How we build and test our tools
Trust is earned in the details. This page explains how we build ToolFiddle's tools, how we check that they are accurate, and how we handle the topics where a wrong answer could matter.
One job, done clearly
Every tool is designed to do a single job well rather than to be a sprawling app you have to learn. Before we build one, we look at what people are actually trying to accomplish, the mistakes they tend to make, and the questions they ask. The tool, the wording, and the help text all follow from that.
How we verify accuracy
For anything that calculates or converts, we start from the recognized formula or standard for that task and implement it directly. We then check the output against worked examples, against established references, and against other reputable tools, using edge cases as well as ordinary inputs. Where a tool relies on a published method, such as a strength formula, a tax band, or a unit definition, we cite or follow the source rather than inventing our own.
Built to run on your device
Our tools run in your browser rather than on a server. That is a privacy choice, and it is also a reliability choice: the same code runs the same way for everyone, there is no hidden server step to go wrong, and you can see that your files are not being sent anywhere. Where a tool genuinely needs outside data to work, we say so plainly on the page.
Honest about limits
A calculator is only as good as the assumptions behind it. For topics that involve health, money, law, or safety, we write the tools to be measured and clear about what they can and cannot tell you, and we point you to a qualified professional when a real decision is at stake. We would rather a tool say "this is an estimate, confirm it" than pretend to be the last word.
Keeping tools current
Rules, rates, and standards change. When we learn that a formula, a rate, or a piece of guidance has moved, we update the affected tool. If you spot something that has fallen out of date or looks wrong, telling us is the fastest way to get it fixed.
Corrections and feedback
We treat corrections seriously. If a result looks off, email us at [email protected] or use the contact page with the tool, your inputs, and what you expected. Real feedback from people using the tools is the single best source of improvements.