RSU vs ISO: Tax Treatment, Risk, and When Each Makes Sense

RSUs and ISOs are the two most common equity compensation types for tech employees, but they work very differently. RSUs vest automatically and are taxed as ordinary income --- simple and predictable. ISOs require an exercise decision, can qualify for capital gains treatment, but carry AMT risk --- complex but potentially more tax-efficient.

Understanding these differences is essential if you are comparing job offers, planning exercise strategies, or simply trying to minimize your tax bill.

How Each Works

RSUs (Restricted Stock Units): Your company promises to deliver shares on a vesting schedule. When shares vest, the full fair market value is added to your W-2 as ordinary income. You pay nothing to receive the shares --- they are free. There is no exercise decision.

ISOs (Incentive Stock Options): Your company grants you the right to buy shares at a strike price (typically the FMV on the grant date). You must choose when to exercise --- buying shares at the strike price. If you hold the shares for 1 year from exercise and 2 years from grant, any gain qualifies for long-term capital gains rates (15-20%) instead of ordinary income rates (up to 37%).

Side-by-Side Comparison

Tax Treatment: A Worked Example

Let's compare the tax impact of $200,000 in equity compensation delivered as RSUs vs ISOs for a single filer with $150,000 in salary.

RSU Scenario

You receive RSUs worth $200,000 at vesting.

The tax is immediate and automatic. No decisions to make. See how RSUs are taxed for a full breakdown.

ISO Scenario (Qualifying Disposition)

You exercise 10,000 ISOs at a $5 strike when the FMV is $25. Spread = $200,000. You hold for the qualifying period and sell at $25 (no additional appreciation).

Higher Income Scenario: $500K ISO Spread at Staff Engineer Compensation

The picture changes at higher income levels. Consider a Staff/L6 engineer earning $250,000 in base salary who exercises ISOs with a $500,000 spread.

ISO path: The $500,000 spread is added to AMT income. At this income level, the AMT exemption ($88,100 for single filers in 2025) begins to phase out --- the exemption is reduced by 25 cents for every dollar of AMTI above $626,350. With a combined AMTI of $750,000+ (salary plus spread), the exemption erodes by roughly $30,900, leaving only ~$57,200 of exemption. The resulting AMT bill is approximately $126,000-$140,000 (28% rate applies above $239,100 of AMTI over exemption). If the shares are held through the qualifying period and sold at the same price, the $500,000 qualifies for LTCG at 20% (the higher rate applies at this income level) = $100,000, offset by AMT credits.

RSU path: The $500,000 is ordinary income on top of the $250,000 salary = $750,000 total W-2. Federal tax attributable to the RSUs (incremental methodology): approximately $175,000 (37% bracket on income above $626,350, 35% below). Plus state tax --- in California, this adds another ~$55,000.

Net comparison: After AMT credit recovery, the ISO path produces a total tax of roughly $100,000-$110,000 versus the RSU path at $175,000+ federal alone. The ISO advantage is larger in dollar terms ($65,000-$75,000 savings) --- but so is the risk. You must fund the $126,000+ AMT bill upfront, and if the stock declines during the holding period, you may sell for less than the AMT you paid. The AMT credit carries forward but does not refund cash immediately.

Decision Framework by Company Stage

Early-Stage Startup (Pre-Series B)

ISOs are typically better. The strike price is very low (often pennies to a few dollars), so the exercise cost is minimal. The spread at exercise may be small, keeping AMT manageable. If the company succeeds, you get capital gains treatment on enormous appreciation.

Why not RSUs here: RSUs at a private company create a tax bill at vesting when you cannot sell the shares. You would owe ordinary income tax on illiquid stock.

Growth Stage (Series B-D)

It depends on the 409A valuation. If the FMV has risen significantly, new ISO grants have a high strike price, reducing the upside advantage. RSUs may be more attractive because they don't require cash to exercise and don't carry AMT risk.

Key question: Is the exercise cost affordable, and is the AMT exposure manageable?

