Where these came from
One person writes and maintains every page here. Almost every tool started as a job he needed done and could not get a straight number for. The calculator that existed answered a slightly smaller question than the one he had, and never said so. That gap is where people overspend: they buy for the question the tool answered, then pay again for the part it skipped.
So the awkward part of a question gets handled here instead of rounded away. A marketplace fee comes off the total the buyer pays, because that is how the platform bills you. A quantity with a wide range around it gets the range, because one confident figure is how you end up short on material or long on cash tied up in it. The about page names the person behind this and walks through the method.
How the arithmetic is checked
Where the math allows it, a tool keeps its arithmetic in a standalone JavaScript module: pure functions, values in and values out, separate from the page that displays them. The reason is practical. A formula buried in page code can only be checked by typing numbers into the page, and in practice nobody does that.
Once the math is separate, it gets tested. A script in this repository runs numerical checks against those modules on every build. Each expected value comes from outside the code: an official unit definition, a calculation done by hand, or a mathematical identity that has to hold. None of the tests restate the formula they check, since that would prove nothing. If a check fails, the build fails and the tool does not ship.
Some numbers can't be verified that way. When a result rests on a cost estimate, a regional price or a supplier's rule of thumb, the page says so in the body text and tells you what the number depends on. Call your supplier before you order on it.
What runs where
The arithmetic runs in your browser. Square footage, prices and salaries stay on your machine, so there is no account to create and no copy of your numbers on a server. The one page that needs a server fetches live exchange rates, and it tells you what it sends and why.
Who runs this
The operator is MetaSecurity LTDA, a Brazilian company, registration (CNPJ) 38.660.173/0001-09. It is a small operation with no newsroom behind the calculators. We publish the registration number because you can check it, and on a page about money a claim you can't check is worth nothing. The same identification is on the privacy policy and the contact page. The about page covers how a tool gets built and what we refuse to publish.