Topic Terms

What is a Tax Deduction

A tax deduction is an eligible expense that reduces your taxable income, which lowers your tax bill by your marginal rate times the deduction amount — not dollar-for-dollar, but still meaningful savings.

A tax deduction is a specific expense, contribution, or amount you're allowed to subtract from your income before calculating the tax you owe. Deductions reduce your taxable income, which in turn reduces your tax liability. The higher your tax bracket, the more valuable a deduction is.

How a deduction affects your taxes:

$$\text{Tax Saved} = \text{Deduction Amount} \times \text{Marginal Tax Rate}$$

For example, a $5,000 deduction saves a taxpayer in the 22% bracket $1,100 in federal taxes ($5,000 × 22%). That same deduction saves a taxpayer in the 32% bracket $1,600.

Deductions vs. Tax Credits

This is one of the most important distinctions in tax planning:

Tax Deduction Tax Credit
What it reduces Taxable income Tax owed directly
Impact Saves (deduction × marginal rate) Saves the full credit amount
Is it dollar-for-dollar? No Yes

A $2,000 deduction in the 22% bracket saves $440 in taxes. A $2,000 credit saves the full $2,000. Credits are generally more valuable — but deductions still add up significantly, especially large ones like the mortgage interest deduction or retirement contributions.

Types of Deductions

"Above-the-line" deductions (adjustments to income) These come before you calculate Adjusted Gross Income (AGI) and are sometimes called "above-the-line deductions." Anyone can take them regardless of whether they itemize:

  • Traditional IRA contributions
  • Student loan interest (up to $2,500)
  • Self-employment tax (50%)
  • HSA contributions
  • Educator expenses (up to $300)

Standard deduction A flat dollar amount ($15,000 for single filers in 2025) that anyone can claim without documentation. See standard deduction.

Itemized deductions A specific list of eligible expenses you can deduct if their total exceeds your standard deduction. These include mortgage interest, SALT (capped at $10,000), charitable contributions, and medical expenses above 7.5% of AGI. See itemized deductions.

Business deductions (Schedule C) Self-employed taxpayers can deduct ordinary and necessary business expenses, including home office, equipment, vehicle use, health insurance premiums, and more.

Investment-related deductions

Common Deductions by Taxpayer Type

Employees:

  • Retirement contributions to 401(k) reduce W-2 income
  • HSA contributions
  • Student loan interest

Self-employed:

  • Business expenses (home office, equipment, mileage, phone)
  • Health insurance premiums
  • Retirement contributions (SEP-IRA, SIMPLE IRA, Solo 401(k))
  • Half of self-employment tax

Homeowners:

  • Mortgage interest (up to $750,000 of qualifying mortgage debt)
  • Property taxes (combined with state income tax up to $10,000 SALT cap)

Investors:

  • Capital losses up to $3,000 against ordinary income
  • Investment interest expense

Documenting Deductions

The IRS can audit any return and disallow deductions without proper documentation. Best practices:

  • Keep receipts for all significant deductible expenses
  • For charitable cash donations over $250, get a written acknowledgment from the charity
  • Log business mileage in a contemporaneous log
  • Keep records for at least 3–7 years after filing

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