The california iso amt calculator pairs the federal 26%/28% AMT brackets with California's non-conforming ISO rules — California taxes the bargain element at exercise even when no AMT is federally owed. For most startup employees, the AMT-free exercise limit is a fraction of vested shares. Run your numbers below.
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Use our ISO AMT Calculator to find the optimal number of shares to exercise without triggering AMT.
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How the California ISO AMT calculator treats ISO exercises
For federal purposes, exercising an Incentive Stock Option (ISO) and holding the shares triggers no regular income tax — but the bargain element (FMV minus strike, times shares) is added to your Alternative Minimum Taxable Income for the year (IRS Publication 525). If your AMT exceeds your regular tax, you owe the difference.
California does not conform to federal ISO treatment. Per the California Franchise Tax Board, California taxes the ISO bargain element at exercise even when no federal regular-tax income event has occurred. California's top marginal rate is 13.3% (a 1%–12.3% progressive bracket plus a 1% Mental Health Services Tax on income over $1,000,000), and ISO exercises feed into that progressive ladder.
The federal AMT calculation has three moving parts (IRS Form 6251):
- The bargain element —
(FMV − strike) × shares exercised— is added to your AMT income for the year. - The AMT exemption is subtracted (a fixed amount that phases out at high incomes — fully phased out at very high AMTI).
- The remainder is taxed at 26% on lower AMT taxable income and 28% on higher AMT taxable income (IRS Form 6251 publishes the exact rate-break threshold each year).
The trap: a California ISO exercise can produce a federal AMT bill and an immediate California ordinary-income hit on the same bargain element. The EquityTax engine handles both layers — the worked examples below show what each scenario costs.
Worked examples (California, 2025)
Each example shows the AMT-free exercise limit and the cost of exercising every vested share, using the EquityTax engine. Numbers match what the calculator returns for the inputs shown.
Example 1 — Early-stage IC, $180K salary, single
Inputs: $180,000 W-2, single filer, 10,000 vested ISOs at $2 strike, $12 current FMV.
Result:
- AMT-free exercise limit: 3,220 shares
- Exercise cost at the limit (strike × shares): $6,440
- Paper gain at the limit (bargain element): $32,200
- Federal regular tax (no exercise): $32,267
- Federal AMT if you exercise all 10,000 shares: $17,627 in additional federal tax
- California state tax impact (regular tax on the same income): $12,647.98 before exercise, applied through your progressive bracket on the bargain element at exercise
At $180K salary and a single filer, you can exercise about a third of your vested ISOs (3,220 of 10,000) without triggering a single dollar of federal AMT. Push past that to all 10,000 and the bargain element jumps to $100,000, AMTI climbs to $280,000, and the federal AMT bill is $17,627 — on top of the $6,440 cash you needed to write the strike-price check.
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Example 2 — Mid-stage IC, $250K salary, MFJ
Inputs: $250,000 W-2, married filing jointly, 20,000 vested ISOs at $2 strike, $40 current FMV.
Result:
- AMT-free exercise limit: 886 shares
- Exercise cost at the limit: $1,772
- Paper gain at the limit: $33,668
- Federal regular tax (no exercise): $38,134
- Federal AMT if you exercise all 20,000 shares: $201,524 in additional federal tax
- California state tax (regular income only): $15,065.96, plus California's non-conforming ISO add-back on the bargain element
This is the classic AMT trap. The 409A FMV jumped from $12 to $40, the AMT-free limit collapsed from 3,220 shares to just 886 — under 5% of vested — and exercising everything generates a $760,000 bargain element that pushes AMTI to $1,010,000. The federal AMT bill alone is $201,524. California then taxes that same bargain element as ordinary income.
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Example 3 — Late-stage / pre-IPO IC, $350K salary, MFJ
Inputs: $350,000 W-2, married filing jointly, 30,000 vested ISOs at $2 strike, $120 current FMV.
