An Explanation of How Tax Deductions Work

Quick answer

• Tax deductions reduce your taxable income, lowering your overall tax bill.
• They can be “above-the-line” (adjustments to income) or “below-the-line” (itemized deductions or the standard deduction).
• You choose between itemizing or taking the standard deduction, whichever saves you more.
• Common deductions include student loan interest, IRA contributions, medical expenses (if itemizing), and state/local taxes.
• Keeping good records is crucial for claiming deductions accurately.

What to check first (before you file or change withholding)

Filing Status

Your filing status (Single, Married Filing Jointly, Married Filing Separately, Head of Household, Qualifying Widow(er)) significantly impacts your tax brackets, standard deduction amount, and eligibility for certain credits. Ensure you’re using the most advantageous status for your situation.

Income Sources

Identify all sources of income, including wages, self-employment earnings, investment gains, retirement distributions, and any other taxable income. Accurate reporting of all income is the first step to determining your tax liability.

Withholding or Estimated Payments

Review your W-4 form with your employer or your estimated tax payments if you’re self-employed or have significant income not subject to withholding. Insufficient withholding can lead to penalties, while over-withholding means you’re giving the government an interest-free loan.

Deductions and Credits

Understand which deductions and credits you might qualify for. Deductions reduce your taxable income, while credits directly reduce your tax liability. Many people overlook valuable deductions and credits each year.

Deadlines and Extensions (General)

Be aware of the tax filing deadline (typically April 15th) and the process for requesting an extension if needed. An extension to file is not an extension to pay; any taxes owed are still due by the original deadline to avoid penalties and interest.

Step-by-step (simple workflow)

1. Gather All Income Documents: Collect W-2s, 1099s (for freelance work, interest, dividends, etc.), and any other statements showing income received throughout the year.

• What “good” looks like: You have a complete list of all income earned, making it easy to report accurately.
• Common mistake: Forgetting about side hustle income or small interest payments.
• Avoid it by: Creating a checklist of potential income sources and reviewing bank statements for deposits.

2. Identify Potential “Above-the-Line” Deductions: Look for adjustments to income, such as contributions to traditional IRAs, student loan interest paid, or self-employment tax deductions.

• What “good” looks like: You’ve identified all eligible adjustments that reduce your Adjusted Gross Income (AGI).
• Common mistake: Not knowing about or claiming deductions like student loan interest.
• Avoid it by: Reviewing IRS Publication 17, “Your Federal Income Tax,” or consulting a tax professional.

3. Calculate Adjusted Gross Income (AGI): Subtract your eligible “above-the-line” deductions from your gross income.

• What “good” looks like: Your AGI is accurately calculated, serving as the basis for many other tax calculations.
• Common mistake: Incorrectly calculating AGI due to missed deductions.
• Avoid it by: Double-checking your math and ensuring all eligible adjustments were included.

4. Determine Your Standard vs. Itemized Deduction: Compare the standard deduction amount for your filing status with the total of your potential itemized deductions.

• What “good” looks like: You know which deduction (standard or itemized) will lower your taxable income more.
• Common mistake: Itemizing when the standard deduction would result in a larger tax saving.
• Avoid it by: Always performing this comparison before filing.

5. Gather Documentation for Itemized Deductions (If Applicable): If itemizing, collect receipts and statements for medical expenses (exceeding the AGI threshold), state and local taxes (SALT), mortgage interest, and charitable contributions.

• What “good” looks like: You have organized records supporting every dollar you intend to itemize.
• Common mistake: Claiming deductions without proper substantiation.
• Avoid it by: Keeping a dedicated folder or digital system for tax-related documents throughout the year.

6. Calculate Total Itemized Deductions: Sum up all eligible itemized expenses.

• What “good” looks like: Your total itemized deductions are accurately calculated and ready to be compared to the standard deduction.
• Common mistake: Including ineligible expenses or miscalculating totals.
• Avoid it by: Carefully reviewing IRS guidelines for each deduction category.

7. Choose the Larger Deduction: Select either the standard deduction or your total itemized deductions, whichever is greater.

• What “good” looks like: You’ve made the choice that maximizes your tax benefit.
• Common mistake: Failing to compare and defaulting to the standard deduction when itemizing would be better.
• Avoid it by: Performing the direct comparison as outlined in Step 4.

8. Calculate Taxable Income: Subtract your chosen deduction (standard or itemized) from your AGI.

• What “good” looks like: You have a clear figure for your taxable income.
• Common mistake: Using AGI instead of taxable income to calculate tax liability.
• Avoid it by: Ensuring this subtraction is the final step before applying tax brackets.

9. Determine Tax Liability: Use the appropriate tax brackets for your filing status and taxable income to calculate your initial tax bill.

• What “good” looks like: Your tax liability is calculated based on the current year’s tax rates.
• Common mistake: Using outdated tax brackets or misapplying them.
• Avoid it by: Consulting the official IRS tax tables for the relevant tax year.

10. Subtract Credits and Payments: Deduct any tax credits you qualify for and the amount of taxes you’ve already paid through withholding or estimated payments.

• What “good” looks like: Your final tax due or refund is accurately calculated.
• Common mistake: Forgetting to subtract credits or overestimating payments made.
• Avoid it by: Carefully listing all credits and payments made.

