You finish your federal return, and the software asks one more question: which state did you live in? Then it opens a second return, with different questions and a different number at the bottom. State income tax is a separate system with its own rules, and it can bill you in the same year the IRS sends you a refund.
Two Systems, Two Returns, Two Deadlines
Your federal return goes to the IRS. Your state return goes to a state agency, usually called the department of revenue, which funds state highways, public universities, prisons and much of K-12 schooling.
The two returns share your numbers and little else: different forms, different credits, different definitions of income. Most states match the federal deadline in mid April, though a few sit later. Some extend your filing time automatically with a federal extension; others want their own form. In both systems, an extension buys time to file, not time to pay.
As of 2026, 41 states and the District of Columbia tax wage income, so for most people filing season means two returns.
The Three Shapes a State Income Tax Takes
A small number of states do not tax wage income at all. You still file federal and have no state return for your wages. That group is not fixed: one state finished repealing its tax on interest and dividends in 2025, and another is stepping its rate toward zero as revenue allows. Check your own state's department of revenue rather than a list online.
Some states charge a flat rate: one percentage on all taxable income.
The rest use graduated brackets, where each rate applies only to income above a threshold, the way the federal ones do. The thresholds are the surprise: a state's top rate can arrive at an income a full-time job clears easily.
No Income Tax Does Not Mean No Taxes
A state without an income tax still needs money, so it collects it elsewhere. Sales and property taxes carry most of the load, with the rest from whatever the state happens to have: oil and gas, gambling, tourism.
Sales tax takes a bigger share of a small income than a large one, and property tax reaches you through your rent even if you own nothing. The phrase tells you which tax you skip, not what your total bill is.
Your State Return Starts With a Federal Number
Most state returns open by copying a figure off your federal return, usually your federal adjusted gross income (your income after a few federal adjustments).
From there the state goes its own way. Additions put back income the state taxes and the federal government does not. Subtractions take out income the state does not tax. Then come the state's own standard deduction, exemptions and credits.
Say your federal AGI is 45,000 dollars. Your state adds nothing, subtracts nothing, and allows a 5,000 dollar standard deduction, leaving 40,000 dollars of taxable income. At a flat 4 percent, the state wants 1,600 dollars on top of the federal bill.
Withholding Shows Up as Two Separate Lines
Look at a pay stub. Federal income tax, Social Security and Medicare each get a line, and state income tax gets its own.
Federal withholding comes from the Form W-4 you filled out on your first day. Many states have their own version, and some just use your W-4. In January, boxes 15 through 20 of your W-2 report your state and local wages and the tax withheld.
The two estimates are made independently, so a federal refund alongside a state bill is normal.
City and County Taxes Are a Third Layer
In roughly 15 states, income tax has a third floor: cities, counties, townships or school districts levying their own. In some states it rides on your state return as an extra line; in others the city collects it, with its own form and due date.
Local tax is usually a flat percentage of wages with no brackets, so it starts near your first dollar. It often follows where you work rather than where you live, and a home city that taxes you too normally credits what the work city took.
Residency, Domicile, and Where a Student Actually Lives
Two words do the work here, and they are not synonyms.
Your domicile is the one place you treat as your permanent home, the place you mean to return to when you are away, and you can have only one. Residency is broader: many states count days, so keeping a home in the state and spending more than half the year there can make you a resident whatever your domicile says. States differ on whether a dorm room counts as keeping a home.
Leaving for college usually does not move your domicile, because you plan to come back. A student from one state attending school in another is normally still a resident at home, and files a resident return there on income from everywhere. Work a campus job and the school state taxes what you earned inside its borders, through a nonresident return.
In-state tuition residency is your school's decision under its own rules, and it does not settle your tax residency.
Two States, One Paycheck
Live in one state and work in another and both have a claim. The work state taxes what you earned inside it. Your home state taxes everything. Left alone, that is the same dollars taxed twice.
The credit for taxes paid to another state is the fix. Your home state credits what you paid the work state, usually capped at what it would have charged itself. In total you pay about the higher of the two rates, not both stacked.
Neighboring states sometimes skip the exercise with a reciprocity agreement, where the work state agrees not to tax commuters. It is not automatic: you give your employer a nonresidence certificate, and some states make you refile every year.
Remote work usually belongs to your home state. A few states, New York most prominently, apply a convenience of the employer rule and tax at-home days as office days when working from home was your choice.
Moving mid year splits the year in two. You file a part year return in each state, and each taxes what you earned while you lived there.
A summer internship in another state is the small version of all this. Withholding starts with your first check, and a few weeks of pay usually owes little or nothing, so the nonresident return is how you get that money back.
