How to calculate tax deductions?

Tax deduction calculation — general framework

A tax deduction reduces taxable income, not tax owed directly. The actual tax saved equals the deduction amount multiplied by your marginal tax rate.

Step-by-step process

1. Establish gross income

   • Sum all income sources (wages, business revenue, investments, etc.).

2. Calculate Adjusted Gross Income (AGI)

   • AGI = Gross income − above-the-line adjustments (e.g., retirement contributions, student loan interest, self-employment tax adjustments).

3. Choose standard vs. itemized deductions

   • Standard deduction: fixed amount set annually by the tax authority (e.g., US 2024: $14,600 single, $29,200 married filing jointly).
   • Itemized deductions: sum of qualifying expenses (mortgage interest, state/local taxes, charitable gifts, medical costs above a threshold).
   • Rule: use whichever is larger.

4. Identify eligible deductible expenses

   • Business expenses: ordinary and necessary costs (supplies, equipment, travel, insurance).
   • Home office: space used exclusively and regularly for business (US simplified method: $5/sq ft, max 300 sq ft).
   • Depreciation: spread the cost of long-lived assets over useful life.
   • Retirement/health accounts: pre-tax contributions (401(k), IRA, HSA, or local equivalents).

5. Apply limitations and thresholds

   • Percentage floors: e.g., US medical expenses deductible only above 7.5% of AGI.
   • Caps: e.g., US state and local tax (SALT) deduction capped at $10,000; charitable gifts generally limited to 60% of AGI.
   • Business use percentages: mixed-use assets deductible only by business-use ratio.

6. Compute taxable income

   Taxable income = AGI − deductions

7. Calculate the tax benefit

   Tax saved = Deduction × marginal tax rate

Worked example

• Gross income: $75,000
• Above-the-line adjustments: $2,000 → AGI = $73,000
• Itemized: mortgage interest $9,000 + SALT $8,000 (under cap) + charitable $3,000 = $20,000
• Standard deduction: $14,600 → itemized is larger
• Taxable income: $73,000 − $20,000 = $53,000
• Benefit vs. standard: ($20,000 − $14,600) × 22% = $1,188 tax saved

Key distinction

• Deduction: reduces taxable income (worth your marginal rate per dollar).
• Credit: reduces tax owed dollar-for-dollar (typically more valuable).

Documentation requirements

• Retain receipts, invoices, mileage logs, and bank statements.
• Standard retention period: 3–7 years depending on jurisdiction.

Notes

• Thresholds, caps, and eligible categories vary by country and tax year; verify against the official tax authority’s current guidance.
• Complex situations (multi-jurisdiction income, depreciation schedules, business structures) typically warrant a licensed tax professional or certified software.