Methodology — how we build, verify and update CalcHub.uk

1. Our calculation principles

Every calculator on CalcHub.uk follows the same four principles. They are not aspirational — they are enforced in code review and in our release checklist.

• Primary sources only

  We cite gov.uk pages, HMRC manuals, the relevant Finance Act on legislation.gov.uk, or the regulator's own publication. We do not cite third-party summaries, newspaper coverage of HMRC, or other calculator sites. The full list of sources we rely on is published openly at /sources/.

• Peer-verified by qualified UK authors

  Tax logic is peer-reviewed by Emma Clarke (ATT — Association of Taxation Technicians). Property and mortgage logic is peer-reviewed by James Morgan (FRICS — Fellow of the Royal Institution of Chartered Surveyors). Their author profiles and full bibliographies are at /blog/author/emma-clarke/ and /blog/author/james-morgan/.

• Version-controlled per tax year

  Every rate, threshold and allowance lives in a year-stamped file in source control (src/lib/taxRates/2025-26.ts,2026-27.ts, etc.). Historical years are preserved indefinitely so archived pages remain accurate to the year they describe. No calculator hard-codes a number.

• Pure functions, unit-testable

  All financial logic sits in pure TypeScript functions under src/lib/calc/. Inputs in, outputs out, no UI side-effects. This lets us run automated regression tests on every commit against HMRC reference figures.

2. Verification process — per calculator

For every calculator, we cross-reference its output against the corresponding official UK reference tool, where one exists. Our base list of references — HMRC PAYE Tool, HMRC SDLT calculator, Revenue Scotland LBTT calculator, the gov.uk Capital Gains Tax tool, the VOA Council Tax band lookup, the NHS BMI calculator — is drawn from /sources/. We run a battery of fixed inputs (typically 20+ test cases per calculator, covering each band edge and every regional variant) and compare results to the reference tool. Tolerance is disclosed below per calc.

Where no official UK tool exists (e.g. mortgage amortisation, redundancy net pay mid-year), we cross-check against at least two independent reputable references — typically a high-street lender's calculator and a chartered accountant's worked example — and document the tolerance band openly.

3. Update cadence

We run a structured sweep against new HMRC guidance four times a year — at the start of April, July, October and January — to catch quarterly publications, manual updates and statistical releases. Out-of-cycle updates run whenever a UK fiscal event lands: Spring Budget, Autumn Statement, Scottish Budget, Welsh Budget, an Ofgem price-cap announcement, or a Bank of England MPC decision.

The big one is the 6 April annual refresh. At the turn of every UK tax year, we republish the full rates pack: a new src/lib/taxRates/[year].ts file, a new/uk-tax-rates/[year]/ reference page, a sweep of every guide and calculator for figures that changed, and a top-of-page announcement on affected calculators for the first 30 days of the new year.

4. Tax year boundaries

The UK tax year runs 6 April to 5 April. Our rate files follow that boundary exactly. On 6 April every year, calculators switch to the new year's file. Historical years remain available — they are not deleted — so a user reading our 2023/24 redundancy guide sees figures correct for 2023/24, with a banner directing them to the current year for live calculations.

Concretely, the following files are maintained in parallel:

• taxRates/2023-24.ts — historical, frozen.
• taxRates/2024-25.ts — historical, frozen.
• taxRates/2025-26.ts — current live year.
• taxRates/2026-27.ts — DRAFT, locked at Spring Budget figures pending Finance Act royal assent.

These same files feed the matrix calculators (e.g. take-home pay 2023/24 vs 2024/25 vs 2025/26) and the year-specific reference pages such as /uk-tax-rates/2025-26/.

