California Form 540 Guide

What Is California Form 540?

Form 540 is the California Resident Income Tax Return — the state equivalent of the federal Form 1040. California residents file Form 540 to report their California taxable income and calculate their California income tax liability. California's income tax rates range from 1% to 13.3%, with the 13.3% rate applying to income over $1 million (for single filers). California also imposes a 1% Mental Health Services Tax on income over $1 million.

Who Must File Form 540

California residents must file Form 540 if their gross income exceeds the filing threshold for their filing status. For 2025, the filing threshold is approximately $19,310 for single filers and $38,620 for married filing jointly. Part-year residents and nonresidents with California-source income file Form 540NR instead of Form 540. California has no standard deduction equivalent to the federal standard deduction — California's standard deduction is much lower ($5,540 for single filers in 2025).

Key Differences from Federal Form 1040

California does not conform to all federal tax law changes. Key differences for 2026 include: California does not allow bonus depreciation (California uses MACRS with no bonus depreciation), California does not conform to the OBBBA's No Tax on Tips or No Tax on Overtime provisions, California's standard deduction is much lower than the federal standard deduction, and California does not allow the federal QBI deduction (Section 199A). These differences mean your California taxable income is often higher than your federal taxable income.

California-Specific Credits

California offers several credits not available at the federal level: the California Earned Income Tax Credit (CalEITC) for lower-income workers, the Young Child Tax Credit ($1,117 per child under 6 for qualifying taxpayers), the Child and Dependent Care Expenses Credit, the Renter's Credit ($60 for single filers, $120 for joint filers), and the Senior Head of Household Credit. KDA reviews every client's return for all available California credits.

Filing Deadlines

California Form 540 is due April 15 — the same date as the federal return. A 6-month extension (to October 15) is available by filing FTB Form 3519 or making an extension payment. Unlike the federal extension, California does not automatically grant an extension — you must either file Form 3519 or pay at least the estimated tax due by April 15. If you owe California tax, you must pay by April 15 even if you file an extension.

Common Form 540 Errors

The most common Form 540 errors KDA corrects: (1) Applying federal bonus depreciation on the California return (California does not conform). (2) Claiming the federal QBI deduction on the California return (not allowed). (3) Using the wrong filing status (California has different rules for registered domestic partners). (4) Missing the California SDI deduction (California SDI withheld from wages is deductible on the California return). (5) Forgetting to report California lottery winnings (taxable in California, unlike some states).