Understanding Tax Deductions in U.S. Federal Taxation

Suppose the government wants to encourage taxpayers to save for retirement, pursue higher education, or pay for medical expenses. Instead of giving cash directly, it often provides tax benefits in the form of deductions. But what exactly is a deduction, and why are some deductions called ‘above-the-line’ while others are ‘below-the-line’? Let’s understand this step by step.

What Is a Tax Deduction?

A tax deduction is an amount that the IRS allows a taxpayer to subtract from income before calculating federal income tax.

In simple words: A deduction reduces the income on which federal income tax is calculated. Higher Deduction  →  Lower Taxable Income  →  Lower Federal Income Tax

Classification of Deductions

There are two broad categories of deductions.

Chart 1 — Above-the-line vs. below-the-line deductions at a glance

A. Above-the-Line Deductions (ALD)

These deductions are claimed before arriving at Adjusted Gross Income (AGI) and are generally available whether or not the taxpayer itemizes deductions.

Deduction	Purpose
Traditional IRA Contributions	Retirement savings
Health Savings Account (HSA) Contributions	Medical savings
Student Loan Interest	Higher education
Self-Employed Health Insurance	Self-employed taxpayers
Certain Business Expenses of Self-Employed Persons	Business deductions
Educator Expenses	Qualified classroom expenses
Alimony Paid *	Subject to applicable tax law
Moving Expenses *	Limited eligibility (primarily certain members of the Armed Forces under current law)

A Note on Alimony

Before the Tax Cuts and Jobs Act (TCJA):

• Alimony paid was generally deductible by the payer.
• The recipient generally included it in taxable income.

For most divorce or separation agreements executed after 31 December 2018, the TCJA generally eliminated the deduction for alimony payments and excluded those payments from the recipient’s taxable income. Older agreements may continue to follow the prior rules unless modified to adopt the TCJA treatment.

A Note on Moving Expenses

The TCJA generally suspended the moving expense deduction for most taxpayers through 2025.

Currently, this deduction is generally available only to eligible active-duty members of the U.S. Armed Forces who move pursuant to military orders.

B. Below-the-Line Deductions (BLD)

After calculating AGI, taxpayers claim either the Standard Deduction or Itemized Deductions — whichever provides the greater benefit.

Itemized Deduction	Remarks
Mortgage Interest	Subject to IRS rules
State and Local Taxes (SALT)	Subject to statutory limitation
Charitable Contributions	Qualified organizations
Medical Expenses	Subject to AGI threshold
Gambling Losses	Deductible only up to the amount of gambling winnings and generally only if the taxpayer itemizes deductions; adequate records are required

Key Tax Law Updates Affecting Deductions

Deduction	Before TCJA	Current High-Level Position
Alimony	Generally deductible	Generally not deductible for most post-2018 agreements
Moving Expenses	Generally deductible if requirements met	Generally suspended except for eligible military personnel
Gambling Losses	Deductible up to gambling winnings	Same general principle continues, subject to current IRS rules
Standard Deduction	Lower	Significantly increased by TCJA (amounts adjusted periodically for inflation)

Case Study: Understanding Deductions in Action

Background

Emma is a marketing executive working in New York. During the 2025 tax year, she earned a salary of $100,000. She wants to determine how deductions reduce her taxable income before calculating her federal income tax.

Assume the following figures (assumed for classroom learning):

Particulars	Amount ($)
Gross Salary	100,000
Traditional IRA Contribution	4,000
Student Loan Interest	1,000
HSA Contribution	2,000
Standard Deduction	15,000

Step 1: Calculate Above-the-Line Deductions

Particulars	Amount ($)
Traditional IRA Contribution	4,000
Student Loan Interest	1,000
HSA Contribution	2,000
Total Above-the-Line Deductions	7,000

Step 2: Calculate Adjusted Gross Income (AGI)

Particulars	Amount ($)
Gross Income	100,000
Less: Above-the-Line Deductions	(7,000)
Adjusted Gross Income (AGI)	93,000

Step 3: Claim Standard Deduction

Particulars	Amount ($)
Adjusted Gross Income	93,000
Less: Standard Deduction	(15,000)
Taxable Income	78,000

Chart 2 — Emma’s taxable income, step by step

Learning Outcome

Without deductions, Emma’s taxable income would have been $100,000. Because of the deductions:

• Above-the-Line Deductions = $7,000
• Standard Deduction = $15,000

Her taxable income is reduced to $78,000.

Tax savings arise because federal income tax is calculated on $78,000 instead of $100,000.

 Q&A

Professor: Did the deductions reduce Emma’s tax directly?

Students: No.

Professor: Then what did they reduce?

Students: They reduced her taxable income.

Professor: Excellent! The tax will now be calculated at $78,000 rather than $100,000. The actual tax saving depends on Emma’s applicable federal income tax bracket.

Memory Sheet

Concept	Effect
Above-the-Line Deduction	Reduces Gross Income to arrive at AGI
Below-the-Line Deduction	Reduces AGI to arrive at Taxable Income
Tax Deduction	Reduces Taxable Income
Tax Credit	Reduces Federal Income Tax Liability
Tax Withholding	Advance payment of tax
Estimated Tax Payment	Advance payment of tax

Takeaway

Think of deductions as reducing the amount of income that is taxed. They come before the tax is calculated. Tax credits come later and reduce the tax itself.