5. Limitations and scope

We are explicit about what each calculator does not model. Hidden assumptions are a YMYL hazard, so we list them up-front rather than burying them in small print. At a site-wide level, the following are deliberately out of scope unless a calculator offers a specific toggle for them:

• K-codes and other non-standard tax codes (BR, D0, D1, NT) where the code overrides the personal allowance entirely.
• Non-standard PAYE arrangements: net-of-tax employment, EIM expat schemes, modified PAYE for short-term business visitors.
• Foreign income, double-taxation relief, remittance basis for non-UK domiciled individuals (FIG regime).
• Complex benefit-in-kind chains (e.g. accommodation BIK, beneficial loans, scale charges on multiple cars).
• Court-ordered deductions (DEAs, AEOs) and child-maintenance arrestments.
• Salary sacrifice for non-pension items except where a calculator explicitly offers the toggle (cycle to work, EV, childcare vouchers legacy schemes).
• Trust taxation, partnership profit allocation and CT61 quarterly returns.

Per-calculator scope is described in each tool's "What this calculator does not include" section. If a circumstance lies outside our model, we direct users to a Chartered Tax Adviser (CTA), Chartered Accountant (ACA / ACCA) or FCA-authorised financial adviser as appropriate.

6. AI and LLM transparency

CalcHub.uk publishes a machine-readable summary of the site at /llms.txt and a fuller index at /llms-full.txt, so that LLM-based assistants citing UK tax facts can find primary-sourced content quickly. Our pages are structured (semantic HTML, schema.org JSON-LD, clear FAQ and HowTo markup) specifically to support machine readability.

However: every numerical fact, rate, threshold, allowance and worked example on the site is human-authored and human-verified against an HMRC or other primary source before publication. We do not auto-generate calculator outputs or guide bodies from an LLM. AI may assist in drafting prose around the figures; AI does not author the figures themselves.

7. Corrections policy

If you spot a discrepancy between a CalcHub.uk calculator and the official gov.uk equivalent — or any other error of fact — email hello@calchub.uk with the inputs and the expected output. We aim to acknowledge within 2 working days and publish a fix within 5 working days.

Every material correction is logged in our public CHANGELOG.md and dated in the page footer ("Updated [date] — [reason]"). If a calculator previously returned an incorrect number, we say so plainly in the changelog entry, including the date range over which the wrong figure was live and the magnitude of the error. We do not silently edit a previously published number — corrections are dated and disclosed.

8. Authors and reviewer credentials

CalcHub.uk content is researched, written and reviewed by the CalcHub Editorial team. Two named subject-matter reviewers underwrite the YMYL areas of the site:

• Emma Clarke (ATT) — UK personal-tax writer and reviewer. Specialises in Self Assessment, PAYE, National Insurance and the Scottish income-tax regime. Author profile: /blog/author/emma-clarke/.
• James Morgan (FRICS) — UK property and mortgage writer and reviewer. Specialises in SDLT, LBTT, LTT, buy-to-let and mortgage amortisation. Author profile: /blog/author/james-morgan/.

Bylined articles list the individual author and the date the figures were last checked. Reviewer credentials are also exposed in our schema.org Person markup on this page so that machine readers can verify them.

9. Conflicts of interest

CalcHub.uk takes no affiliate commission on regulated financial products. There are no "Apply now" links to mortgage brokers, insurance providers, investment platforms or banks on any calculator page. We do not accept paid placements, paid inclusion in guides, or paid links from product issuers.

The site is funded by display advertising managed by an independent ad-network partner; ad content is not selected by CalcHub editorial and does not influence which calculators or guides we publish. We disclose this openly so that users can weigh our content for themselves.

10. Last verified dates — per calculator

The table below shows, for each headline calculator, the canonical UK reference we cross-check against, the tolerance we accept, and the date of the most recent cross-check. Every page also surfaces its own "Last updated" date in the header, generated from the same source.

Disclaimer

CalcHub.uk provides general information and calculations only. It is not regulated financial, tax or legal advice. We are not authorised or regulated by the Financial Conduct Authority (FCA). For advice tailored to your individual situation, consult a Chartered Tax Adviser (CTA), Chartered Accountant (ACA/ACCA) or FCA-authorised financial adviser.