Flat Tax vs. Bracket States: Why Identical Incomes Owe Different State Tax

State income tax comes in two structures. Flat-rate states charge the same percentage on every dollar of taxable income, no matter how much you earn. Bracket states scale the rate up as income rises, the same stacking mechanism the federal system uses. Two filers with identical $80,000 taxable incomes can owe noticeably different amounts depending on which structure their state uses — and the state with the higher headline rate isn't always the one that costs more.

Here's the arithmetic behind both, worked with two real states for 2025.

How a flat-rate state computes tax

Colorado taxes all wage income at a single 4.4% flat rate, regardless of income level. On $80,000 of taxable income:

$80,000 × 4.4% = $3,520.00

That's the entire calculation. In a flat-rate state, marginal rate and effective rate are always the same number — 4.4% on the last dollar earned is also 4.4% on the whole return, because every dollar is taxed identically.

How a bracket state computes tax

California uses ten brackets running from 1% to 13.3% for 2025 taxable income, stacking the way federal brackets do. For the same $80,000 of taxable income, a single filer's tax is built slice by slice:

• 1% on the first $10,756 = $107.56
• 2% on the next $14,743 (to $25,499) = $294.86
• 4% on the next $14,746 (to $40,245) = $589.84
• 6% on the next $15,621 (to $55,866) = $937.26
• 8% on the next $14,740 (to $70,606) = $1,179.20
• 9.3% on the remaining $9,394 (to $80,000) = $873.64

Total California tax: $3,982.36.

Why the marginal rate you hear about isn't what you pay

This is where the two states get interesting to compare. This California filer's top marginal rate — the rate on their last dollar — is 9.3%, more than double Colorado's flat 4.4%. But their effective rate, total tax divided by taxable income, is $3,982.36 ÷ $80,000 = 4.98%. That's barely above Colorado's flat rate, not double it.

The gap between the two filers' actual bills is $462.36 — real, but nowhere near what comparing "4.4% vs. 9.3%" headline rates would suggest. A state's top bracket rate describes what happens to income at the very top of that bracket schedule, not what a typical filer at $80,000 actually owes. This is the same marginal-vs-effective distinction that applies to federal brackets: the rate you're "in" and the rate you actually pay are different numbers, and news coverage of state tax rates almost always quotes the former.

The gap widens as income rises

The $462.36 difference between Colorado and California at $80,000 looks small because both filers are still near the bottom of California's rate schedule. Run the same two states at $300,000 of taxable income instead:

• Colorado: $300,000 × 4.4% = $13,200.00
• California: the first five brackets contribute the same $3,108.72 as before, plus 9.3% on the remaining $229,394 (from $70,606 to $300,000) = $21,343.64, for a total of $24,452.36

At $300,000, California's filer owes $11,252.36 more than Colorado's — roughly 24 times the $462.36 gap at $80,000 — even though their marginal rate didn't change at all between the two examples; both the $80,000 and $300,000 California filers are taxed at 9.3% on their last dollar, because both fall inside the same $70,606–$360,659 bracket. What changed is how much of their income sits above the point where California's rate schedule catches up to and passes Colorado's flat rate. The size of the flat-vs-bracket gap depends heavily on where your income falls, not just on which two states you're comparing.

Taxable income itself isn't defined the same way

This example holds taxable income fixed at $80,000 in both states to isolate the rate structure. In practice, the two states also define taxable income differently before any rate applies. Colorado conforms to the federal standard deduction — $15,000 for a single filer in 2025 — while California sets its own, much smaller standard deduction of $5,202 for a single filer. So a Colorado and a California filer with the same gross income won't arrive at the same taxable income to begin with; the deduction gap is a second source of difference layered on top of the rate structure itself.

Comparing your own income across states

Every state falls into one of these two structures — flat or bracketed — and a handful of states charge no wage income tax at all. Rather than compare two states by hand, the State Income Tax Comparison tool runs this same bracket-by-bracket or flat-rate calculation for your actual income across all 50 states and DC at once, ranked from lowest to highest effective rate, with every state's rate cited to its own revenue department.

If you want the full bracket table or flat rate for one specific state instead, the State Income Tax Calculator covers all 50 states individually.