Flat vs Progressive Income Tax
A flat tax applies one rate to all taxable income. A progressive (graduated) tax applies rising rates to successive slices. Most U.S. states that tax income use graduated brackets; a growing minority have moved to a flat rate.
Flat taxes are still progressive in effect
Almost every flat-rate state still exempts a slice of income through a standard deduction or personal exemption. That exemption is worth the same in dollars to everyone, so it is worth proportionally more to a low earner — which makes the effective rate rise with income even though the statutory rate never moves.
A state with a flat 4% rate and a $15,000 deduction charges a $30,000 earner an effective 2%, and a $300,000 earner an effective 3.8%. Flat on paper, gently progressive in practice.
The recent direction of travel
A substantial number of states have either moved from graduated brackets to a single flat rate, or legislated multi-year reductions in an existing flat rate, often with revenue triggers that fire only if collections hold up.
Because those step-downs happen annually, a page published even one year ago is likely to show the wrong rate. This is the most common error in state tax content, and it is why every figure here carries the year it applies to and the date we last checked it.
What each structure actually changes
For most households the structure matters less than the level. A graduated state whose top bracket starts at $20,000 behaves like a flat tax for nearly every full-time worker. A flat state with a large exemption can be cheaper for low earners than a graduated one with none.
Read the bracket thresholds, not the bracket count. Both are on every state page here.