State tax conformity is the mechanism by which a state links its income tax rules to the federal Internal Revenue Code, letting you use figures from your federal return as the starting point for your state return. Forty-one states and the District of Columbia levy a broad-based individual income tax, and nearly all of them tie their calculations to the IRC in some way. How tightly they tie, and which federal provisions they reject, is what determines how much extra work your state return actually takes.
The Three Methods States Use to Conform
States take one of three approaches to the federal code, and knowing which one applies to you tells you how closely your state return will track the federal rules.
Rolling Conformity
Roughly half of income-tax states automatically adopt changes to the IRC as Congress enacts them. When a new federal tax law takes effect, rolling-conformity states absorb the changes without any separate state legislation. For taxpayers, this is the simplest arrangement: whatever the federal rules say this year, the state follows.
Fixed-Date (Static) Conformity
The remaining states peg their tax codes to the IRC as it existed on a specific date. If Congress changes the federal rules after that date, the state doesn’t automatically follow. The legislature must pass a bill to update its conformity date or selectively adopt the new provisions. Before assuming a new federal deduction or credit applies at the state level, check whether your state has updated its conformity date.
Selective Conformity
Some states cherry-pick which federal provisions to adopt, regardless of whether they otherwise use rolling or fixed-date conformity. A state might follow federal definitions of gross income while rejecting federal bonus depreciation rules. This hybrid approach gives legislators fine-grained control over the state’s tax base and adds complexity for the person preparing the return. One line of your federal return may carry over cleanly while the next one doesn’t.
Where Your State Return Starts
Even among states that conform, the jumping-off point for calculating state taxable income differs. About 31 states and the District of Columbia begin with federal adjusted gross income, which is your gross income after above-the-line deductions like student loan interest and retirement contributions but before your standard or itemized deduction. Five states start further down the federal return at federal taxable income, which already reflects the standard or itemized deduction.
The difference matters. A state that starts from AGI typically applies its own standard or itemized deduction rules. A state starting from taxable income has already inherited the federal deduction structure and makes adjustments from there. A handful of states don’t use either federal starting point; they build income from scratch, though they still reference federal forms like W-2s and 1099s for specific income items.
States Without an Income Tax
Eight states impose no broad-based individual income tax at all: Alaska, Florida, Nevada, New Hampshire, South Dakota, Tennessee, Texas, and Wyoming. If you live exclusively in one of these states, conformity doesn’t affect your personal filing. It becomes relevant again only if you earn income in a state that does tax individuals, because that state’s conformity method will govern how those earnings are taxed.
Federal Provisions States Commonly Reject
Even rolling-conformity states frequently decouple from specific federal provisions that would cost too much revenue or clash with local policy. These are the areas most likely to create gaps between your federal and state returns.
Bonus Depreciation
Under IRC Section 168, businesses can deduct a percentage of certain asset costs in the first year rather than spreading the deduction over the asset’s useful life. The Tax Cuts and Jobs Act set that percentage at 100% for property placed in service through 2022, but the rate has been dropping by 20 points each year since then. For 2026, the federal bonus depreciation rate is 20%.1Office of the Law Revision Counsel. 26 USC 168 – Accelerated Cost Recovery System
Many states never adopted 100% bonus depreciation, and others decoupled during the years when the federal write-off was largest. In those states, you may need to add back part or all of the federal bonus depreciation on your state return and then claim smaller depreciation deductions spread over the asset’s recovery period. The result is a timing difference: you get the same total deduction eventually, but the state spreads it out while the federal return front-loads it.
The SALT Deduction Cap
The state and local tax deduction was originally capped at $10,000 by the Tax Cuts and Jobs Act. Legislation signed in 2025 raised that cap to $40,000 ($20,000 for married-filing-separately filers), with the higher cap indexed for inflation starting in 2026 (approximately $40,400). The cap phases down for taxpayers with modified adjusted gross income above roughly $500,000, and it is scheduled to reset to $10,000 after 2029.2Internal Revenue Service. Topic No. 503, Deductible Taxes
The federal cap has limited direct effect on most state returns because states generally don’t allow a deduction for their own income tax on the state return. But the interaction still matters. Some states start from federal taxable income, which already reflects the capped SALT deduction, and those states may then require adjustments so that the full amount of state and local taxes paid is properly accounted for. If your state starts from federal AGI, the SALT cap typically has less direct impact because the federal SALT deduction hasn’t been applied at that stage.
