Who builds and maintains CalcOpenly
CalcOpenly is built and maintained by its project maintainers, who implement the calculators and run the automated checks described below. Each calculator exposes its method, worked examples and source references. Reach the maintainers through the contact and corrections page. We do not claim independent professional review or attach a clinician's or financial adviser's name to a calculator without an actual review.
Precision and rounding
Core arithmetic uses decimals with 50 significant digits (decimal.js), so sums such as 0.1 + 0.2 come out as exactly 0.3. Graphs and some numerical methods use floating-point approximations. Arithmetic on fractions keeps exact numerators and denominators as long as the answer is rational: 1/3 × 3 is 1, and 0.(3) is read as one third. Irrational steps such as square roots, logarithms and trigonometry are evaluated with extra guard digits and rounded back to 50.
Displayed values follow the stated rounding rule: money to its currency's minor unit, half-to-even; scientific quantities to a chosen number of significant figures; counts of things you buy rounded up. Some models also require intermediate rounding. “Copy exact” copies the stored result before display formatting.
Inputs you can type naturally
Number fields accept digit grouping in any style (2,50,000 or 250,000), suffixes such as 20k, 1.5 lakh or 2 cr, fractions and mixed numbers, scientific notation, currency symbols and small expressions like 10% of 50. Measurement fields take units, including 5'11", 180 cm, 12 st 3 and 37°C, and convert them with the exact legal definitions of each unit.
Checked against independent answers
Each calculator ships at least three worked examples — 1,901 across the site — whose expected answers come from a source other than the calculator itself: a separate Python implementation using exact decimal or fractional arithmetic, a published table, a standard's test vector or a textbook worked example. They are covered by the automated test suite, and you can run a calculator's own checks from its page.
Sources and limits
Every calculator lists the standards, papers or official pages its method comes from. Health, finance and tax calculators describe populations and published rules, not your circumstances; each carries a note saying so.
How sources are selected
We prefer the document that defines a method: an official standard for a unit, a government publication for a tax rule, or the original research paper for a model. Each calculator links to its references and describes the formula it implements. General reference pages provide context; a dated regulation or study should identify which version or population the result applies to.
Sources can disagree because their assumptions differ. A different compounding period, quartile convention, unit system or clinical population can produce a different valid answer. The calculator's inputs and method should make those choices visible. A precise arithmetic result is not a guarantee that the model describes a person, market or building accurately.
Editorial updates and corrections
Before changing a method, compare it with the cited source, check units and boundary cases, and run the independent worked examples. For a reported bug, the preferred correction includes an example that reproduces the error so future changes can be checked against it. Report a correction with fictional inputs, the expected result and a source; do not share private financial or health data in a public issue.
Content dates track recorded changes to the calculator and its explanation, rather than the time a deployment runs. The changelog highlights releases and substantive changes; version control retains the detailed implementation history. A recent update date does not mean that a professional reviewed a tool or that every underlying source was republished that day.
Privacy
Calculations run in your browser. History, saved calculators and settings stay in this browser's storage and can be exported or cleared from the History page. The currency converter downloads daily reference rates; nothing you type is sent with that request.
Google Analytics is optional and stays off until you allow it. Our page-view events omit query strings and calculator inputs. Shared calculator URLs can still contain inputs and are sent to the host when opened. Read the privacy policy to manage consent and learn about storage and external services.