Federal and state income tax are calculated by two entirely separate sets of rules — but most people want one combined answer: "what do I actually owe, total?" Here's the step-by-step way to get there.
Step 1: Understand They're Separate Calculations, Sharing Some Inputs
Your federal return and your state return both start from your income, but apply completely different brackets, deductions, and rules on top of it. Some states use their own standard deduction; a handful (like Pennsylvania, New Jersey, and Connecticut) don't have a standard deduction at all and tax from the first dollar. This is why you can't estimate state tax by just applying a rough percentage to your federal result — the two need to be run separately.
Step 2: Find Out What Category Your State Falls Into
Every state falls into one of three categories:
- No state income tax — Alaska, Florida, Nevada, New Hampshire, South Dakota, Tennessee, Texas, Washington, Wyoming. If you live here, your federal calculation is the whole picture.
- Flat rate — every dollar of taxable income is taxed at the same percentage, regardless of how much you make.
- Progressive brackets — similar structure to federal tax, with the rate increasing at higher income tiers.
Open the State Tax calculator and click your state on the map (or pick it from the dropdown list below the map) — the page will tell you immediately which category it's in and show the actual bracket table or flat rate that applies.
Step 3: Calculate Your Federal Tax First
Start with the federal side: wages, self-employment income if any, filing status, and deductions, run through the main calculator. This gives you federal income tax, FICA/self-employment tax, and your federal refund or amount owed.
Step 4: Calculate Your State Tax Separately
On the State Tax tab, enter your income for that state and click "Estimate State Tax." You'll get the state tax amount, your effective and top state rates, and a bracket breakdown if your state uses progressive brackets.
Step 5: Combine Both for Your True Total Tax Rate
Add your federal total tax and your state tax together, then divide by your gross income, to see your actual combined effective rate — this is the number that reflects what you're really paying, not just the federal piece in isolation. For someone in a high-tax progressive state, the state portion can add several percentage points on top of the federal rate; for someone in a no-tax state, it's zero.
Step 6: Special Situations — Moving or Working Across State Lines
Two common complications:
- You moved partway through the year: you generally file as a part-year resident in both your old and new state, splitting income based on when you lived where. State rules on exactly how to split vary, and this is worth extra care or a preparer's help the first time you do it.
- You live in one state but work (or worked) physically in another: the state where you physically perform work can tax that income even as a non-resident, and the rules for when that applies differ significantly state by state — some have low physical-presence thresholds, others (notably New York) apply a stricter "convenience of the employer" rule. This is genuinely one of the more complicated corners of state tax law and isn't something a single-state calculator can fully automate — if this applies to you and there's real money at stake, it's worth the time of a CPA familiar with multi-state returns.
Step 7: Don't Forget State Estimated Payments If You're Self-Employed
If you owe quarterly estimated federal tax as a freelancer, most states with an income tax expect their own quarterly estimated payments too, on a similar (though not always identical) schedule. Check your specific state's Department of Revenue for its exact deadlines rather than assuming they match the federal ones exactly.
Common Mistakes
- Assuming a state uses the same standard deduction as federal — many states set their own, and several use none at all.
- Estimating state tax as "federal tax times some percentage" instead of running the actual state brackets.
- Forgetting state-level estimated payments when already juggling federal quarterly payments as a freelancer.
- Not addressing part-year-resident or multi-state-work situations carefully after a mid-year move.
Quick FAQ
Do all states use the same tax year deadlines as federal? Most states' filing deadlines match the federal deadline, but always double-check your specific state — a few set their own.
What if I moved states mid-year? You'll typically file part-year-resident returns in both states, splitting income by the time you lived in each — the exact method varies by state, so check your new state's Department of Revenue instructions or consult a preparer for your first split-year return.