A persistent worry among American workers is that a raise could push them into a higher bracket and leave them worse off. The structure of the system makes that outcome impossible.
Brackets apply to slices, not to totals
A progressive income tax divides taxable income into bands, and each band is taxed at its own rate. Income is not taxed at a single rate determined by the total.
Crossing into a higher bracket means only the dollars above that threshold are taxed at the higher rate. Everything below it continues to be taxed as before.
So an additional dollar of income always leaves something after tax, because no bracket rate reaches the full amount and none applies retroactively to earlier income.
Marginal and effective rates differ
The marginal rate is the rate applied to the next dollar earned, and it is the number people usually mean when naming their bracket.
The effective rate is total tax divided by total income, and it is always lower than the marginal rate in a progressive system with multiple bands.
Confusing the two produces large errors. Applying the top bracket rate to an entire income overstates the liability considerably.
Deductions and credits work differently
A deduction reduces the amount of income that is taxed, so its value depends on the marginal rate at which that income would have been taxed.
A credit reduces the tax itself, dollar for dollar, which makes its value the same regardless of which bracket the taxpayer occupies.
This asymmetry explains why credits are the more powerful instrument for lower-income filers, whose marginal rates make deductions worth less.
Cliffs exist elsewhere in the system
The bracket structure has no cliffs, but benefit programs and certain credits do phase out as income rises, and a few end abruptly at a threshold.
Where such a threshold exists, additional income can reduce a benefit by more than it adds in pay, which is a real effect but not a bracket effect.
These provisions are specific, they vary by program, and they change as rules are revised, which makes them a question for a professional rather than a rule of thumb.
Withholding creates the misleading paycheck
Payroll withholding estimates annual liability from the current paycheck, and a bonus or a mid-year raise can be withheld at a rate that overshoots.
The paycheck then looks as though the raise was consumed by tax, even though the annual reconciliation returns the excess at filing.
What changes at filing is the refund, not the underlying liability, which is why the paycheck is a poor place to judge how much tax a raise actually caused.