California Paycheck Tax Calculator (2026, RSUs and Bonuses)

California takes more out of a high earner’s paycheck than any other state, and it does so through three separate mechanisms: a nine-rate income tax topping out at 12.3%, a 1% surcharge on taxable income over $1 million, and an uncapped 1.3% State Disability Insurance tax on every wage dollar for 2026. The calculator above applies the state rate structure; this guide explains the two problems it cannot fix for you, underwithholding on equity compensation and exit planning.

The underwithholding problem is mechanical. California withholds bonuses and stock compensation at a flat 10.23%. A tech employee whose RSU vesting pushes total income into the 11.3% bracket, or past $1 million into 13.3% territory, is underwithheld on every supplemental dollar by design. Stack the same effect on the federal side, where supplemental withholding runs 22% until year-to-date supplemental wages pass $1 million against a 37% top bracket, and April surprises in the tens of thousands are routine.

California applies nine rates: 1%, 2%, 4%, 6%, 8%, 9.3%, 10.3%, 11.3%, and 12.3%. Brackets are indexed for inflation each year; the state publishes final 2026 thresholds late in the year, but the structure is stable, with the 2025 single-filer top bracket beginning around $721,000. Taxable income over $1 million adds the 1% Mental Health Services Tax regardless of filing status, producing the 13.3% top statutory rate.

On top of the income tax, every wage dollar pays 1.3% SDI for 2026 (up from 1.2% in 2025), with no wage cap. SDI is not income tax and does not appear in bracket tables, but it is withheld from the paycheck and it scales without limit, which is why the table below shows combined marginal rates.

California requires employers to withhold a flat 10.23% on bonuses and stock compensation (a 6.6% rate applies to other supplemental pay like severance). For anyone whose total taxable income lands in the 11.3%, 12.3%, or 13.3% zones, every vest is underwithheld by one to three-plus percentage points at the state level alone.

The federal side compounds it: supplemental withholding is 22% until year-to-date supplemental wages exceed $1 million, against a 37% top federal bracket. A senior engineer with heavy vesting can be underwithheld by 10 or more combined points on hundreds of thousands of dollars of equity income.

The fix is quarterly estimated payments or a DE 4 adjustment requesting additional withholding. California’s estimated payment schedule is front-loaded (30% of the year’s requirement in Q1, 40% in Q2, nothing in Q3, 30% in Q4), so a mid-year vest can require a catch-up payment sooner than the federal calendar suggests.

California uses its own withholding certificate, Form DE 4. If you never file one, the employer computes state withholding from your federal W-4 elections, which maps poorly onto California’s brackets for high earners. Filing a DE 4 with an additional per-period withholding amount is the simplest lever for closing a known RSU gap without managing quarterly estimates.

Married taxpayers with two incomes should note that California’s brackets for joint filers are wide, but two large incomes stack quickly into the 9.3%-plus zones that per-paycheck withholding tables underestimate. The same annualization problem behind the federal two-earner trap applies, at California scale.

California taxes residents on worldwide income and nonresidents on California-source income. Moving ends the residency layer prospectively, but income sourced to California working days keeps its character: RSUs vesting after the move are taxed by California on the portion earned during California workdays between grant and vest, bonuses for California performance periods remain partly Californian, and income from a California business or rental stays fully taxable.

California also audits high-income departures aggressively, weighing domicile facts: home, spouse and dependents, time, professional ties, and where the taxpayer’s life actually happens. A clean break with documentation beats a technically-argued one. The move-year return (Form 540NR, part-year) and a workday allocation schedule for post-move vesting are where the planning turns into filings.

There is no special exit tax on leaving under current law, but the trailing-source rules do much of the same work for equity-heavy earners. Model the vest-by-vest allocation before assuming the move zeroes the California line.

The allocation math in miniature: an engineer is granted RSUs while working in San Francisco, moves to Austin two years into a four-year vest, and a tranche vests a year later. Of the workdays between grant and that vest, roughly two-thirds were California days, so about two-thirds of the tranche’s value is California-source income taxable on a 540NR even though she is a Texas resident on vest date. Her employer’s stock administrator typically applies the same ratio to state withholding. The remaining third is sourced to Texas and escapes state tax entirely. Each later tranche carries a smaller California fraction as Texas workdays accumulate, which is why the trailing tax fades over the vesting horizon rather than ending at the border.

A single-income W-2 household with modest or no equity compensation, income inside the 9.3% bracket, and no side income is usually withheld accurately by default; the 10.23% supplemental rate overshoots slightly for them and produces small refunds. The planning population is the opposite profile: heavy RSU vesting, dual high incomes, 1099 or K-1 income with zero withholding, and anyone whose total income approaches the $1 million MHST line.

California produces the biggest withholding-to-liability gaps we see anywhere, and they cluster in predictable profiles. Dual-income tech couples where both employers withhold as if the other salary did not exist. Anyone with vesting concentrated in one or two quarters, because the front-loaded estimate schedule punishes late-year catch-up. Physicians and professionals adding 1099 income to a California W-2, where the state estimate, the federal estimate, and SDI-free self-employment income all run on different rules. Sellers of appreciated stock or a business, where a single event can add the Mental Health Services Tax and the federal net investment income tax at once. And planned leavers, for whom the projection doubles as the domicile file.

A realistic scenario: a married couple in San Jose earns $350,000 and $280,000 in salary with $250,000 of combined RSU vesting, most of it in March and September. Their combined withholding runs at roughly the 10.23% supplemental rate on the equity while their true marginal state rate sits in the 11.3% zone, and the September vest lands after the quarter when California wanted 70% of the year’s estimates already paid. The projection sets a DE 4 additional-withholding amount in April and a small September estimate, converting a five-figure April balance plus penalty into a near-zero reconciliation.