How we calculate & review
NumberBench tools are built to give clear, honest estimates and to show their work. This page explains where our numbers come from, how often we update them, and what they can and can't tell you.
Our approach
Every calculator runs entirely in your browser — your inputs are never sent to us or stored. Each tool shows the formula it uses in its “How it's calculated” section, along with a worked example, common mistakes, and the assumptions behind the result. Our goal is that you can see exactly how a number was produced and decide whether it fits your situation.
Data sources
| State income tax | 2026 single-filer and married-filing-jointly brackets and standard deductions from the Tax Foundation, “2026 State Individual Income Tax Rates & Brackets.” Excludes local income taxes, credits, and itemized deductions. |
| Property & sales tax | Statewide average effective property tax rates and statewide base sales tax rates (2025). Local rates vary by county/city and are entered by you. |
| Health tools | BMI uses the standard weight/height² formula; body fat uses the U.S. Navy circumference method; calorie/TDEE uses the Mifflin–St Jeor equation with standard activity multipliers. |
| Finance tools | Loan, mortgage, and amortization figures use standard time-value-of-money formulas. FICA uses the published Social Security (6.2%) and Medicare (1.45%) rates. |
| Manufacturing tools | Cost, DFA, OEE, takt/cycle-time, and scrap calculators use widely accepted industrial-engineering formulas, documented on each page. |
How often we update
Tax brackets and rates are reviewed at least once a year, after each tax year's figures are published, and the “updated” date on each page reflects its last review. Formula-based tools (loans, geometry, health) change only if the underlying standard changes.
Limitations
Our calculators are estimates for general information and education — not financial, legal, tax, medical, or professional advice. Tax results assume a single income and standard deduction, and exclude local taxes, credits, and special situations. For decisions that matter, confirm the figures with a qualified professional or the relevant authority (for example, your state Department of Revenue).
Expert review
We are committed to having credentialed professionals review the tools that carry the most responsibility. Finance and tax calculators are slated for review by a licensed CPA, and manufacturing calculators by a professional engineer.
Corrections
Found a number that looks off? We want to know. Contact us with the calculator and the inputs you used, and we'll review it promptly.