Net Operating Losses
Under IRC Section 172, net operating losses arising after 2017 can be carried forward indefinitely but generally cannot be carried back, with narrow exceptions for farming businesses and certain insurance companies. The deduction in any given year is capped at 80% of taxable income.3Office of the Law Revision Counsel. 26 USC 172 – Net Operating Loss Deduction
States often set their own NOL rules. Some impose a fixed carryforward period rather than allowing indefinite carryforwards. Others cap the annual deduction at a lower percentage of income, or limit the dollar amount that can be used in a single year. A few states still allow carrybacks that the federal code no longer permits. When your state’s rules differ, you’ll need to keep two separate NOL schedules, one federal and one state.
Interest on U.S. Treasury Securities
Interest earned on federal savings bonds and other U.S. Treasury obligations is included in your federal adjusted gross income but is exempt from state and local income taxes.4TreasuryDirect. Tax Information for EE and I Bonds Subtract that interest when calculating your state taxable income. It’s an easy adjustment to miss if your brokerage lumps Treasury interest together with other investment income on a single 1099. Missing the subtraction means overpaying state tax on income the state has no right to tax.
State Credits Don’t Follow Federal Rules Automatically
Conformity to the IRC’s income definitions doesn’t mean a state follows federal tax credit rules. Most states that offer their own earned income tax credit calculate it as a percentage of the federal credit, so your federal EITC drives the state number. But eligibility can differ. Several states extend the credit to filers using an Individual Taxpayer Identification Number rather than a Social Security number, and some have expanded eligibility for younger childless workers beyond what federal rules allow.
State child tax credits are more independent still. Most states that offer one have built it from scratch rather than linking to the federal child tax credit. That lets each state set its own income thresholds, refundability rules, and eligibility criteria. Qualifying for a federal credit doesn’t guarantee you qualify for the state version, and vice versa. Check the state’s credit rules separately rather than assuming they mirror your federal return.
Reconciling the Two Returns
Complete your federal Form 1040 first. Nearly every state return begins with a number pulled from it, either federal AGI (line 11) or federal taxable income (line 15), depending on your state.5Internal Revenue Service. About Form 1040, U.S. Individual Income Tax Return
From that starting point, your state return will have an adjustment schedule where you enter two types of modifications:
- Additions cover items that are deductible or excluded on the federal return but taxable in your state. Bonus depreciation claimed federally in a state that doesn’t allow it is the most common example.
- Subtractions cover items that are taxable federally but exempt at the state level. U.S. Treasury bond interest and certain retirement income are typical subtractions.
State revenue department websites publish the specific adjustment schedules and instructions you need. Those schedules list every line item where the state departs from the federal code, so you can work through them systematically rather than guessing which federal numbers need to change. Match each adjustment to the corresponding line on your federal return so the numbers stay traceable. Filing software handles most of this mapping automatically; if you file on paper, the adjustment schedule must be attached to your state return to avoid processing delays.
What Happens After a Federal Audit or Amended Return
If the IRS changes your federal return, whether through an audit, an amended return you file, or a math-error correction, most states require you to report that change and file a corresponding state amendment. Deadlines vary widely, from as little as 30 days to as long as two years depending on the state. Missing your state’s window can produce penalties and interest even when the underlying tax is small.
A few points that trip people up:
- In most states, the reporting clock doesn’t start when the IRS first contacts you. It starts when the federal change becomes final, meaning you’ve either agreed to the adjustment, let the appeal window lapse, or exhausted your court options.
- A federal adjustment to one income item shouldn’t open your entire state return to re-examination, but not every state’s law is that clean. Some states limit their review to items affected by the federal change when the normal statute of limitations has already expired.
- Interest on any additional state tax typically runs from the original due date of the return, not from the date of the federal adjustment. Annual interest rates on state deficiencies commonly fall in the 7% to 15% range, so delays add up.
Estimated Tax Payments When Conformity Gaps Exist
Conformity gaps affect estimated payments during the year, not just the final return. If you know your state doesn’t allow a large federal deduction, such as bonus depreciation on a business asset you purchased, your state liability will be higher than a mirror of the federal calculation would suggest. Failing to account for that in your quarterly payments can trigger a state underpayment penalty.
Most states have their own estimated tax safe harbor rules, and those don’t always match the federal thresholds. At the federal level, you avoid penalties by paying at least 90% of the current year’s tax or 100% of last year’s tax (110% if your AGI exceeded $150,000). Your state may use similar percentages or set different ones. Run the numbers through both obligations separately, especially in any year with a major gap between federal and state rules.