Tax Deduction
An amount subtracted from gross income to lower taxable income, reducing the tax owed by the deduction amount times the marginal rate. Includes mortgage interest, charitable gifts, and others.
How deductions work
The basic mechanism:
- Reduce taxable income by the deduction amount.
- Tax savings = deduction × marginal rate.
- Different from tax credit — credit reduces tax dollar-for-dollar; deduction reduces income that's then taxed.
A $1,000 deduction saves $220 for a 22% marginal-rate filer. A $1,000 credit saves $1,000.
Standard vs. itemized
Two approaches:
- Standard deduction — fixed amount based on filing status; simple.
- Itemized deductions — sum of specific eligible deductions.
- Take whichever is larger — most filers take standard since 2017 increase.
The choice affects which deductions are usable.
Common itemized deductions
Major categories:
- Mortgage interest — on primary residence.
- State and local taxes (SALT) — capped at $10,000 currently.
- Charitable contributions — to qualified organizations.
- Medical expenses — above 7.5% of AGI.
- Investment expenses (limited).
Each has rules and limits.
Above-the-line deductions
Some deductions reduce income before AGI calculation:
- Traditional IRA contributions.
- HSA contributions.
- Student loan interest (capped, phases out).
- Self-employment tax (half).
- SEP/Solo 401(k) contributions.
These are valuable because they're available regardless of standard vs. itemized choice.
Why deductions matter
Several reasons:
- Reduce tax liability — direct savings.
- Affect optimal financial decisions — mortgage decisions, charitable timing.
- Lower AGI — may qualify you for income-based credits and benefits.
- Behavioral incentives — government uses deductions to encourage activities.
Understanding deductions affects many financial planning choices.
Common misunderstandings
Frequent errors:
- Confusing deductions with credits.
- Believing deductions are "free money" — they save marginal rate × amount.
- Not realizing standard deduction may be higher.
- Missing above-the-line deductions even when taking standard.
These errors leave money on the table.
What individuals should know
Practical takeaways:
- Compare standard vs. itemized annually.
- Above-the-line deductions are always valuable.
- Charitable bunching — concentrating donations into one year to exceed standard threshold.
- Track expenses that may itemize — medical, state tax, charity.
Tax deductions are foundational to filing. Understanding the mechanics helps optimize the savings available, though the gains are typically smaller than commonly believed.