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UK Tax Tools

Methodology & Sources

Last reviewed: 2026-04-19

Primary sources

All tax rules, bands, thresholds, and National Insurance rates used across UK Tax Tools come from primary government sources — GOV.UK, HMRC, the annual Finance Acts, and Scottish Government rate resolutions where Scottish income tax differs. We do not rely on secondary commentary for numbers.

Update cadence

  • New tax year (6 April) — all calculators are audited and dual-year support (current + upcoming) is maintained once rates are confirmed.
  • Autumn Budget / Spring Statement — affected calculators and guides are reviewed within 48 hours of announcement.
  • HMRC rate revisions mid-year — applied as soon as the new rate is published.
  • "Last updated" dates — displayed at the foot of every calculator page, sourced from the file's git history.

Where the math lives

Calculation logic lives in tested TypeScript files under src/utils/, separated from UI. Every number shown can be traced back to a pure function with a test suite that exercises band boundaries, allowance tapers, and worked examples from HMRC where available. UI components pass inputs to these functions and render the result — they contain no tax logic of their own.

Verification

Where HMRC publishes a worked example (income tax, PAYE, NI, CGT, IHT), we add it to our automated test suite so future changes cannot regress against the published answer. We also run a pre-deploy review on flagship calculators (Income Tax, Take-Home Pay, Inheritance Tax) after any rate change.

Handling ambiguous rules

Some HMRC rules depend on circumstances we can't observe (residency, allowances transferred, marginal relief edge cases). In those cases the calculator uses the most common interpretation and flags the assumption inline, either in the input labels or the results panel.

Per-calculator sources and assumptions

Income Tax / Take-Home Pay / Personal Allowance

Sources

  • HMRC — Income Tax rates and Personal Allowances (gov.uk/income-tax-rates)
  • Scottish Government — Scottish Income Tax rates (gov.scot)
  • Finance Act 2024 — frozen personal allowance £12,570 until 2028

Assumptions

Personal Allowance taper at £100k, Marriage Allowance where claimed, Scottish rates applied when Scotland is selected.

National Insurance (employee, self-employed, employer)

Sources

  • HMRC — National Insurance rates and categories (gov.uk/national-insurance-rates-letters)
  • Autumn Statement 2024 — employee Class 1 main rate 8%, Class 4 main rate 6%
  • Autumn Budget 2024 — employer Class 1 rate 15% from April 2025, Secondary Threshold £5,000

Assumptions

Category A assumed unless otherwise selected; Apprenticeship Levy applied where relevant in employer cost calculator.

Dividend Tax

Sources

  • HMRC — Tax on dividends (gov.uk/tax-on-dividends)
  • Finance Act 2024 — dividend allowance £500

Assumptions

Dividends stacked on top of other income; allowance consumed by lowest-rate slice first.

Capital Gains Tax

Sources

  • HMRC — Capital Gains Tax rates (gov.uk/capital-gains-tax/rates)
  • Autumn Budget 2024 — main rates raised to 18%/24% from 30 October 2024
  • HMRC — Business Asset Disposal Relief (gov.uk/business-asset-disposal-relief)

Assumptions

Annual exempt amount £3,000 (2025/26); residential vs other rates split; BADR 14% for disposals on/after 6 April 2025.

Stamp Duty (SDLT / LBTT / LTT)

Sources

  • HMRC — SDLT rates (gov.uk/stamp-duty-land-tax/residential-property-rates)
  • Revenue Scotland — LBTT rates and bands (revenue.scot)
  • Welsh Revenue Authority — LTT rates and bands (gov.wales)

Assumptions

First-time buyer relief, higher-rate surcharge (3% SDLT / 6% LBTT ADS / 4% LTT higher), and non-resident surcharge applied when selected.

Inheritance Tax

Sources

  • HMRC — Inheritance Tax thresholds and rates (gov.uk/inheritance-tax)
  • IHTM14611 — 7-year gift taper relief
  • IHTA 1984 s7 — annual exemption £3,000

Assumptions

NRB £325k, RNRB £175k with £2m estate taper, transferable NRB between spouses, 7-year taper (0/20/40/60/80% relief at 0-3/3-4/4-5/5-6/6-7 years).

Student Loan (Plans 1, 2, 4, 5, Postgraduate)

Sources

  • Student Loans Company — Repayment thresholds (slc.co.uk)
  • HMRC — SL3 guidance

Assumptions

9% above threshold for Plans 1/2/4/5; 6% above threshold for Postgraduate; simultaneous deduction when eligible for both.

Pension / Tax Relief / Annual Allowance

Sources

  • HMRC — Pension tax relief (gov.uk/tax-on-your-private-pension/pension-tax-relief)
  • HMRC — Tapered annual allowance (gov.uk/guidance/pension-schemes-work-out-your-tapered-annual-allowance)

Assumptions

Standard annual allowance £60,000, tapered for threshold income > £200k and adjusted income > £260k, MPAA £10,000.

High Income Child Benefit Charge / Marriage Allowance

Sources

  • HMRC — HICBC (gov.uk/child-benefit-tax-charge)
  • HMRC — Marriage Allowance (gov.uk/marriage-allowance)

Assumptions

HICBC taper 1% per £200 of Adjusted Net Income between £60k–£80k from April 2024; Marriage Allowance transfer of 10% PA where eligible.

IR35 / CIS / Contractor Comparison

Sources

  • HMRC — Off-payroll working (gov.uk/topic/business-tax/ir35)
  • HMRC — Construction Industry Scheme (gov.uk/what-is-the-construction-industry-scheme)

Assumptions

Deemed direct payment calculation for inside-IR35 (5% admin allowance removed post Apr 2017 public / Apr 2021 private); CIS 20% (registered) and 30% (unregistered) deductions.

Not financial advice

UK Tax Tools provides general information only. It is not tax, legal, or financial advice. For a binding answer, speak to a qualified accountant or a member of the Chartered Institute of Taxation.

Report an error

We publish corrections within three working days of verification. A useful report includes the page URL, the inputs used, the number our calculator returned, and the HMRC source that contradicts it.