Methodology
How Calclens builds, verifies, and maintains its calculators — and why you can trust the numbers.
Our principles
Every Calclens calculator follows the same set of rules:
- Formulas come from authoritative sources — government bodies (HMRC, FCA, NHS, ONS), official standards (British Standards, ISO), or peer-reviewed academic work. Never from anonymous web pages or other calculator sites.
- Calculation runs in your browser — not on a remote server. The maths is open to inspection in the page source, and your numbers never leave your device for the calculation itself.
- Each formula is documented on its calculator’s page — typically under a “How it works” section, with worked examples that you can verify by hand.
- British conventions by default — GBP, UK tax bands, NHS health guidance, miles with kilometres alongside where useful. We say so when a calculator is region-neutral.
- Updates are continuous — when rates change (tax thresholds, VAT rates, NHS guidance), we update the calculator. Each page shows its last-updated date.
How a calculator gets built
Each calculator goes through a six-step process before it goes live:
1. Research the formula
We identify the authoritative source — for example, HMRC’s published PAYE tables for an income tax calculator, or NHS BMI thresholds for a body-mass calculator. We do not derive formulas from competitor calculators or unsourced web pages.
2. Validate against a known answer
We pick a worked example from the source (or, where the source doesn’t provide one, a recognised textbook or government worksheet) and confirm our implementation returns the same number to the appropriate precision.
3. Build the widget
The calculator is written as a small self-contained widget in vanilla JavaScript. It runs in your browser, formats numbers using Intl.NumberFormat for UK locale (correct commas, currency symbols, rounding), and works without an internet connection once the page has loaded.
4. Write the explanation page
Each calculator gets a 1,200- to 1,800-word page that includes an introduction, a “how it works” section with the formula in plain notation, three to five worked examples, and a frequently asked questions section. This is the content layer that helps you understand the number — not just receive it.
5. Write the AI prompt
Every calculator has its own dedicated prompt for the optional AI explanation feature. The prompt is tested against typical inputs to make sure the response is useful, accurate to the calculation, and free of recommendations for specific products or providers. See our how AI works page for details.
6. Cross-check before publishing
Before a calculator goes live, we re-test edge cases (zero values, very large values, negative inputs where applicable), check the page on a 320-pixel-wide mobile viewport, validate the structured data, and run a Lighthouse audit. Pages that don’t score 90+ on mobile are sent back for fixes.
How we handle updates
Rates, thresholds, and guidance change. Our approach:
- Tax-year-sensitive calculators (income tax, VAT, stamp duty, salary take-home) are reviewed in the weeks before the start of the new UK tax year on 6 April, and again in November if there’s an autumn budget that affects them.
- Health calculators are reviewed when NHS or NICE publishes updated guidance for the formula or thresholds we use.
- General formulas (compound interest, percentage, area, etc.) don’t change, so they don’t get rate-driven updates — but they may be revised for clarity or accessibility.
- Errors are fixed as soon as they’re confirmed. If a fix changes a published number, we say so on the page and in the build log.
Every calculator page shows its last-updated date near the top. If you spot a date that looks stale relative to a known change in UK rates, please tell us.
What we don’t do
Some practices common to other calculator sites that we deliberately avoid:
- We do not run affiliate links. No mortgage calculator on Calclens recommends a specific lender or earns a commission on a click-through.
- We do not show display ads. The site is free to use; future revenue is planned to come from optional paid PDF reports, not from advertising.
- We do not personalise results based on tracking. Calclens has no login, no profile, and no cross-site identifier. Your inputs aren’t stored against an identity.
- We do not present AI explanations as advice. Every AI response on a YMYL topic includes a disclaimer pointing you toward a qualified professional for decisions that matter.
Reporting an error
If a Calclens calculator gives a result that conflicts with a published government figure or a recognised standard, we want to know. The fastest way to reach us is the contact page. Please include the calculator’s URL, the inputs you used, the result we showed, and the result you expected (with a link to the source).
See our sources
We publish a categorised list of the references we use to build each calculator. It’s the next stop if you want to verify a formula yourself.