Updated October 24, 2026. Quick answer: Line 18 of California Form 540 is where you enter your California itemized deductions or your California standard deduction. The booklet says “Your California income tax will be less if you take the larger of:” and the form’s line 19 then subtracts the line 18 amount to reach taxable income. This page walks through what the booklet and the form print for line 18.
What this line does
The booklet’s instructions for this line are headed “California Itemized Deductions or California Standard Deduction”. They open with “Decide whether to itemize your charitable contributions, medical expenses, mortgage interest paid, taxes, etc., or take the standard deduction.” and name two choices: “Your California itemized deductions.” and “Your California standard deduction.”
On the form, line 18 sits on the page headed “Side 2 Form 540 2025”, in the part headed “Taxable Income”. Its own wording is “Enter the larger of Your California itemized deductions from Schedule CA (540), Part II, line 30; OR Your California standard deduction shown below for your filing status:”
The line before it is line 17, the California adjusted gross income line, and the line after it is line 19, covered on California Form 540 line 19. For the whole sequence see California Form 540 line by line.
Who it applies to
The booklet sets out several conditions for how to fill in line 18. Spouses and registered domestic partners filing separately are told: “If married/or an RDP and filing separate tax returns, you and your spouse/RDP must either both itemize your deductions” and the sentence goes on to say that holds even if one spouse’s itemized deductions are less than the standard deduction, and that the two may instead both take the standard deduction.
A taxpayer who can be claimed by someone else is handled separately: “If someone else can claim you as a dependent, you may claim the greater of the standard deduction or your itemized deductions.” The form repeats the warning on line 18 itself: “If Married/RDP filing separately or the box on line 6 is checked, STOP. See instructions.”
For net disaster losses, the booklet draws a line between itemizers and everyone else: “For California, deductions for disaster losses are only allowed for those individual taxpayers who itemized their deductions.”
What the instructions say
| Line | What the form says |
|---|---|
| Choosing between the two | “Your California income tax will be less if you take the larger of:” |
| Itemized deduction limits | “California itemized deductions may be limited based on federal AGI. To compute limitations, use Schedule CA (540).” “RDPs, use your recalculated federal AGI to figure your itemized deductions.” |
| Disaster losses | “On federal tax returns, individual taxpayers who claim the standard deduction are allowed an additional deduction for net disaster losses.” “For California, deductions for disaster losses are only allowed for those individual taxpayers who itemized their deductions.” |
| Married or RDP, filing separately | “If married/or an RDP and filing separate tax returns, you and your spouse/RDP must either both itemize your deductions” “(even if the itemized deductions of one spouse/RDP are less than the standard deduction) or both take the standard deduction.” |
| Someone else can claim you | “If someone else can claim you as a dependent, you may claim the greater of the standard deduction or your itemized deductions.” “To figure your standard deduction, use the California Standard Deduction Worksheet for Dependents.” |
| Itemized deductions | “Figure your California itemized deductions by completing Schedule CA (540), Part II, line 1 through line 30. Enter the result on Form 540, line 18.” |
| Itemizing only on the California form | “If you did not itemize deductions on your federal income tax return but will itemize deductions for your Form 540, first complete federal Schedule A (Form 1040), Itemized Deductions.” “Then check the box on Side 5, Part II of the Schedule CA (540) and complete Part II.” “Attach both the federal Schedule A (Form 1040) and California Schedule CA (540) to the back of your tax return.” |
| Standard deduction | “Find your standard deduction on the California Standard Deduction Chart for Most People.” “If you checked the box on Form 540, line 6, use the California Standard Deduction Worksheet for Dependents.” |
| Chart caution | “Do not use this chart if your parent, or someone else, can claim you (or your spouse/RDP) as a dependent on their tax return.” |
| Chart compared with federal | “The California standard deduction amounts are less than the federal standard deduction amounts.” |
| Form, line 6 box and separate filers | “If Married/RDP filing separately or the box on line 6 is checked, STOP. See instructions.” |
| Dependents worksheet, who uses it | “Use this worksheet only if your parent, or someone else, can claim you (or your spouse/RDP) as a dependent on their return.” |
| Dependents worksheet, line 3 | “Enter the larger of line 1 or line 2 here” |
| Dependents worksheet, line 4 | “Single or married/RDP filing separately, enter $5,706.” “Married/RDP filing jointly, head of household, or qualifying surviving spouse/RDP, enter $11,412” |
| Dependents worksheet, line 5 | “Standard deduction. Enter the smaller of line 3 or line 4 here and on Form 540, line 18” |
The Standard Deduction Chart for Most People is a two column chart of filing statuses and amounts. This page does not reproduce it; it is printed in the booklet under the line 18 instructions, and the form prints the amounts for each filing status beneath the line 18 wording as well.
The figures the form prints
Two dollar figures sit inside single sentences of the dependents worksheet. Worksheet line 4 reads “Single or married/RDP filing separately, enter $5,706.” and “Married/RDP filing jointly, head of household, or qualifying surviving spouse/RDP, enter $11,412”. Line 5 of the same worksheet then sends the smaller of line 3 or line 4 to line 18.
The form’s own line 18 wording points to the amounts printed below it: “Your California standard deduction shown below for your filing status:” Check which one matches the filing status you used.
Where the result goes next
Line 19 picks up your line 18 amount: “Subtract line 18 from line 17. This is your taxable income.” The booklet’s line 19 and line 31 instructions explain the steps after that.
If you are comparing with the federal return, the federal side is covered on Form 1040 line 12, and the state overview is on California retirement taxes.
Edition and line numbers
This page reads the California 540 Forms and Instructions, 2025 Personal Income Tax Booklet, and Form 540, California Resident Income Tax Return (2025), for tax year 2025, the current file the Franchise Tax Board served on 2026-10-24. Check the edition printed on your own form and use that edition’s line numbers.
Common mistakes
- Choosing differently from your spouse or registered domestic partner on separate returns. The booklet says “you and your spouse/RDP must either both itemize your deductions” or both take the standard deduction.
- Using the standard chart when someone else can claim you. The booklet warns “Do not use this chart if your parent, or someone else, can claim you (or your spouse/RDP) as a dependent on their tax return.”
- Assuming the California amounts match the federal ones. The booklet states “The California standard deduction amounts are less than the federal standard deduction amounts.”
Related: California retirement taxes, Form 1040 line 12, Form 1040 lines 14 and 15, and California tax brackets.
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- California 540 Forms and Instructions, 2025 Personal Income Tax Booklet, Franchise Tax Board: www.ftb.ca.gov
- Form 540, California Resident Income Tax Return (2025), Franchise Tax Board: www.ftb.ca.gov
- Related official page (not quoted): Franchise Tax Board, 2025 Personal Income Tax Booklet page: www.ftb.ca.gov
Read from the state revenue documents linked above on October 24, 2026. General information, not tax advice. Your own facts decide the outcome, and a preparer or the state revenue department is the right place to confirm anything consequential.