---
title: "California ISO AMT Calculator (2026): The AMT Trap and How to Avoid It"
slug: ca-iso-amt-trap
publishedAt: 2026-06-16T17:00:00.000Z
updatedAt: 2026-06-17T16:41:54.706Z
author: "Mike Navarro"
authorSlug: mike-navarro
category: "State Tax"
tags: ["California", "ISO", "AMT", "Stock Options", "State Tax"]
excerpt: "California ISO AMT calculator for 2026: see your AMT-free exercise limit, the trap when you exercise too many shares, and worked numbers for three startup grants."
canonical: https://myequitytax.com/blog/ca-iso-amt-trap
---
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.
[**Open the California ISO AMT calculator with these inputs prefilled →**](/calculator/iso?state=CA&shares=10000&fmv=12&strike=2&salary=180000)
## 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](https://www.irs.gov/publications/p525)). 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](https://www.ftb.ca.gov/file/personal/income-types/stock-options.html), 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](https://www.irs.gov/forms-pubs/about-form-6251)):
1. The **bargain element** — `(FMV − strike) × shares exercised` — is added to your AMT income for the year.
2. The **AMT exemption** is subtracted (a fixed amount that phases out at high incomes — fully phased out at very high AMTI).
3. The remainder is taxed at **26% on lower AMT taxable income and 28% on higher AMT taxable income** ([IRS Form 6251](https://www.irs.gov/forms-pubs/about-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.
[Run this scenario in the calculator →](/calculator/iso?state=CA&shares=10000&fmv=12&strike=2&salary=180000)
### 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.
[Run this scenario in the calculator →](/calculator/iso?state=CA&shares=20000&fmv=40&strike=2&salary=250000)
### 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 →](/calculator/iso?state=CA&shares=30000&fmv=120&strike=2&salary=350000)
## 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.
1. **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.
2. **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](https://www.irs.gov/forms-pubs/about-form-6251).
3. **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.
4. **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 →](/calculator/iso?state=CA)
## 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](https://www.ftb.ca.gov/file/personal/income-types/stock-options.html)), 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](https://www.ftb.ca.gov/file/personal/income-types/stock-options.html) — California ISO and AMT guidance, non-conforming treatment.
- EquityTax California ISO AMT Calculator (internal engine, last verified 2026-05-09).
*Estimate only — not financial or tax advice. Consult a qualified CPA before making decisions about exercising stock options, selling equity, or other financial moves.*