About Calculator Index

Calculator Index exists to answer one kind of question well: "What does the math say?"

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. Calculator Index offers 90+ free calculators that give you the answer instantly — and then show you exactly how it was computed.

What makes Calculator Index different

• Every formula is shown. No black boxes. Each calculator page displays the exact formula it uses, a worked example with real numbers, and the source or standard it comes from (IRS tax brackets, WHO BMI categories, the Mifflin-St Jeor equation, standard amortization math, and so on).
• 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.
• Free, forever. No sign-ups, no paywalls, no locked features. The site is supported by advertising, which lets us keep every tool free.
• Dated and maintained. Tax- and rate-dependent calculators state which year's figures they use, and each 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 topics. They are educational tools — not 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. See our full disclaimer.

Found an error? Tell us.

If a calculator ever gives a result you believe is wrong, we genuinely want to know. Contact us with the calculator name and the inputs you used, and we'll investigate and fix it.