How We Build and Check Our Calculators

A calculator is only useful if you can trust the number it gives you. This page explains where our formulas come from, how we test them and what we do when something is wrong.

Formulas from published sources

We do not invent formulas. Each tool is built on a method that is published and widely used: standard amortization and compound interest equations for loans and savings, the definitions used by health organizations for measures such as BMI, the Mifflin-St Jeor equation for resting energy needs, and the official definitions of units from bodies such as NIST and the International System of Units (SI). When experts use more than one method, the tool page says which one we use and, where it helps, lets you choose. Every tool page describes its formula in plain language so you can check the result yourself.

Automated tests with known answers

Every calculator has its own set of automated tests. The tests feed the calculation known inputs and compare the output with reference values: worked examples from textbooks and standards, results checked by hand, and simple cases whose answer is obvious (a loan with zero interest, a conversion from a unit to itself). We also test the awkward cases on purpose: empty fields, zero, negative numbers, very large values and inputs that would divide by zero. The whole test suite runs again before changes go live, so a fix in one place cannot quietly break another tool.

Calculations run in your browser

The numbers you type are processed by code running on your own device. They are not sent to our servers, not stored and not shared. This keeps the tools fast and private, and it means a tool keeps working even on a slow connection once the page has loaded. A few network tools, such as the IP and DNS lookups, need to ask a server by their nature; their pages say so.

Rounding and estimates

Calculations use full precision internally and are rounded only for display. Money is shown to the cent and rounded half away from zero, so totals in a schedule can differ from a lender's statement by a cent or two. Other results are shown with a sensible number of decimal places for the task. Many results are estimates by design: a loan payment does not include fees your lender may add, and a calorie target is a starting point, not a prescription. Where an assumption matters, the tool states it next to the result.

Health and finance results

Our health tools are for general information and education. They are not a diagnosis and cannot replace advice from a doctor, dietitian or other qualified professional who knows your situation. Our finance tools illustrate how a calculation works; they are not financial, tax or legal advice, and real offers depend on rates, fees and rules we cannot see. Please check important decisions with a professional.

How content is reviewed

Tool descriptions, frequently asked questions and guides may be drafted with the help of software, but nothing is published automatically. The site owner reviews every text and every calculator before it goes live, checks the examples against the tool itself and edits or rejects anything that is unclear or inaccurate. Pages are revisited when standards change or when readers point out a problem.

Found an error? Tell us

If a result looks wrong, please contact us and include the tool, the values you entered and the result you expected. We check every report, correct confirmed errors as quickly as we can and add a test for the case so the same mistake cannot return.

You can read more about the site on our About page.