TaxChecker documents IRS primary sources used for federal tax constants, due dates, and educational explanations. Source traceability is not IRS certification, approval, or endorsement.
IRS publications
| Source | Tax year | Date accessed | Used for |
|---|---|---|---|
| Publication 15 (Circular E), Employer's Tax Guide | 2025 | 2026-06-16 | Payroll and FICA calculations |
| Publication 334 — Tax Guide for Small Business | 2025 | 2026-06-16 | Self-employment tax guides and educational resources |
IRS forms
| Source | Tax year | Date accessed | Used for |
|---|---|---|---|
| About Schedule SE (Form 1040) | 2025 | 2026-06-16 | Self-employed and 1099 tax calculators |
| Form 1040-ES (2025) | 2025 | 2026-06-16 | Quarterly estimated tax due dates |
Revenue procedures
| Source | Tax year | Date accessed | Used for |
|---|---|---|---|
| Revenue Procedure 2024-25 | 2025 | 2026-06-16 | HSA contribution limits |
| Revenue Procedure 2024-40 | 2025 | 2026-06-16 | Federal income tax brackets and rates; Standard deduction amounts |
IRS notices
| Source | Tax year | Date accessed | Used for |
|---|---|---|---|
| Notice 2024-80 — 2025 Retirement Plan Limits | 2025 | 2026-06-16 | SEP IRA limits (planned calculators); Solo 401(k) limits (planned calculators) |
IRS topics
| Source | Tax year | Date accessed | Used for |
|---|---|---|---|
| Topic no. 554, Self-employment tax | 2025 | 2026-06-16 | Self-employment tax rates and rules |
IRS guides
| Source | Tax year | Date accessed | Used for |
|---|---|---|---|
| Limited Liability Company (LLC) — IRS | 2025 | 2026-06-16 | LLC vs S Corp comparison calculator and entity education |
| Questions and Answers for the Additional Medicare Tax | 2025 | 2026-06-16 | Additional Medicare tax thresholds |
| S Corporations — IRS Small Business Guide | 2025 | 2026-06-16 | S Corp tax calculator and entity comparison content |
Source update policy
TaxChecker reviews IRS primary sources when tax-year constants change, when calculators are added or updated, and when educational resources reference new federal rules. Internal review against IRS publications is not IRS certification or endorsement.
- Identify the IRS publication, form, notice, or revenue procedure that governs the constant or rule.
- Update the shared tax-year configuration used by the tax engine.
- Re-run calculator and resource content review for affected pages.
- Update last reviewed dates on affected calculator and resource registry entries.
- Document the change on the Methodology and Sources pages.
Methodology references
- Methodology — how estimates are produced and what is excluded
- Editorial standards — content, review, and correction policies
- About — mission, independence, and federal scope
Estimates only — not tax advice, legal advice, or financial advice. TaxChecker is not affiliated with the IRS. Consult a qualified tax professional for your situation.
Last updated 2026-06-16