A calculator is only useful when its method is understood and its results have been tested. This page explains the process used to build and review tools on The Useful Site.
Start with the question
Development begins with the problem the visitor is trying to solve. Before writing code, I identify the required inputs, the result that would actually help and whether the answer can be calculated responsibly.
If a question needs information that cannot reasonably be collected by a simple browser tool, I do not pretend otherwise. It may offer a limited estimate, require the user to supply a published coefficient or explain why professional assessment is needed.
Find the method
The method may come from straightforward geometry, financial mathematics, physical relationships, official guidance, manufacturer documentation or published research.
Primary sources are preferred where they are available. Examples include GOV.UK, HMRC, the NHS, Microsoft documentation, standards organisations, regulators and original research papers.
Write the calculation visibly
Important assumptions are visible on the page, or represented as fields you can adjust. Hidden constants are avoided where they could materially change the answer.
Where several accepted approaches exist, the page says which one it uses. Specialist tools may ask the user to enter a coefficient or threshold from the source appropriate to their circumstances.
Test ordinary examples
Each calculator is tested with realistic values that a visitor might enter. The result is compared with a manual calculation, spreadsheet, trusted reference example or independent implementation where practical.
Test the edges
Testing should also cover values that expose mistakes:
- zero and blank entries;
- negative values where they make no sense;
- very small and very large figures;
- decimal values and rounding;
- inconsistent units;
- reversed dates or impossible ranges;
- mobile input and keyboard use.
Check the explanation
A correct formula can still produce a poor page if the result is not explained. I check whether the units are clear, whether an estimate is labelled as an estimate and whether the visitor is warned about the main limitations.
Review high-impact subjects more carefully
Health, finance and forensic-science tools require extra care because a reader may be tempted to treat a simple estimate as professional advice or evidence.
These pages make the educational role clear and point towards appropriate professional or official help where a decision carries significant consequences.
Browser-based privacy
Calculator logic is normally written in JavaScript and runs on the visitor's device. The values entered into a calculator are not deliberately submitted to the website as saved calculator records.
This statement is about calculator entries. The site may still use essential hosting logs, consent controls and advertising technologies as explained in the Privacy and Cookie policies.
Maintenance
Rates, allowances, guidance and product behaviour can change. I review and update pages that rely on changeable information when the underlying source changes.
My publication checklist
| Check | Question |
|---|---|
| Purpose | Does the tool solve a real and clearly stated problem? |
| Method | Can the formula or model be explained and supported? |
| Testing | Have normal, unusual and invalid inputs been checked? |
| Interpretation | Will the visitor understand what the result does—and does not—mean? |
| Accessibility | Are labels, focus, contrast and mobile layout usable? |
| Privacy | Does the calculation avoid collecting inputs unnecessarily? |
| Maintenance | Is there a plan to review information that may change? |