How we verify our calculators

A calculator that is wrong is worse than no calculator, because it is wrong with confidence. Here is what we do so you can trust the number, and check it yourself.

We start from primary sources

When a calculator depends on a published figure, a tax bracket, a VA funding fee, a conversion factor, an interest rule, we go to the source that sets it, not to another calculator. Conversion factors use the exact internationally agreed definitions. Tax and benefit numbers come from the agency that publishes them. We note the effective date on the page so you know how current the figure is.

Every formula ships with a worked example

Each calculator explains its formula in plain language and walks a real example with real numbers. That example is not decoration. The numbers in it have to match what the calculator itself produces, so the explanation and the tool can never quietly disagree. On math, statistics, and geometry pages we go further and show the steps using the numbers you entered, generated by the same code that produced the answer.

We test by running the code, not by trusting it

Before a calculator ships, we run it through an automated test harness that feeds it inputs and checks the outputs against values we worked out independently. Across the site that is several thousand individual assertions. A flagship tool like the derivative calculator was checked against dozens of textbook problems and against hundreds of randomly generated expressions compared to an independent numerical method. If a single case disagrees, the calculator does not ship until it is fixed.

We test the edges, not just the easy cases

Zero, empty fields, negative numbers, very large numbers, the boundary between one rule and the next: these are where calculators usually break. We test them on purpose. Divisions are guarded so a blank field returns nothing rather than an error, and range based results state their assumptions instead of pretending to a precision they do not have.

Estimates are labeled as estimates

Some questions do not have one exact answer. Retirement, longevity, project costs, and anything that depends on the future are estimates, and we present them as honest ranges with the assumptions stated, not as false certainty. Health and finance tools carry a clear disclaimer because they are not a substitute for a professional who knows your situation.

When we get something wrong, we fix it and say so

We are not immune to mistakes, and figures go out of date every year. If you find an error, tell us and we will correct it. Notable data refreshes and fixes are recorded on our updates page so you can see what changed and when.