How marginal tax brackets work across income tiers

Quick answer: A marginal tax rate applies only to the income within a specific bracket, not to your entire income — as your income rises and crosses into higher brackets, only the portion above each threshold is taxed at the higher rate.

Unfortunately, none of the documents provided to me contain a general explanation of how the federal marginal-rate bracket system itself works (i.e., the mechanics of tiered brackets). The documents I have cover specific topics — education credits, mortgage interest, casualty losses, employer withholding, and state sales/income tax charts — but not the core explanation of marginal-bracket taxation.

What I can offer as brief general background (not from these documents, so treat this as general orientation only):

• The U.S. federal income tax uses a "graduated" or "marginal" bracket system. Each bracket has a rate that applies only to income earned within that bracket's range.
• For example, if brackets were 10% up to $X and 12% for income above $X, someone earning above $X would pay 10% on the first $X and 12% only on the amount over $X — not 12% on all of their income.
• Your "marginal rate" is the rate on your last dollar earned; your "effective rate" (total tax ÷ total income) is always lower than your top marginal rate.
• One document does illustrate marginal-rate concepts in a related context: Publication 15-A discusses 2026 "Marginal Rate Tables" used for withholding elections on rollover distributions, referring to the "2026 Marginal Rate Tables (inclusive of all related text above and within the tables)"¹ — though this is a withholding-specific table, not a full explanation of how income tax brackets work generally.

What it depends on:

• Which tax year's bracket thresholds and rates apply to you (these are adjusted annually and were also affected by the One Big Beautiful Bill Act enacted July 2025 — you should confirm current-year figures rather than relying on older figures).
• Your filing status (single, married filing jointly, etc.), since bracket thresholds differ by status.
• Whether you're asking about federal brackets, or a state's brackets — for example, New Mexico's chart shows its restructured 2025 brackets run roughly 1.5% to 5.9% for individuals², which also works on a graduated basis.

Because I don't have the specific bracket thresholds and rates for your tax year and filing status on hand in these documents, it's worth confirming the current-year numbers with a CPA who can also show you your actual effective rate.