About Calculator Index
Calculator Index exists to answer one kind of question well: "What does the math say, and how do you know?"
Whether you're weighing a mortgage, planning a calorie deficit, checking a grade, or estimating concrete for a weekend project, the math behind the decision shouldn't be a mystery — and it shouldn't require trusting a number you can't check. We built Calculator Index to give you both: an instant answer, and the exact formula, source, and worked example behind it, on the same page.
Who builds this site
Calculator Index is an independent, ad-supported publisher. We are not a bank, lender, insurer, or medical provider, and we don't sell financial products or accept payment to feature or rank any result. Our only commercial relationship is Google AdSense, which places ads alongside the free tools — disclosed in full in our privacy policy. That structure is deliberate: it means a calculator's formula is never influenced by what would generate the most revenue.
Our sourcing standard
Every calculator on this site is built on a named, publicly checkable source rather than an internal guess. We hold ourselves to three rules:
- Name the source on the page. Each calculator states which standard, formula, or agency figure it's built on — for example, current-year IRS federal tax brackets, the World Health Organization's BMI categories, the Mifflin-St Jeor metabolic equation, or the U.S. Navy body fat method. You should never have to guess where a number came from.
- Show the arithmetic, not just the result. Every calculator page includes the underlying formula and a fully worked numeric example, so you can independently verify the output by hand or with your own calculator.
- Date everything that can change. Tax brackets, contribution limits, and similar figures are tied to a specific year and reviewed annually. Every page carries a "last reviewed" date so you can judge how current the figures are before relying on them.
How we review and update calculators
The site's calculators fall into two categories, reviewed on different schedules:
- Rate- and year-dependent calculators — income tax, take-home pay, retirement contribution limits, and similar tools — are checked each year against newly published IRS and agency figures and updated before the new tax year takes effect. The effective year is stated directly on each of these pages.
- Formula-based calculators — BMI, statistics, unit conversions, loan amortization — rely on fixed, published formulas that don't change with policy. These are reviewed periodically for clarity and correctness as we expand the site, even though the underlying math is stable.
When we correct an error or update a source figure, we revise the "last reviewed" date on that page so returning visitors can see the change reflects current information, not a stale calculation.
What makes Calculator Index different
- Every formula is shown. No black boxes. Each page displays the exact formula, a worked example with real numbers, and the source it comes from.
- Fast and private by design. All calculations run in your browser. Nothing you type is sent to a server, stored, or shared. Pages are lightweight static HTML that load in well under a second — you can verify this yourself by checking your browser's network activity while using any calculator.
- Free, forever. No sign-ups, no paywalls, no locked features. The site is supported entirely by advertising, which lets us keep every tool free to use.
- Dated and maintained. Tax- and rate-dependent calculators state which year's figures they use, and every page shows when it was last reviewed.
Accuracy and limitations
We work hard to keep every calculator accurate and current, and we cite the methodology on each page. That said, calculators produce estimates based on the numbers you enter and simplified models of complex, individualized topics. They are educational tools — not personalized financial, medical, tax, or legal advice. For decisions that matter, use our results as a starting point and confirm the details with a qualified professional who knows your full circumstances. See our full disclaimer for the complete terms.
Found an error? Tell us.
If a calculator ever gives a result you believe is wrong, we genuinely want to know — accuracy is the entire premise of this site. Contact us with the calculator name and the exact inputs you used, and we will investigate and, if warranted, correct it and update the page's review date.