Where the Number Comes From

Your federal AGI is calculated on your Form 1040. It’s your total gross income (wages, interest, dividends, business income, capital gains, retirement distributions, rental income, and everything else) minus specific “above the line” adjustments like IRA contributions, student loan interest, self-employment tax deduction, and health insurance deductions for the self-employed. These adjustments are defined throughout IRC Section 62.

California doesn’t rebuild this calculation from scratch. The state starts with your federal AGI and then makes its own modifications through Schedule CA additions (Line 14) and Schedule CA subtractions (Line 15). This is called the “federal conformity” approach under Cal. Rev. & Tax. Code Section 17071, and it saves you from having to reconstruct your entire income picture under a separate set of California rules.

The practical effect: your federal return has to be finished and correct before you can start your 540. That might sound obvious, but every year we see clients who try to file their California return before their federal return is finalized. If your 1040 changes — say, you get an amended W-2 or realize you forgot a 1099 — your 540 has to change too.

Why California Doesn’t Just Use Federal Taxable Income

A reasonable question: why not skip ahead and just start with federal taxable income instead of AGI? Because California has its own deduction rules, its own exemptions, and its own exclusions. The state needs to back up to AGI so it can apply those differences properly.

For example, the federal standard deduction for 2025 is $15,000 for single filers. California’s is $5,540. If California started with federal taxable income, it would be inheriting a deduction that doesn’t match state law. By starting at AGI, California can apply its own deduction amount on a later line.

Same logic applies to exemptions. The feds eliminated personal exemptions under TCJA. California kept them. Starting at AGI lets each system apply its own rules downstream without conflict.

Federal Exclusions California Doesn’t Recognize

Here’s where it gets interesting. Some items that reduce your federal AGI aren’t recognized by California. When that happens, your California AGI (after Schedule CA adjustments) ends up higher than your federal AGI. These are the biggest ones:

• HSA contributions — The federal deduction for Health Savings Account contributions (IRC Section 223) doesn’t exist in California. The state never conformed to HSA tax treatment. Your HSA contributions, employer contributions, and earnings inside the account are all taxable for California purposes. This gets added back on Schedule CA.
• HSA earnings — Not just contributions. Interest, dividends, and capital gains inside your HSA are federally tax-free but California-taxable. You have to track them separately.
• Tuition and fees deduction — When this deduction was available federally, California didn’t always conform. Check the specific tax year.
• Moving expenses — Federally, only military moves are deductible post-TCJA. California still allows moving expense deductions for everyone under Cal. Rev. & Tax. Code Section 17072, which means this one actually works in the taxpayer’s favor — it’s a subtraction on Schedule CA.

The HSA issue is by far the most common problem we see at our Los Angeles practice. California has roughly 7 million HSA account holders, and every one of them has a Schedule CA adjustment. If you use tax software, it usually handles this automatically. If you’re filing manually, it’s easy to miss.

What Happens After Line 13

Once your federal AGI is on Line 13, the next two lines adjust it:

• Line 14 — California additions: Income California taxes but the feds don’t. HSA contributions and earnings are the most common item here. Out-of-state municipal bond interest is another big one.
• Line 15 — California subtractions: Income the feds tax but California doesn’t. Social Security benefits are the headline item — California fully exempts them. California lottery winnings are another.

Line 13 + Line 14 – Line 15 gives you your California adjusted gross income on Line 17. That’s the number that feeds into the rest of the 540: your deductions, your taxable income, and ultimately your California tax liability.

A small but counterintuitive point: it’s entirely possible for your California AGI to be lower than your federal AGI, even though California is known as a high-tax state. If you’re a retiree collecting Social Security and living off California lottery winnings (unlikely but legal), your California AGI could be dramatically lower than federal. California’s additions and subtractions cut both ways.

Common Errors on Line 13

• Using the wrong 1040 line — Your federal AGI is on Form 1040 line 11. Not line 15 (taxable income) and not line 24 (total tax). This mix-up happens more than you’d think.
• Forgetting to update after amending — If you file an amended federal return (1040-X), your Line 13 on Form 540 changes. You’ll need to file a California amended return (540X) too.
• Rounding errors — California allows you to round to whole dollars on the 540. If your federal AGI is $87,432.67, you can enter $87,433. Just be consistent throughout.
• Community property allocation — Married filing separately in a community property state like California means you split community income 50/50 under Cal. Family Code Section 760, even on Line 13. This can produce a very different AGI split than what appears on each spouse’s federal return if they also filed separately.

Sources & References

• FTB Form 540 Instructions (2024 Tax Year)
• Cal. Rev. & Tax. Code Section 17071 — Federal Conformity
• IRC Section 62 — Adjusted Gross Income Defined
• IRC Section 223 — Health Savings Accounts
• IRS Form 1040 — U.S. Individual Income Tax Return
• IRS Form 1040-X — Amended U.S. Individual Income Tax Return