Common mistakes (and what happens if you ignore them)

Mistake	What it causes	Fix
Forgetting to claim eligible deductions	Higher tax bill, missed savings	Amend your tax return (Form 1040-X) for the relevant year.
Incorrectly calculating AGI	Wrong taxable income, potentially incorrect tax bill	Re-calculate AGI, ensuring all eligible “above-the-line” deductions are included. Amend return if necessary.
Itemizing when standard is better	Higher tax bill	Re-file using the standard deduction. For past years, amend to switch if applicable and beneficial.
Not having receipts for itemized deductions	IRS disallowance of deductions, higher tax bill	If possible, reconstruct records or obtain duplicates. If not, you may have to forgo the deduction for that year.
Missing “above-the-line” deductions	Higher AGI, higher tax bill	Amend your tax return to claim the missed deductions.
Not understanding SALT limitations	Overstated deductions, higher tax bill	Adjust your SALT deduction to the legal limit. Amend return if you previously overstated.
Claiming personal expenses as business deductions	IRS penalties, interest, and potential audit	Reclassify personal expenses. Amend returns to correct any misclassifications.
Not keeping records for multiple years	Difficulty amending returns, potential audit issues	Establish a system for organizing and storing tax documents for at least three years after filing.
Incorrectly categorizing expenses	Disallowed deductions, potential penalties	Review IRS guidelines for expense categorization. Amend returns if errors are found.
Failing to claim student loan interest	Higher taxable income, higher tax bill	Amend your tax return to claim the student loan interest deduction.

Decision rules (simple if/then)

• If your total eligible itemized deductions are greater than the standard deduction for your filing status, then itemize your deductions because this will reduce your taxable income more.
• If you paid significant medical expenses that exceed 7.5% of your Adjusted Gross Income (AGI), then consider itemizing your deductions because these expenses can be deductible.
• If you made contributions to a traditional IRA, then you may be able to deduct those contributions, reducing your AGI.
• If you paid interest on a student loan, then you can likely deduct a portion of that interest, reducing your AGI.
• If you are self-employed, then you can deduct one-half of your self-employment taxes, reducing your AGI.
• If you are a homeowner who pays mortgage interest, then this interest is often deductible if you itemize, reducing your taxable income.
• If you made charitable contributions, then these are deductible if you itemize, reducing your taxable income.
• If your state and local taxes (property, income, or sales tax) exceed the statutory limit, then you can only deduct up to that limit, and it counts towards your itemized deductions.
• If you are unsure whether to itemize or take the standard deduction, then calculate both and choose the option that results in lower taxable income.
• If you received a significant tax bill due to not having enough withheld, then adjust your W-4 with your employer or increase your estimated tax payments for the next year because this prevents underpayment penalties.
• If you are a small business owner, then keep meticulous records of all business expenses because these are often deductible and reduce your taxable business income.
• If you are considering certain home improvements or major life events, then research potential deductions or credits associated with them before making decisions because this can impact your tax strategy.

FAQ

Q1: What is the difference between a deduction and a credit?

A deduction reduces your taxable income, thereby lowering the amount of income subject to tax. A credit directly reduces your tax liability dollar-for-dollar.

Q2: How do I know if I should itemize deductions or take the standard deduction?

You should compare the total of your eligible itemized deductions to the standard deduction amount for your filing status. Whichever amount is larger is the one you should use.

Q3: What are “above-the-line” deductions?

These are adjustments to income that are subtracted from your gross income to arrive at your Adjusted Gross Income (AGI). Examples include IRA contributions, student loan interest, and half of self-employment taxes.

Q4: Can I deduct my everyday work commute?

Generally, no. Personal commuting expenses are not deductible. However, unreimbursed employee expenses for business travel away from your tax home may be deductible if you itemize and meet specific criteria.

Q5: What is the limit on state and local tax (SALT) deductions?

There is a limit on the amount of state and local taxes you can deduct. Check current IRS guidelines for the specific threshold.

Q6: How long do I need to keep tax records for deductions?

The IRS generally recommends keeping tax records for at least three years from the date you filed your return or the due date of the return, whichever is later. Some records, like those for property, should be kept longer.

Q7: Can I deduct expenses for a home office?

You may be able to deduct home office expenses if you use a portion of your home exclusively and regularly for business. There are specific rules and calculations for this deduction.

Q8: What if I discover I missed a deduction on a past tax return?

You can file an amended tax return using Form 1040-X to claim missed deductions and potentially get a refund. There are time limits for filing amended returns.

What this page does NOT cover (and where to go next)

• Specific tax forms and detailed instructions for filling them out.
• Next: Refer to IRS publications and tax software guides.
• State and local tax laws, which vary significantly.
• Next: Consult your state’s department of revenue or a local tax professional.
• Tax implications of specific investment vehicles (e.g., cryptocurrency, complex options trading).
• Next: Seek advice from a financial advisor or tax specialist experienced in investments.
• Advanced tax planning strategies for high-net-worth individuals or businesses.
• Next: Consult with a Certified Public Accountant (CPA) or a tax attorney.
• The process of claiming tax credits (e.g., child tax credit, education credits), which is a separate but related topic.
• Next: Research tax credits available to individuals and families.