Short-term capital gains in California are taxed as ordinary income at the state’s regular marginal rates, which run from 1% up to 13.3% once the 1% Mental Health Services Tax on income above $1 million is included.1Franchise Tax Board. Capital Gains and Losses That state tax sits on top of federal tax, which also treats short-term gains as ordinary income, so a California resident in the top brackets can lose more than half of a short-term gain to combined taxes.
How California Treats the Gain
A gain is short-term when you sell a capital asset you held for one year or less. The holding period starts the day after you acquire the asset and runs through the day you sell.2Internal Revenue Service. Topic No. 409, Capital Gains and Losses
California draws no distinction between short-term and long-term gains. Both are folded into your regular taxable income and taxed at whatever marginal bracket that total falls into.1Franchise Tax Board. Capital Gains and Losses For a single filer in 2025, the 9.3% bracket runs from about $72,725 to $371,479, the top ordinary bracket of 12.3% starts above roughly $742,953, and married-joint thresholds are roughly double those numbers. Brackets adjust for inflation each year.3California Legislative Information. California Revenue and Taxation Code 17041
Above $1 million of taxable income, California adds a 1% Mental Health Services Tax, bringing the top effective rate to 13.3%.1Franchise Tax Board. Capital Gains and Losses That $1 million line is not indexed for inflation, and a single large short-term trade can push you across it in a year you would not otherwise clear it.
A rough number to anchor the range: a $10,000 short-term gain earned by someone already in the 9.3% bracket costs about $930 in California tax. The same gain in the hands of someone already above the $1 million line costs $1,330 in state tax alone.
What the Federal Side Adds
The federal government also taxes short-term gains as ordinary income, at rates from 10% to 37%. For 2025, a single filer reaches the 37% bracket at $626,351 of taxable income.4Internal Revenue Service. Federal Income Tax Rates and Brackets The federal preferential rates of 0%, 15%, and 20% apply only to long-term gains, so a short-term seller loses that break entirely.2Internal Revenue Service. Topic No. 409, Capital Gains and Losses
Higher-income taxpayers face an additional 3.8% net investment income tax on gains once modified adjusted gross income exceeds $200,000 for single filers, $250,000 for joint filers, or $125,000 for married filing separately. Those thresholds are fixed in the statute and have never been indexed for inflation.5Internal Revenue Service. Questions and Answers on the Net Investment Income Tax
Combined Rate for a California Resident
Stack the three layers and a top-bracket California resident can face:
- Federal ordinary income tax up to 37%
- Net investment income tax of 3.8%
- California state tax up to 13.3%
That is a potential combined marginal rate of 54.1% on a short-term gain. Few taxpayers sit at the top of every bracket at once, but the direction is clear: a taxpayer with $1.5 million of taxable income who books a $200,000 short-term gain will hand over more than half of it. Holding the position past the one-year mark saves nothing at the California level, but at the federal level it can cut the rate roughly in half.
Estimated Payments and the Millionaire Trap
No one withholds California tax from a brokerage trade, so a short-term gain typically has to be paid through estimated tax. California requires estimated payments if you expect to owe at least $500 for the year, or $250 if married filing separately.6Franchise Tax Board. Estimated Tax Payments Payments use Form 540-ES, with quarterly due dates of April 15, June 15, September 15, and January 15 of the following year.7Franchise Tax Board. 2026 Instructions for Form 540-ES Estimated Tax for Individuals
Underpayments are penalized on Form 5805. The penalty is calculated based on when you actually received the income, so a gain realized in March generates a larger penalty than the same gain realized in November.8Franchise Tax Board. 2023 Instructions for Form FTB 5805 Underpayment of Estimated Tax by Individuals and Fiduciaries
The state offers safe harbors that prevent the penalty even if your final bill is bigger than you planned for:
- Pay at least 90% of the current year’s tax, or 100% of last year’s tax.
- If last year’s adjusted gross income was over $150,000 (or $75,000 if married filing separately), the prior-year safe harbor rises to 110% of last year’s tax.
- If your current-year California AGI is $1,000,000 or more (or $500,000 if married filing separately), the prior-year safe harbor is gone. You must base payments on 90% of the current year’s tax.9Franchise Tax Board. 2025 Instructions for Form 540-ES Estimated Tax for Individuals
That last rule is where short-term gains cause the most trouble. A single trade can push you past $1 million and knock out the safety of paying based on last year’s smaller tax. If you realize a large gain partway through the year, recalculate your estimated payment for the next deadline instead of waiting until you file.
QSBS: A Federal Break California Ignores
Federal law lets investors exclude up to 100% of the gain from selling Qualified Small Business Stock held at least five years under Internal Revenue Code Section 1202.10Office of the Law Revision Counsel. 26 USC 1202 California does not conform. The state requires you to report the full gain as taxable income even if you excluded 100% of it federally, and it does not recognize the Section 1045 deferral for rolling QSBS proceeds into new qualifying stock.11Franchise Tax Board. 2024 Instructions for California Schedule D (540)
Founders and early employees regularly miss this. A multimillion-dollar QSBS sale can carry a zero federal tax bill and still generate six figures of California tax. The adjustment is made on California Schedule D, where you enter the full gain regardless of the federal exclusion. QSBS requires a five-year hold, so it will not apply to a short-term gain, but a sale that fails the QSBS holding period drops back to ordinary treatment at both levels.
Offsetting a Gain with Losses
California follows the federal capital loss rules. Short-term losses offset short-term gains first. If losses exceed gains overall, up to $3,000 of the net loss can be deducted against ordinary income each year ($1,500 for married filing separately), and any remaining loss carries forward indefinitely.11Franchise Tax Board. 2024 Instructions for California Schedule D (540)
Selling a losing position to offset a gain works dollar for dollar on both returns. California also conforms to the federal wash sale rule, so if you repurchase the same or a substantially identical security within 30 days before or after the sale, the loss is disallowed.12Franchise Tax Board. California Conformity to Federal Law
If You Moved In or Out of California
Part-year residents pay California tax on everything earned during the months they lived in California, plus any California-sourced income earned while they lived elsewhere.13Franchise Tax Board. Part-Year Resident and Nonresident
For non-residents, California sources include the sale of real property located in California. Gains on stocks, bonds, and other intangible assets are generally not California-sourced for a non-resident.13Franchise Tax Board. Part-Year Resident and Nonresident So if you move out of California before selling an appreciated stock, the state typically cannot reach that gain. Selling California real estate after the move is a different story: California still taxes that profit.
People moving into California face the mirror problem. The state may require you to adjust your basis so it only taxes appreciation that occurred during residency, not gains that accrued before you arrived. FTB Publication 1100 walks through the allocation.14Franchise Tax Board. FTB Pub. 1100 – Taxation of Nonresidents and Individuals Who Change Residency
How to Report the Gain
Reporting starts with the federal return. You calculate net short-term gains and losses on federal Schedule D, which feeds into your Form 1040. The federal result then flows onto California Form 540, the resident return.15Franchise Tax Board. 2025 Instructions for Form 540 California Resident Income Tax Return
If your California figures differ from your federal figures, you file Schedule CA (540) to make adjustments and may need California Schedule D to reconcile the capital gain or loss difference. The most common trigger for California Schedule D is a federal exclusion the state does not recognize, such as QSBS.16Franchise Tax Board. 2025 Personal Income Tax Booklet – California Forms and Instructions 540