Startup Equity Tax Guide (2026): ISO vs NSO, 83(b) Elections, and AMT Explained

The IRS taxes startup equity differently depending on option type, when you exercise, and how long you hold. This guide covers the three things that determine how much tax you'll pay — and how to minimize it.

How Startup Equity Tax Works

Startup equity is taxed at two moments: when you exercise (buy your shares at the strike price) and when you sell (at exit, IPO, or secondary sale). The amount you owe at each moment depends on your option type.

Here's the basic formula:

• Spread: The difference between the strike price you pay and the fair market value (FMV) of the shares
• Exercise tax: Tax on the spread (for some option types)
• Sale tax: Capital gains tax on the difference between FMV at exercise and sale price

The type of option you have — ISO or NSO — determines which of these taxes apply and at what rate.

ISO vs NSO: The Tax Difference That Can Cost You Thousands

The two most common stock option types are treated very differently by the IRS. Understanding the difference is the single most important thing you can do for your tax bill.

The Real-World Impact

Say you have 10,000 options with a $1 strike price. The current FMV is $10/share. You exercise and later sell at $50/share.

With ISOs (held for qualifying period): You pay long-term capital gains on the full $49/share gain. At a 15% rate, that's $73,500 in tax on $490,000 of gains.

With NSOs: You pay ordinary income tax on the $9/share spread at exercise ($9,000 x your marginal rate, say 32% = $2,880). Then capital gains on $40/share of appreciation. Total tax: roughly $92,880.

The ISO saves you nearly $20,000 in this scenario. The difference grows dramatically with larger grants and higher valuations.

The 83(b) Election: File It or Lose Money

If your equity grant is subject to vesting, Section 83(b) of the tax code gives you a powerful option: you can elect to pay tax on the value of your shares now — when the value is low — instead of paying tax as each tranche vests, when the value could be much higher.

How to File an 83(b) Election

1. Complete the Section 83(b) election form (available from your company or online)
2. Mail it to the IRS within 30 days of your grant date (certified mail recommended)
3. Send a copy to your employer's HR or legal team
4. Include a copy with your next tax return
5. Keep a copy with proof of mailing for your records

AMT: The Hidden Tax on ISO Exercises

The Alternative Minimum Tax (AMT) is a parallel tax system that catches many ISO holders by surprise. When you exercise ISOs, the spread between your strike price and the FMV counts as income for AMT purposes — even though it's not regular taxable income.

Here's how it works:

1. You exercise 10,000 ISOs at $1 strike price, FMV is $20/share
2. The $190,000 spread is added to your AMT income calculation
3. If your AMT calculation exceeds your regular tax, you pay the difference
4. You could owe tens of thousands in tax — on shares you haven't sold

AMT Strategies

• Exercise early when the spread is small (low FMV = low AMT)
• Exercise in January to give yourself 15 months before the tax is due (April of the following year)
• Exercise in batches across multiple years to stay under the AMT threshold
• Track your AMT credit — you can recover some AMT in future years when you sell

5 Tax Strategies for Startup Equity

1. Exercise Early When FMV Is Low

The earlier you exercise, the lower the FMV (and spread). A lower spread means less tax at exercise and a larger portion of your gains qualify for capital gains treatment. Early exercise is the single biggest tax-saving move you can make.

2. File Your 83(b) Election

If your shares are subject to vesting, file the 83(b) election. It's free to file, takes 30 minutes, and can save you tens of thousands. The only downside is if the company fails — you've prepaid tax on worthless shares (which is typically near $0 anyway).

3. Hold ISOs for Qualifying Disposition

To get the full ISO tax benefit, hold shares for at least 2 years from the grant date AND 1 year from the exercise date. Selling before both periods expires triggers a "disqualifying disposition" — the spread gets taxed as ordinary income instead of capital gains.

4. Manage Your AMT Exposure

Before exercising a large ISO grant, calculate your potential AMT liability. Use the Equity Tax Calculator to model different exercise scenarios. Consider exercising in smaller batches across tax years.

5. Consider State Taxes

California, New York, and other high-tax states add significant state income tax on top of federal. A $100,000 spread in California could cost $37,000+ in federal tax plus $13,000+ in state tax. Factor state taxes into your exercise decisions, especially if you're planning to move.

Common Questions

What happens if I don't exercise my options?

If you leave your company, you typically have 90 days to exercise your vested options. After that, you lose them. Unexercised options cost you nothing in taxes — you only owe tax when you exercise.

Should I exercise my options before a funding round?

Exercising before a funding round (when the 409A valuation is still low) can significantly reduce your tax bill. The spread — and therefore your tax — is calculated based on the FMV at the time of exercise. After a funding round, the 409A typically increases, making exercise more expensive.

What if my company never exits?

If you exercised and paid tax on the spread, you may be able to claim a capital loss on your tax return. However, the rules are complex and the loss may be limited. This is why early exercise (when the spread is small) is lower risk.

How do RSUs compare to options for tax?

RSUs are taxed as ordinary income when they vest — there's no exercise step. You pay income tax on the full market value at vest. Unlike options, there's no strategy to minimize this tax. The 83(b) election is generally not available for RSUs (it can apply to restricted stock awards, but not RSUs).