About CalculatorLab

CalculatorLab is a free collection of calculators for everyday questions: money, work and pay, dates and time, units, cooking and math. Every page gives you the answer, the formula behind it, worked examples, the assumptions it makes and what it does not do.

How we build and check a calculator

  1. Sources first. Each formula and every rate or definition is taken from a primary source where one exists: national standards bodies such as NIST, government agencies such as the U.S. Department of Labor, the IRS or state tax departments, and federal regulations. Sources are listed at the bottom of every calculator.
  2. Exact arithmetic. Unit conversions use exact definitions (for example 1 inch = 2.54 cm). Money and fractions are calculated without the rounding errors of ordinary computer arithmetic where it matters, and rounding rules are stated on the page.
  3. Independent tests. Every calculator has an automated test suite with hand-checked examples, a second, separately written method for the same answer, and randomized checks. Every worked example you see on a page is re-checked by that suite before each release.
  4. Browser checks. Before release each page is tested in a real browser on phone and desktop sizes, including keyboard use and automated accessibility checks (WCAG 2.1 AA).
  5. Compared with other calculators. We compare each tool with several popular alternatives and only publish when it is at least as accurate and more useful. A tool must score at least 90 out of 100 on our release checklist before it goes live.

What our calculators are not

The results are estimates for information. They are not financial, tax, legal, payroll or medical advice, and they cannot know the rules that apply to your exact situation. Pages that touch regulated topics, such as overtime, sales tax or mortgages, say which rules they use and where to confirm them.

Corrections

If a result looks wrong, please tell us with the page and the numbers you entered. We check every report against the sources, fix confirmed errors, and update the review date on the page.

Privacy

Calculations run in your browser. We do not ask for an account and we do not sell data. See the privacy policy for the analytics we use and your choices.