How is state income tax filing different from federal?
Two governments, two returns
The United States has parallel tax systems: the federal income tax administered by the IRS, and state taxes administered by each state's own revenue agency. Filing your federal return does not file your state return, and the two agencies are entirely separate organizations — a point that surprises many first-time filers. Tax software often bundles the two into one workflow, which is convenient but obscures the fact that two distinct returns are going to two distinct governments, each with its own rules, deadlines, and correspondence.
How much states vary
State income tax regimes differ more than most people expect:
- Some states levy no individual income tax at all, funding themselves through other taxes instead.
- Among states that do tax income, some use graduated brackets like the federal system, while others apply a single flat rate to taxable income.
- States make independent choices about what to tax: whether Social Security benefits are taxed (a choice separate from the federal rules), whether retirement income gets special treatment, which deductions and credits exist, and how closely the state's definition of income tracks the federal one.
- Some cities and counties add local income taxes on top, administered under state frameworks.
Because of this variation, generalizations about "state taxes" are nearly useless. The authoritative source for your situation is your own state's tax agency, and the practical way to find it is the Federation of Tax Administrators' directory of state tax agencies, which links to the official revenue department of every state. Going through the directory also protects you from look-alike sites; state tax agencies have inconsistent, hard-to-guess web addresses, and search results for them attract impostors.
Where the returns connect
Despite being separate, the two returns are linked in practice. Most state returns start from a number on your federal return — often federal adjusted gross income — and then apply state-specific additions and subtractions. This has two consequences worth knowing. First, you generally need to complete the federal return before the state one, which is why software does them in that order. Second, changes ripple: if your federal return is amended or adjusted, the state return that was built on it may need amending too, and states generally require you to report federal changes.
Your W-2 reflects the split directly: federal wages and withholding occupy the early boxes, while Boxes 15 through 20 carry the state and local versions. State withholding follows the same settle-up logic as federal withholding — an estimate paid through the year, reconciled at filing.
Deadlines and residency wrinkles
State filing deadlines usually track the federal date but are set independently, and a federal extension does not automatically extend every state's deadline — each state publishes its own rules. People who live in one state and work in another, or who move mid-year, may owe part-year or nonresident returns in more than one state; states have agreements and credit mechanisms to limit double taxation, but the details are state-specific. Multi-state situations are among the most individual in all of tax filing, and they are a strong candidate for professional help. For the federal side of the calendar and process, the IRS's filing overview and USA.gov's tax portal remain the anchors.