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income stacks on top of your other income, so the rate that applies to that slice is your , often higher than the flat rate your employer withholds. That difference is why a can be withheld at one rate but cost you more when you file.
Why this happens
The tax system is progressive: income is taxed in layers, and your usually sits on the top layer above your salary.
The flat supplemental rate is a payroll convenience, not a calculation of your .
Deductions, credits, and filing status shift where your brackets fall, so your effective and marginal rates differ.
State income tax, where it applies, adds its own brackets or flat rate on top.
What to check
- Your estimated total taxable income for the year, including the .
- Which federal bracket the top of your income falls into.
- The flat rate used to on the , for comparison.
- Your state's tax structure, brackets or flat rate.
- Whether large deductions meaningfully change your marginal layer.
Common mistake
Treating the flat rate as your tax rate. The withheld amount is not always the final tax. Your on the can be higher, which is the gap people feel in April.
Example scenario (hypothetical)
Illustration only, not your tax situation.
Example: Morgan's salary already reaches a higher bracket, and a $25,000 sits entirely on top of it. If that top slice is taxed around 32% federally plus state tax, but only a lower flat rate was withheld, Morgan should expect to owe the difference at filing.
When to get help from a tax pro
- You are near a bracket boundary and timing income matters.
- You have large itemized deductions or credits.
- You want to model a multi- year precisely.
- You have income in more than one state.
Related calculators
Related pages
For learning, not filing
Grants, employers, and states all differ. Use your own documents and a qualified tax professional before you make decisions from this guide.