California wealth tax fiscal impact calculator

Interactive tool analyzing the fiscal impact of California's proposed 2026 Billionaire Tax Act (Initiative 25-0024), a one-time 5% tax on net worth above $1 billion.

Live app: https://california-wealth-tax.vercel.app

Scope

This repo is intentionally narrow. It models the fiscal impact of the wealth tax under user-set assumptions about:

• baseline one-time wealth tax revenue
• avoidance
• departures
• annual return hazard for remaining movers
• annual California-taxable income as a share of taxed wealth
• time horizon and discounting

California income tax is not entered directly. The app derives it from PolicyEngine's ca_income_tax calculation using a precomputed lookup at billionaire-scale income levels.

The preset buttons are calibrated to the headline figures in:

• Galle, Gamage, Saez & Shanske (2026)
• Rauh, Jaros, Kearney, Doran & Cosso (2026)

They are simplified calibrations, not full replications of either paper.

Preset calibration notes

• Saez headline: the backed-out parameter is the gross one-time score. The app sets it to $109.5B so that, after a 10% avoidance assumption, collected wealth-tax revenue is about $98.6B, close to the paper's roughly $100B static headline.
• Rauh headline: the backed-out parameter is the annual taxable-income yield on wealth. The app sets it to 3.6% so that, given a $67.2B gross score, 15% avoidance, 30% departures, zero return migration, a 3% discount rate, and a perpetuity horizon, the simplified model lands at about -$24.7B.
• In both presets, other values like the discounting setup, return path, and the mapping from annual taxable income to California income tax come from this app's simplified model layer, not from a full paper replication.

Features

• One-page fiscal impact calculator
• Shareable scenario URLs
• Annual return-hazard migration model
• Income-to-wealth assumption instead of a free dollar income input
• Year-by-year cash-flow chart alongside the discounted summary
• Waterfall chart from baseline revenue to net fiscal impact
• PolicyEngine-backed California income-tax lookup

Data

Precomputed from PolicyEngine

• data/income_tax_lookup.json — California income tax at billionaire-scale annual incomes, used to map derived annual taxable income to annual CA income tax

External references

• Rauh et al. replication code: https://github.com/bjaros20/wealth_tax
• Forbes real-time billionaires API: https://github.com/komed3/rtb-api
• PolicyEngine US model: https://github.com/PolicyEngine/policyengine-us

Architecture

app/
├── page.js                 # Main calculator page
├── components/
│   ├── Slider.js           # Reusable slider + exact input
│   └── WaterfallChart.js   # Recharts waterfall
├── globals.css             # PE-flavored design tokens + Tailwind
└── layout.js               # Metadata + fonts

lib/
├── calculator.js           # Pure JS fiscal impact model
├── incomeTaxLookup.js      # Maps annual taxable income to CA income tax
├── scenarioUrl.js          # URL parsing/serialization for shared scenarios
└── waterfall.js            # Waterfall chart presentation helpers

scripts/
└── precompute.py           # Generates the income-tax lookup from PolicyEngine

The app is fully static. All PolicyEngine-dependent values are precomputed into JSON and loaded client-side. There are no runtime API calls.

Development

npm install
npm run dev
npm run build

Regenerating the income-tax lookup

Requires policyengine-us installed:

cd ~/PolicyEngine/policyengine-us
.venv/bin/python ~/PolicyEngine/california-wealth-tax/scripts/precompute.py

Related work

• State tax progressivity explorer — separate repo for cross-state rate structure and progressivity analysis