Primary IRS forms and publications
Tax questions rarely fit one formula. These official sources define the scope of the site's supported tools and provide the fuller instructions needed for filing, withholding, estimated payments, and eligibility.
Links reviewed . Check the tax year and revision shown on IRS.gov before relying on a document.
2025 Form 1040 instructions (opens in a new tab)
Return-level rules, filing status, income, deductions, credits, other taxes, and payments.
2025 Schedule 1-A (opens in a new tab)
Qualified tips, qualified overtime, qualifying car-loan interest, and the enhanced senior deduction.
2025 Schedule A instructions (opens in a new tab)
Itemized deductions, including the state-and-local-tax limitation.
2025 Schedule SE instructions (opens in a new tab)
Self-employment tax, wage-base coordination, Additional Medicare Tax reference, and the deductible portion.
Publication 334, Tax Guide for Small Business (opens in a new tab)
Broader federal tax guidance for sole proprietors and self-employed people. It does not replace return-specific forms.
Publication 505, Tax Withholding and Estimated Tax (opens in a new tab)
Estimated-tax and withholding rules that require information beyond a narrow single-form calculator.
Publication 15-T, Federal Income Tax Withholding Methods (opens in a new tab)
Employer withholding methods and worksheets. Paycheck withholding is not ordinary-income tax divided by pay periods.
IRS Tax Withholding Estimator (opens in a new tab)
The official household-level tool for W-4 and withholding questions.
IRS 2026 inflation adjustments (opens in a new tab)
Official 2026 brackets, standard deductions, and other inflation-adjusted planning figures.
See methodology for calculator-by-calculator scope and the source update map for the 2025/2026 boundary.