How U.S. Federal Income Tax Works
The federal income tax is progressive: income is divided into bands, and each band is taxed at its own rate. A common misconception is that moving into a higher bracket taxes all your income at that rate. It does not. Only the income above the bracket threshold is taxed at the higher rate.
Someone earning $105,000 as a single filer in 2025 does not pay 24% on everything. Their taxable income after the standard deduction is $89,250, which falls in the 22% bracket — and even then, the first $11,925 is taxed at 10%, the next portion at 12%, and only the remainder at 22%. Their effective rate is far below their marginal rate.
Marginal vs. Effective Rate
| Marginal rate | Effective rate | |
|---|---|---|
| Definition | The rate on your next dollar of income | Total tax divided by total income |
| Used for | Deciding whether extra income or a deduction is worthwhile | Understanding your actual tax burden |
| At $85,000 single | 22% | about 11% federal |
The marginal rate is the one that matters for decisions — a $1,000 pre-tax 401(k) contribution saves $220 at a 22% marginal rate. The effective rate is the one that describes reality.
2025 Federal Tax Brackets
Single filers — applied to taxable income (after deductions):
| Rate | Taxable income |
|---|---|
| 10% | $0 to $11,925 |
| 12% | $11,925 to $48,475 |
| 22% | $48,475 to $103,350 |
| 24% | $103,350 to $197,300 |
| 32% | $197,300 to $250,525 |
| 35% | $250,525 to $626,350 |
| 37% | $626,350 to and above |
Married filing jointly:
| Rate | Taxable income |
|---|---|
| 10% | $0 to $23,850 |
| 12% | $23,850 to $96,950 |
| 22% | $96,950 to $206,700 |
| 24% | $206,700 to $394,600 |
| 32% | $394,600 to $501,050 |
| 35% | $501,050 to $751,600 |
| 37% | $751,600 to and above |
Standard Deduction, 2025
| Filing status | Standard deduction |
|---|---|
| Single | $15,750 |
| Married filing jointly | $31,500 |
| Married filing separately | $15,750 |
| Head of household | $23,625 |
You take either the standard deduction or your itemized deductions, whichever is larger. Since the 2017 tax reform roughly doubled the standard deduction, around 90% of filers take it. Itemizing is generally worthwhile only with substantial mortgage interest, state and local taxes (capped), charitable giving, or large medical expenses.
Federal Income Tax by Income Level
Assuming the standard deduction and no other adjustments:
| Gross income | Tax (single) | Effective rate | Tax (married joint) | Effective rate |
|---|---|---|---|---|
| $30,000 | $1,472 | 4.9% | $0 | 0.0% |
| $50,000 | $3,872 | 7.7% | $1,850 | 3.7% |
| $75,000 | $7,949 | 10.6% | $4,743 | 6.3% |
| $100,000 | $13,449 | 13.4% | $7,743 | 7.7% |
| $150,000 | $25,067 | 16.7% | $15,898 | 10.6% |
| $200,000 | $37,067 | 18.5% | $26,898 | 13.4% |
| $300,000 | $69,035 | 23.0% | $50,134 | 16.7% |
| $500,000 | $139,035 | 27.8% | $104,046 | 20.8% |
Note how far the effective rate sits below the marginal rate at every level, and how much the joint filing status is worth at middle incomes.
FICA: Social Security and Medicare
These are separate from income tax and are charged on wages from the first dollar — there is no deduction.
| Tax | Employee rate | Wage base (2025) |
|---|---|---|
| Social Security | 6.2% | First $176,100 |
| Medicare | 1.45% | No limit |
| Additional Medicare | 0.9% | Wages above $200,000 (single) / $250,000 (joint) |
Employers match the 6.2% and 1.45%, so the true cost of employment is higher than the payslip shows. Self-employed people pay both halves as self-employment tax, at 15.3%, though half is deductible.
Filing Status
| Status | Who qualifies |
|---|---|
| Single | Unmarried, and not qualifying as head of household |
| Married filing jointly | Married couples combining income on one return — usually the lowest total tax |
| Married filing separately | Married couples filing individually — usually higher tax, but useful for liability separation or income-driven student loan repayment |
| Head of household | Unmarried, paying over half the cost of a home for a qualifying dependent — wider brackets and a larger deduction than single |
| Qualifying surviving spouse | Widowed with a dependent child, for two years after the year of death — uses joint brackets |
Deductions vs. Credits
A deduction reduces taxable income, so it is worth your marginal rate. A $1,000 deduction saves $220 at a 22% marginal rate. A credit reduces the tax itself, so a $1,000 credit saves $1,000 regardless of bracket. Credits are worth roughly four to five times as much as an equivalent deduction for a typical filer.
Some credits are refundable — they can produce a refund larger than the tax paid. The Earned Income Tax Credit and part of the Child Tax Credit work this way. Non-refundable credits can only reduce tax to zero.
Ways to Legally Reduce the Bill
- Traditional 401(k) and IRA contributions — reduce taxable income now, taxed on withdrawal.
- HSA contributions — the only account that is untaxed going in, growing, and coming out for medical expenses.
- FSA contributions — pre-tax dollars for medical or dependent care, but use-it-or-lose-it.
- Employer health premiums — typically paid pre-tax and already excluded from taxable wages.
- Tax-loss harvesting — realised investment losses offset gains, and up to $3,000 of ordinary income per year.
- Charitable giving — only if you itemize; bunching several years of donations into one can make itemizing worthwhile.
Frequently Asked Questions
Will a raise push me into a higher bracket and cost me money?
No. Only the portion above the threshold is taxed at the higher rate, so more gross income always means more net income. (Benefit cliffs in means-tested programs can create genuine exceptions, but the tax brackets themselves never do.)
Why is my paycheck withholding different from this result?
Withholding is an estimate based on your W-4 and is calculated per pay period as if that period repeated all year. Bonuses are often withheld at a flat 22%. The reconciliation happens when you file.
Does this include state tax?
Only as a flat percentage estimate you enter yourself. Real state taxes vary from zero in nine states to progressive systems with their own brackets and deductions.
Which tax year does this use?
The 2025 federal tax year. Brackets, the standard deduction and the Social Security wage base are adjusted for inflation each year, so confirm current figures with the IRS before filing.