups grant ISOs because the strike price is low, the upside story is the recruiting pitch, and the company has no cash for salary-like equity. The employee accepts illiquidity and risk; the ISO's tax treatment is the sweetener. The classic startup outcome pattern is binary: the options end up either worthless or transformative, and the tax planning (early exercises, AMT sizing, the holding clock) only matters in the second case.

Public companies grant RSUs because their employees can sell at will, which makes equity function as salary paid in shares. Certainty is the point; nobody wants their rent money struck at last year's price. In between sit late-stage private companies, which lean on double-trigger RSUs so employees do not owe tax on shares they cannot sell.

If you are moving between worlds, from a startup with ISOs to a public company with RSUs or the reverse, the planning problems swap completely: the ISO holder manages exercise timing and AMT; the RSU holder manages withholding shortfalls and concentration. Our guide to receiving stock options covers the first week's decisions when a new grant lands, and the equity comp hub maps the whole territory.

1. Valuing options at face value in an offer.

"40,000 options" is not $40,000 of anything. An option's value is the spread above strike times the probability the spread materializes, minus the tax and the cash you must commit to exercise. When comparing offers, discount option grants hard, and discount them harder the later the stage and the higher the strike.

2. Treating RSUs as a reason to hold the stock.

After vest, holding RSU shares is identical to buying the stock at that day's price with bonus cash. The tax is already paid either way. Concentration in your employer, which also pays your salary, is a risk decision, not a tax one; selling at vest costs almost nothing in tax and buys diversification.

3. Exercising ISOs without pricing the AMT first.

The AMT bill arrives with the return, unwithheld, in the exercise year. Sizing the exercise so the spread stays near the AMT crossover point, or splitting it across January exercises in consecutive years, routinely saves five figures against an unplanned lump exercise. The sizing method is on the ISO AMT page.

4. Leaving a job without an option plan.

Most ISO plans give you 90 days after termination to exercise or forfeit, and exercising within that window means finding strike cash plus AMT all at once, often before a liquidity event. If a departure is even possible, model the exercise decision before you resign, not during the exit interview.