Last updated: 10/11/2026
1. Formula and sources
We start from the standard mathematical formula or, when the calculation depends on a rule, from the official text. The formula is always shown under «How it is calculated» and the rule is linked under «Official sources». These are the US sources we cite:
- 15 U.S.C. § 260a: daylight saving time
- 2 U.S.C. § 7: time of election
- IRS: Frequently asked questions on gift taxes
- IRS Revenue Procedure 2025-32: 2026 inflation adjustments (tax brackets, standard deduction, credits)
- 26 U.S.C. § 24: child tax credit
- 26 U.S.C. § 151: deduction for seniors (subsection (d)(5)(C))
- 26 U.S.C. § 225: qualified overtime compensation deduction
- Iowa Legislative Services Agency: State General Sales Tax Rates as of January 1, 2026 (data from the Tax Foundation)
- 26 U.S.C. § 224: qualified tips deduction
- IRS news release IR-2025-117: guidance on Trump Accounts
- IRS Notice 2025-68: Trump Accounts
- IRS Publication 15-T (2026): Federal Income Tax Withholding Methods
- IRS Publication 15 (2026): Employer's Tax Guide
- IRS Topic No. 560: Additional Medicare Tax
- IRS Topic No. 554: Self-Employment Tax
- IRS Topic No. 751: Social Security and Medicare Withholding Rates
- IRS: About Form W-4, Employee’s Withholding Certificate
- IRS: One Big Beautiful Bill Act tax deductions for working Americans and seniors
- IRS: 401(k) limit increases to $24,500 for 2026, IRA limit increases to $7,500
- IRS: Retirement topics, 401(k) and profit-sharing plan contribution limits
- IRS: Working Families Tax Cuts, provisions for individuals and workers
- IRS Notice 2025-67: 2026 limitations for retirement plans
- SSA: 2027 Cost-of-Living Adjustment (COLA) Fact Sheet
- SSA: Social Security Announces 3.6 Percent Benefit Increase for 2027
- SSA Office of the Chief Actuary: Primary Insurance Amount (PIA formula)
- SSA Office of the Chief Actuary: Benefit Formula Bend Points
- SSA Office of the Chief Actuary: Social Security Benefit Amounts (AIME and PIA)
- SSA Office of the Chief Actuary: Early or Late Retirement? (reduction for early retirement)
- SSA: Delayed Retirement Credits
- SSA Office of the Chief Actuary: Cost-of-Living Adjustments (COLA history)
- SSA: Social Security Credits
- SSA: my Social Security account
- FTC Guides Against Deceptive Pricing, 16 CFR Part 233
- Fair Labor Standards Act, 29 U.S.C. § 206: minimum wage
- Fair Labor Standards Act, 29 U.S.C. § 207: maximum hours and overtime
- IRS: Standard mileage rates (2026: Jan. 1 – June 30 and July 1 – Dec. 31)
- IRS Revenue Procedure 2025-19: 2026 HSA contribution limits and HDHP amounts
- IRS Revenue Procedure 2026-24: 2027 HSA contribution limits and HDHP amounts
- IRS Publication 463: Travel, Gift, and Car Expenses (business mileage, commuting, parking and tolls)
- IRS Publication 969: Health Savings Accounts and Other Tax-Favored Health Plans
- 5 U.S.C. § 6103: federal holidays
- U.S. Department of Labor: Vacation Leave (the FLSA does not require paid vacations or holidays)
- U.S. Office of Personnel Management: federal holidays
- National Heart, Lung, and Blood Institute (NIH): BMI calculator and categories
- ACOG Committee Opinion No. 700: Methods for Estimating the Due Date
- Consumer Financial Protection Bureau: What is private mortgage insurance?
- Consumer Financial Protection Bureau: When can I remove private mortgage insurance (PMI)?
- Consumer Financial Protection Bureau: What is a debt-to-income ratio?
- Consumer Financial Protection Bureau: Loan Estimate explainer
- Investor.gov (U.S. Securities and Exchange Commission): compound interest calculator
- NIST Special Publication 811: Guide for the Use of the International System of Units
- NIST Office of Weights and Measures: unit conversion
- FuelEconomy.gov (U.S. Department of Energy and EPA)
- Mifflin MD et al. (1990), American Journal of Clinical Nutrition 51(2):241-247
- Tanaka H et al. (2001), Journal of the American College of Cardiology 37(1):153-156
Federal statutes are linked to the United States Code as published by the Legal Information Institute of Cornell Law School; IRS figures to the IRS publication or revenue procedure that sets them.
2. Test cases
We check every calculator against several cases worked out by hand, including edge cases: zeros, negatives, very large values, legal caps and dates such as February 29. The example on each page is one of those cases and matches what the tool calculates.
3. Precision and rounding
Calculations use the browser's full precision and are only rounded for display. There can be one-cent differences with tools that round at intermediate steps. When a rule sets its own rounding, we apply it.
4. Reviews and updates
Each page shows the date of its last review. Calculators that depend on rules are reviewed when the rule changes and at least once a year, when the IRS publishes the new inflation adjustments. Corrections are logged on the corrections page.
5. Limits
A calculator simplifies reality and does not know your whole situation (your state, your employer's plan, your full tax return…). Results are estimates: for important decisions, check them with the official agency or a qualified professional.
6. Privacy
Everything is calculated in your browser. What you type is not sent to any server.