Result:
- AMT-free exercise limit: 220 shares
- Exercise cost at the limit: $440
- Paper gain at the limit: $25,960
- Federal regular tax (no exercise): $62,134
- Federal AMT if you exercise all 30,000 shares: $1,022,284 in additional federal tax
- California state tax (regular income only): $24,365.96, with California's non-conforming ISO treatment applied on top
A pre-IPO exercise is where AMT becomes life-altering. Of 30,000 vested shares, only 220 can be exercised AMT-free at this income and FMV — every share beyond pushes you into the trap. Exercising all 30,000 creates a $3,540,000 bargain element, the AMT exemption fully phases out, and the federal AMT bill is $1,022,284. California stacks its 13.3% top-bracket exposure on top under its non-conforming rules.
Run this scenario in the calculator →
Strategies for California option holders
The single biggest lever is timing. Splitting an exercise across two tax years can keep both years below the AMT crossover and turn a six- or seven-figure tax bill into a small one — sometimes zero.
- Find your AMT-free limit for this year, exercise at most that number, and roll the rest into next year's exercise window. Example 2 above caps at 886 shares — exercise that, then come back January 1.
- Same-year sale (disqualifying disposition) — if you sell ISO shares in the same calendar year you exercise, the bargain element is taxed as ordinary income (no AMT preference). This loses long-term capital gains treatment but kills the AMT trap. See IRS Form 6251 Line 2i.
- If you exit a startup mid-year, model the post-IPO scenario — the AMT-free limit shrinks fast as FMV climbs. The Example 1 → Example 3 progression (3,220 → 886 → 220 shares) shows what an FMV ramp does to your exercise capacity.
- California estimated payments — California does not let you defer ISO state tax to federal AMT timing. If you exercise, plan to cover the California liability on FTB Form 540-ES on the same dates as federal 1040-ES.
Open the calculator with your inputs →
FAQ
What is California's ISO AMT treatment in 2025?
California does not conform to federal ISO rules. California taxes the bargain element at exercise as ordinary income, even when the federal side defers the regular-tax event. California's top marginal rate is 13.3% (per the California FTB), and ISO exercises feed into the progressive bracket.
How many ISOs can I exercise without triggering AMT in California?
It depends on your W-2 income, filing status, and the spread between FMV and strike. The EquityTax calculator uses binary search to find your exact AMT-free limit. In the worked examples above, the limit ranged from 220 shares (pre-IPO, $120 FMV) to 3,220 shares (early-stage, $12 FMV) on the same $2 strike.
Can I avoid California ISO tax with a same-year sale?
A same-year sale is a disqualifying disposition that removes the AMT adjustment federally (no Form 6251 Line 2i add-back) and is taxed as ordinary income in both jurisdictions. You give up long-term capital gains treatment, but the AMT trap goes away. Run the same-year-sale scenario in the calculator to see the net difference.
Should I exercise before or after IPO in California?
The Example 1 → Example 3 progression shows the cost of waiting: when FMV climbed from $12 to $120 on the same $2 strike, the AMT-free limit fell from 3,220 to 220 shares. Pre-IPO exercises usually give you a much larger AMT-free capacity, but they also carry private-company illiquidity risk. Model both scenarios in the calculator before deciding.
Sources
- IRS Form 6251, Alternative Minimum Tax (Individuals) — exemption amounts, phase-out thresholds, AMT rates.
- IRS Publication 525, Taxable and Nontaxable Income — ISO bargain-element treatment.
- California Franchise Tax Board — California ISO and AMT guidance, non-conforming treatment.
- EquityTax California ISO AMT Calculator (internal engine, last verified 2026-05-09).
Tax Disclaimer: This content is for educational purposes only. Always consult with a licensed tax professional or certified public accountant before making financial decisions related to equity compensation, tax planning, or investment strategies.
Estimate only — not financial or tax advice. Consult a qualified CPA before making decisions about exercising stock options, selling equity, or other financial moves.