State vs Federal Income Tax
Federal income tax is one system with one set of rules for the whole country. State income tax is fifty-one separate systems, nine of which do not exist. For most households the federal bill is several times the state bill.
What they share
Most states start from a federal number — adjusted gross income or federal taxable income — so the federal definition of income effectively sets the state base. When Congress changes what counts as income, most states follow automatically.
Both use marginal brackets, both allow a deduction before the rates apply, and both use the same four filing statuses.
What they do not share
Rates and thresholds are entirely independent. Federal rates reach 37%; the highest state top rate is well below that, and nine jurisdictions charge nothing.
Federal brackets and the standard deduction are indexed to inflation every year. Many states index nothing at all — Virginia’s bracket thresholds have not moved since 1990, which quietly raises real tax every year as wages grow.
Deductions and credits diverge sharply. A deduction that exists federally frequently does not exist at state level, and vice versa.
How they interact
State income tax is deductible on a federal itemised return, but only within the combined state-and-local tax cap, which limits the benefit for people in high-tax states.
A handful of states run this the other way and allow a deduction for federal income tax paid — Alabama and Missouri among them. That materially lowers the real state burden and is not reflected in headline rate comparisons, including ours.