Employees reading an offer letter — trying to understand what "ISO" or "NSO" next to their grant actually means for their personal taxes.
What Does ISO vs. NSO Mean on My Offer Letter?
You're not designing an equity plan — you're trying to understand the one you were just handed. Here's what it means for your taxes.
Understand the tax difference before you sign anything or exercise a single share.
If your offer letter mentions Incentive Stock Options (ISOs) or Non-Qualified Stock Options (NSOs), the label matters more than it looks. It determines when the IRS taxes you, how much, and whether that tax hits as ordinary income or capital gains.
Tax Treatment at Exercise: Ordinary Income vs. AMT
With an NSO, the difference between the strike price and fair market value at exercise is taxed immediately as ordinary income, with withholding taken out like a paycheck. With an ISO, there's no regular income tax at exercise — but that same spread can trigger the Alternative Minimum Tax (AMT), which catches many employees off guard.
Tax Treatment at Sale: Qualifying vs. Disqualifying Dispositions
Sell ISO shares after meeting the holding-period rules (two years from grant, one year from exercise) and the entire gain is taxed at long-term capital gains rates — a "qualifying disposition." Sell earlier and part of the gain reverts to ordinary income — a "disqualifying disposition." NSOs don't have this distinction; the exercise-day income is already taxed, and only post-exercise gains get capital-gains treatment.
Quick Reference: What Your Letter Means for You
| Event | ISO | NSO |
|---|---|---|
| At exercise | No regular tax, possible AMT | Taxed as ordinary income immediately |
| At sale (holding period met) | Long-term capital gains | Capital gains on post-exercise growth only |
| Withholding | None at exercise | Withheld like a paycheck |
- Check your grant type first — it's usually stated explicitly in your offer letter or plan documents
- AMT is the ISO trap most employees don't see coming until tax season
- Disqualifying dispositions can turn expected capital gains into ordinary income overnight
Frequently Asked Questions
If my offer letter says ISO, do I owe taxes when I exercise?
Not regular income tax, but you may owe Alternative Minimum Tax depending on the spread between your strike price and current fair market value. This is the single most common surprise for employees exercising ISOs.
- Run an AMT estimate before a large exercise, not after
- The spread at exercise is what triggers AMT exposure
- A tax advisor can model this against your specific numbers
What's the real difference between an ISO and NSO for me personally?
An NSO taxes you as ordinary income the moment you exercise. An ISO defers regular tax until sale but can trigger AMT at exercise instead. Which is better depends on your tax bracket and how long you plan to hold the shares.
- NSOs mean immediate withholding, no surprises later
- ISOs can be more tax-efficient if held to a qualifying disposition
- Your specific numbers, not a general rule, should drive the decision
The Lovie Advantage
Most explainers stop at the definition. Founders who set up their equity through Internal Link: Lovie Cap Table Management give their employees a cap table that already shows each person's grant type, strike price, and vesting status in one place — so you're not reconstructing this from a PDF offer letter when tax season arrives.
Planning an early exercise instead of waiting? See Internal Link: Early Exercise for ISOs vs. NSOs for the 83(b) election timeline. For the official guidance on how the IRS treats each type, IRS Publication 525 on taxable and nontaxable income is the primary source worth reading directly.
See How Your Grant Type Affects You
Understand the tax difference before you sign anything or exercise a single share. Start Free with Lovie