Free online calculators

What Calculator Index is, and how it works

Calculator Index is a free reference library of 105 calculators covering personal finance, health and fitness, everyday math, and household planning. Every calculator runs entirely in your browser — nothing you type is uploaded, logged, or stored on a server — and every result is shown alongside the formula, the source standard, and a worked example, so you can check the math yourself instead of taking a number on faith.

We built the site because most calculator tools online show you an answer and nothing else. That's fine for a quick tip split, but it falls apart the moment the stakes are higher — a mortgage payment, a tax estimate, a calorie target. Our approach is the opposite: the calculator is the fast path, and the explanation underneath it is there for anyone who wants to understand why the number came out the way it did, or verify it against the original source.

Where the numbers come from

Every formula on this site is tied to a named, checkable standard rather than an internal assumption:

• Tax and finance calculators use current-year IRS federal tax brackets, standard deduction amounts, and conventional amortization mathematics — the same formulas lenders and accountants use.
• Health calculators use published clinical formulas: the Mifflin-St Jeor equation for metabolic rate, World Health Organization BMI categories, the U.S. Navy circumference method for body fat, and the Karvonen method for heart-rate training zones.
• Everyday and math calculators use standard, textbook arithmetic — percentage, ratio, and statistical formulas that don't change with policy or season.

Where a figure is tied to a specific tax year or published rate table, the calculator page states the year and source directly, and carries a "last reviewed" date so you can judge how current it is.

How pages are reviewed and kept current

Tax brackets, contribution limits, and similar figures change annually. When they do, we update the affected calculator, note the new effective year on the page, and revise the "last reviewed" date at the bottom. Calculators built on fixed formulas (BMI, statistics, unit conversions) don't need annual updates, but we periodically re-check them for accuracy and clarity as we add new content to the site.

What this site is not

Every calculator here produces an estimate based on simplified models and the numbers you provide — not personalized financial, medical, tax, or legal advice. For a decision that matters, use our results as a starting point and confirm the details with a licensed professional who can account for your full situation. Our full reasoning on this is in the disclaimer, and the complete story of why we built the site is on the About page.

Beyond the calculators themselves, the guides section walks through the reasoning behind common financial and health decisions — how much house you can afford, why compound interest rewards starting early, the real difference between BMR and TDEE — in plain language, with links back to the relevant calculator so you can run your own numbers.