Filing in the United States usually means filing twice, and the two returns are not simply the same calculation at different rates. The states set their own rules.
Two separate taxing authorities
The federal government and each state levy income tax under their own authority. Neither is a subdivision of the other, and each writes its own statute.
That independence means a state can define income, deductions, and credits differently, and can tax categories of income the federal system treats another way.
A handful of states levy no broad income tax at all, funding themselves through sales, property, or severance taxes instead, which changes the shape of a household's total burden rather than removing it.
Most states start from federal numbers
Rather than build a system from scratch, states typically begin with a figure taken from the federal return, then apply their own additions and subtractions.
This is called conformity, and it saves enormous administrative effort because taxpayers and software have already computed the starting number.
Conformity is partial and dated, though. States choose which version of the federal rules they follow and when they update, so a federal change may or may not flow through immediately.
Residency is the pivotal question
State taxation depends on residency and on where income was earned, which are different tests and can point at different states in the same year.
Someone who works in one state and lives in another may owe in both, with a credit mechanism intended to prevent the same income being fully taxed twice.
Reciprocity agreements between some neighboring states simplify this, but they exist only where two particular states have agreed, not as a general rule.
Remote work strained the framework
Rules about where work is performed were written for an era when the location of an employee and their office were the same fact.
Widespread remote work separated them, and states have responded differently, some looking to the employer's location and others to where the person physically sat.
The result is genuine inconsistency between states, and the treatment of a given situation depends on the specific states involved and on rules that continue to change.
Local taxes add a third layer
Some cities and counties levy their own income taxes on top of state and federal, administered separately and with their own filing requirements.
These are easy to miss because they are geographically narrow, and someone moving into such a jurisdiction may not be told about the obligation.
The practical consequence is that comparing states by headline rate alone understates the difference, since the structure below the rate varies just as much.