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Inspectable by design

Open Methodology

Most consumer-finance calculators are black boxes — you put in numbers, you get a number out, and there is no obvious way to check the work. Every calculator on UK Tax Drag is openly inspectable, and this page explains how to read the math for yourself.

By · Reviewed

The principle

If you cannot read a calculator's logic, you cannot trust the calculator's output. The site treats that as a hard rule. Every page that performs a calculation:

How to inspect any calculator

  1. Open the calculator — pick any one, e.g. Tax Calculator.
  2. Open browser DevTools — F12 in most browsers, or right-click → Inspect.
  3. Open the Sources / Debugger tab and look in uktaxdrag.co.uk/assets/. The calculator's logic lives there as a readable .js file (e.g. tdx-tax-calculator-page.js).
  4. Read the math — the rate constants, threshold numbers, and step order are all visible. Where a comment says "// HMRC: …" the source is named explicitly.
  5. Compare with HMRC — if you have an HMRC tool that does the same job (e.g. their income-tax estimator), put the same inputs in both. Differences are signal.

What "open" actually means here

The site uses no obfuscated bundle, no minifier, no source-map-stripped build. The JavaScript that runs in your browser is the same JavaScript a developer would read on disk. There is no hidden server-side calculation: every figure on a calculator page is computed in your own browser, on the visible inputs, using the visible code.

Why we don't minify: minification typically saves a few kilobytes and costs every reader the ability to verify the calculation. For a small site about money, that trade is wrong.

Per-calculator methodology pages

For the most-used calculators, there is a dedicated methodology page that walks the formula in plain English (e.g. how Income Tax is layered with NI, how the personal-allowance taper is applied above £100,000, how SDLT bands compose). Each calculator's About this calculator footer links to its methodology page where one exists.

If you want a methodology page added for a specific calculator, ask through the contact form.

Reproducibility

Every calculator output should be reproducible by hand on a single sheet of paper given the same inputs and the published rates. If you ever find a figure on the site you cannot reproduce — even after reading the methodology page — that is a defect. Please report it; the calculation will be reviewed and either corrected or the methodology page will be updated to explain the discrepancy.

What we openly do not publish

Some operational pieces are not publicly inspectable for good reason — for example the admin key that protects messages sent through the contact form. It is used by a Netlify Function and kept out of the client because it is a server-side credential, not calculation logic. The site sends no email, so no email-sending credentials are in use.

See also: Independence statement · Site-wide methodology · Editorial policy · Changelog.

Editorial accountability
Open Trust Centre →

Pages are checked against the primary sources listed on each page and reviewed by the Editor; corrections are logged in the changelog. Pages are drafted with the help of AI tools (how AI is used). No affiliate revenue. No sponsored content. No paid placements.

Editorial standards Editorial process Corrections policy How we make money The Editor Methodology

UK Tax Drag is an independent publication by Finsolve Consulting Limited, not affiliated with or endorsed by HMRC, GOV.UK or any government body.