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Basic Overview of Income Tax Calculation

One of the biggest complaints we hear from young adults is that they do not understand income taxes. Most young people are not taught this topic in school and thus are left on their own when they enter adulthood. This post gives a quick overview of the US income tax calculation.

Basic Formula

Income from whatever source derived

- Exclusions = Gross Income

- Deductions FOR adjusted gross income = Adjusted Gross Income (AGI)

- Deductions FROM adjusted gross income (greater of itemized or standard)

- Qualifying Business Income (QBI) deduction = Taxable Income


  • Income: this technically includes all sources of income, whether taxable, legal, or not.

  • Exclusions: any item that tax law has deemed not to be taxable; examples:

Some employee fringe benefits

Gifts and inheritances

Life insurance proceeds

Certain scholarships

Personal physical injury settlements

Health plan premiums

  • Gross Income: everything of value received by a taxpayer during the taxable year unless it is established that it is excluded; examples:

Compensation including salary, commissions, etc.





Gains derived from selling property

Income from an interest in an estate or trust

  • Deductions FOR Adjusted Gross Income: generally, expenses conducted with a trade or business, as well as alimony paid for agreements made prior to 2019; examples:

Trade and business deductions

Reimbursed employee expenses

Losses from the sale of property

Deductions attributable to rents and royalties

Contributions to retirement plans

One-half of self-employment taxes paid

Contributions to health savings accounts

  • Adjusted Gross Income (AGI): measure of income that falls between gross and taxable income; this is the measure of income that are used to impose limitations, i.e. itemized deductions, Roth IRA contributions, ESA contributions, taxation of Series EE bonds, ability to apply certain credits, etc.

  • Deductions FROM Adjusted Gross Income: any allowable itemized deductions or the standard deduction

  • Itemized Deductions: Congress allows taxpayers to deduct the following personal expenses. If the sum of the itemized deductions is greater than the applicable standard deduction, the taxpayer will use the itemized deduction.

Medical expenses (must exceed threshold of 10% of AGI)

Casualty losses if the taxpayer suffered losses in a Federally declared disaster area (in excess of 10% of AGI)

Taxes (limited to $10,000)

State, local, and foreign income and real property taxes

State and local personal property taxes

State and local sales taxes if an election is made to deduct these taxes instead of deducting state and local income taxes

Residential interest and investment interest (limited)

Charitable contributions (limited)

  • Standard Deduction: the deduction amounts are below. Most Americans use the standard deduction. Note that the standard deductions are higher for blind, elderly or blind and elderly taxpayers.

Heads of households – $18,350

Unmarried individuals other than heads of house-holds – $12,200

Married people filing separate returns – $12,200

Married couples filing joint returns and surviving spouses – $24,400

  • Qualifying Business Income (QBI) Deduction: QBI includes certain types of income from S-Corporations, LLCs, partnerships sole proprietorships, real estate properties being reported on Schedule E, and trusts and estates that own an interest in real estate (these are collectively called pass-through businesses). The law allows up to a 20% deduction from AGI of qualified business income (QBI). Taxpayers may claim this deduction whether they claim itemized deductions or the standard deduction. Note that there are limitations based on the kind of business, and some have income limitations. For more information, please visit the IRS website.

  • Taxable Income: the remaining amount left after the itemized or standard deduction and the QBI deduction (if applicable) are subtracted from AGI. A taxpayer’s taxable income is used in conjunction with the tax brackets to arrive at gross taxes. Tax credits and prepayments are then subtracted from the gross tax amount to arrive at net taxes due or refund due. The net tax amount is compared to the amount of taxes that have been withheld during the year (which is based on the employee’s W-4). If too much was withheld, the taxpayer owes money to Uncle Sam. If not enough was withheld, the taxpayer is entitled to a refund.

At its core, the income tax code relies on the basic formula outlined above. The details and intricacies of how each component of the formula is defined is what makes calculating income taxes so complicated. Taxpayers do not need to have a deep understanding of the tax code, however, each taxpayer would benefit from a high level understanding of the basic formula so that they at the very least know the reasoning behind some of their hard-earned money going to the government. A little knowledge also goes a long way to minimizing both surprises and the tax burden itself.

Two final things:

1) Lauren is not a CPA, so please get information from one in addition to your own personal research.

2) This information was gathered primarily from Boston University’s FP104E Tax Planning Course (2020).

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