I'm trying to understand if (or in what ways) state income tax is calculated significantly different from federal income tax. I realize this might be somewhat state specific, but is it generally the same? Specifically the two states i'm currently interested in for possible ER are Colorado and Idaho. A google search for colorado income tax says "Colorado's state income tax rate is a flat 4.63 percent of your federal taxable income". Does this effectively mean if I pay 0 dollars in federal tax, I will pay 0 colorado tax? If this is not true, what cases would cause me to not have federal tax but to have CO tax? Same essential question goes for Idaho, google says the tax ramps up to 7.8%, but makes no mention of being related to federal rates. So, is the situation in ID significantly different? Like most people trying to retire early, I will probably have some combination of IRA conversions, capital gains, and dividends, so i'm mostly interested in if/how these categories would be treated differently at the state level.