I do know that most states tax non-residents on only the income from that state's sources and at that state's tax rate.
California is a little different.
California also only taxes non-residents on the income sourced form California, but not at a set rate.
California looks at ALL of your yearly income, not just the California income.
It calculates a tax rate from all your income and than calculates the California tax rate based on that rate (for all your income). The California tax rate is a percentage of your total rate based on the ratio of California income to all your income.
So one non-resident that earned 30,000 in California is not taxed the same as another (otherwise identical) non-resident that also earned 30,000 if one earned more in their home state than the other earned in their home state.
This is why the California State return imports ALL your income first.
According to the State of California:
"Nonresidents of California who received California sourced income in 2024, or moved into or out of California in 2024, file Form 540NR. California taxes all income received while you resided in California and the income you received from California sources while a nonresident.
If you file Form 540NR, use Schedule CA (540NR), column A through column D to compute your total adjusted gross income as if you were a resident of California for the entire year. Use column E to compute all items of total adjusted gross income you received while a resident of California and those you received from California sources while a nonresident. You determine your California tax by multiplying your California taxable income by an effective tax rate. The effective tax rate is the tax on total taxable income, taken from the tax table, divided by total taxable income. You may also qualify for California tax credits, which reduces the amount of California tax you owe.
If you were a resident of California for all of 2024, get a California Resident Personal Income Tax Booklet and file Form 540 or Form 540 2EZ.
For more information on the taxation of nonresidents and part-year residents, get FTB Pub. 1100, Taxation of Nonresidents and Individuals Who Change Residency."
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