Tax Calculation Flow

Follow the money from your total income down to your final refund or amount owed. Every step explained in plain English.

Step 1

Total Income

Form 1040, Line 9

This is everything you earned during the year from all sources—before any deductions or adjustments. It's the starting point for calculating your tax.

What's Included
  • Wages & Salaries (W-2)
  • Interest Income (1099-INT)
  • Dividend Income (1099-DIV)
  • Business Income (Schedule C)
  • Capital Gains (Schedule D)
  • IRA/Pension Distributions
  • Social Security Benefits
  • Rental Income (Schedule E)
  • Unemployment Compensation
  • Other Income (Schedule 1)
minus Adjustments
Step 2

Adjusted Gross Income (AGI)

Form 1040, Line 11

Your AGI is your total income minus specific "above-the-line" deductions. This is a crucial number because many tax benefits are based on your AGI.

Adjustments That Reduce Income
  • Student Loan Interest (up to $2,500)
  • HSA Contributions
  • Traditional IRA Contributions
  • Self-Employment Tax (half)
  • Self-Employed Health Insurance
  • Educator Expenses ($300)
  • Alimony Paid (pre-2019)
  • Moving Expenses (military)
minus Deductions
Step 3

Taxable Income

Form 1040, Line 15

This is the amount that's actually subject to federal income tax. You get here by subtracting either the standard deduction or itemized deductions from your AGI.

Deductions You Choose
  • Standard Deduction ($15,000 single / $30,000 MFJ)
  • OR Itemized Deductions (Schedule A)
  • QBI Deduction (20% of qualified business income)
apply Tax Rates
Step 4

Total Tax

Form 1040, Line 24

This is your total tax liability for the year—what you owe the government based on your taxable income, minus any non-refundable credits, plus any additional taxes.

How It's Calculated
  • Tax from Tax Tables (based on taxable income)
  • Child Tax Credit (up to $2,000/child)
  • Other Non-Refundable Credits
  • Self-Employment Tax (Schedule SE)
  • Additional Medicare Tax
  • Net Investment Income Tax
  • Early Retirement Withdrawal Penalties
minus Payments
Step 5

Total Payments & Refundable Credits

Form 1040, Line 33

These are amounts already paid toward your tax bill or credits that can generate a refund even if you owe no tax. This total is compared to your Total Tax.

What Counts as Payment
  • Federal Withholding (from W-2s, 1099s)
  • Estimated Tax Payments
  • Earned Income Credit (EIC)
  • Additional Child Tax Credit
  • American Opportunity Credit (refundable part)
  • Premium Tax Credit
  • Amount from Extension Payment
equals Result
Step 6

Refund or Amount You Owe

Form 1040, Lines 35a or 37

The final answer! Compare your Total Payments to your Total Tax. If you paid more than you owe, you get a refund. If you paid less, you have a balance due.

The Final Calculation
  • If Payments > Tax = REFUND (Line 35a)
  • If Tax > Payments = AMOUNT OWED (Line 37)

The Complete Formula

Total Income Adjustments = AGI Deductions = Taxable Income
Total Tax Payments = Refund / Owed

Example: See It In Action

A single taxpayer with W-2 wages, taking the standard deduction

📥 Income & Deductions

W-2 Wages $75,000
Interest Income $500
Total Income (Line 9) $75,500
Student Loan Interest −$1,200
AGI (Line 11) $74,300
Standard Deduction −$15,000
Taxable Income (Line 15) $59,700

📤 Tax Calculation

Tax (from tax tables) $8,601
Non-Refundable Credits −$0
Other Taxes $0
Total Tax (Line 24) $8,601
Federal Withholding $9,500
Refundable Credits $0
REFUND (Line 35a) $899

Key Concepts to Understand

📊 Marginal Tax Rates

The U.S. uses a progressive tax system. You don't pay 22% on all your income—only on the portion in that bracket. Your "effective" rate is lower than your "marginal" rate.

💰 Refundable vs Non-Refundable

Non-refundable credits can only reduce your tax to $0. Refundable credits (like EIC) can generate a refund even if you owe no tax—they're essentially cash back.

📋 Standard vs Itemized

Most taxpayers take the standard deduction because it's simpler and often larger. Only itemize if your mortgage interest, state taxes, and charity exceed the standard amount.

🎯 Why AGI Matters

Many credits and deductions phase out based on your AGI. Lowering your AGI with adjustments (HSA, IRA) can unlock additional tax benefits.

💼 Self-Employment Tax

If you're self-employed, you pay both the employee AND employer share of Social Security/Medicare (15.3%). This is separate from income tax.

📅 Withholding vs Tax

Your refund or amount owed has nothing to do with how much tax you "really" paid. It's just the difference between what was withheld and what you actually owe.

Ready to Dive Deeper?

Now that you understand the flow, explore our detailed guides for each section of Form 1040.