Late-Stage / Pre-IPO

RSUs are usually better. The FMV is high, so ISO exercise costs are substantial. The AMT exposure on a large spread can be enormous. RSUs vest and are taxed automatically --- and you will soon be able to sell shares to cover the tax. For exercise timing in this stage, see our guide to exercising stock options before an IPO.

Public Company

RSUs dominate. Almost all public companies issue RSUs. There is no exercise cost, no AMT risk, and shares can be sold immediately to cover taxes. The simplicity and guaranteed value outweigh the potential tax savings from ISOs.

The AMT Factor

The biggest risk with ISOs is triggering an AMT bill that you cannot afford. The spread at exercise (FMV minus strike price) is added to your AMT income. If this pushes you above the AMT exemption, you owe 26-28% on the excess.

2025 AMT exemption amounts:

• Single: $88,100
• Married filing jointly: $137,000

For an in-depth walkthrough, see our AMT calculator guide. Use our ISO AMT calculator to model your specific scenario.

RSU Withholding: A Different Problem

While RSUs avoid AMT, they have their own tax challenge: withholding shortfalls. Your employer withholds at the 22% supplemental rate, but your actual marginal rate may be 32-37% federal plus state taxes. This gap means you may owe thousands at tax time.

See our RSU withholding guide for strategies to manage this.

Multi-Year Planning: Combining RSU and ISO Strategy

If you hold both RSUs and ISOs (common during a company's transition from private to public), coordinating them across tax years can save significant money.

Year-by-year approach:

• High RSU vest years: When a large RSU tranche vests (especially cliff vests or refresh grants stacking), your ordinary income is already elevated. Exercising ISOs in the same year compounds the AMT exposure because both your regular tax base and your AMT income are high. Defer ISO exercises to a lower-income year if possible.
• Low RSU vest years: If you have a gap between cliff vests, or between the end of one grant and the start of a refresh, that lower-income year is ideal for ISO exercises. The AMT exemption is more likely to shield the spread, and your regular tax rate is lower, reducing the AMT crossover point.
• Departure planning: If you are considering leaving the company, exercise ISOs before your departure (while you still have ISO status). After 90 days post-departure, unexercised ISOs convert to NSOs, losing their preferential tax treatment entirely.

Practical tip: Use our ISO AMT calculator to model each year independently, plugging in your projected W-2 income (salary plus expected RSU vests) to find the AMT-free exercise limit for that year. Then sum the annual limits across your planning horizon to determine how many ISOs you can exercise over 3-5 years without triggering AMT.

For employees with both RSUs and ISOs, the goal is to flatten your total income across years rather than letting it spike. RSU vest dates are fixed, but ISO exercise dates are in your control --- use that asymmetry to your advantage.

Frequently Asked Questions

Can I choose between RSUs and ISOs?

Rarely. The company decides which type to grant based on your role, company stage, and tax considerations. Some companies allow a choice (often between ISOs and NSOs, not RSUs). If you have a preference, raise it during offer negotiation.

What if I hold ISOs and the company transitions to RSUs?

Your existing ISO grants remain as ISOs. New grants would be RSUs. You may hold both simultaneously, each with its own tax rules. This is common during a company's transition from private to public.

Which has more upside potential?

ISOs have more upside in a high-growth scenario because you can buy shares at a low strike price and potentially pay only capital gains rates on the appreciation. RSUs capture the full share value but are always taxed as ordinary income at vest, capping the after-tax return.

Do RSUs ever trigger AMT?

No. RSU income is ordinary income for both regular tax and AMT purposes. There is no AMT preference item associated with RSUs. Only the spread on ISO exercises creates an AMT preference.

What about early-exercise ISOs with an 83(b) election?

If your company allows early exercise of unvested ISOs, you can file an 83(b) election to start the holding period clock immediately. This can be advantageous at very early-stage companies where the FMV equals the strike price (zero spread = zero AMT). But you risk forfeiting shares if you leave before vesting.